Technical Field

The present invention relates to apparatus for an aerosol generating device, in particular, apparatus for supplying a varying current to a circuit configured to inductively heat a susceptor arrangement.

Background

Smoking articles such as cigarettes, cigars and the like bum tobacco during use to create tobacco smoke. Attempts have been made to provide alternatives to these articles by creating products that release compounds without combusting. Examples of such products are so-called “heat not bum” products or tobacco heating devices or products, which release compounds by heating, but not burning, material. The material may be, for example, tobacco or other non-tobacco products, which may or may not contain nicotine.

Summary

According to a first aspect of the present invention there is provided apparatus for an aerosol generating device, the apparatus comprising: an inductive heating circuit configured to inductively heat a susceptor arrangement arranged to heat an aerosol generating material, to thereby generate an aerosol, the inductive heating circuit comprising: an inductive element configured to inductively heat the susceptor arrangement when a varying current flows through the inductive element; and a switching arrangement for causing the varying current to flow through the inductive element when a first DC voltage is supplied to the inductive heating circuit; a voltage regulator configured to receive an input voltage from a voltage supply and output the first DC voltage across the inductive heating circuit; and a control arrangement configured to adjust the first DC voltage output by the voltage regulator, thereby to adjust a power supplied to the heating circuit for heating the susceptor arrangement. The control arrangement may be configured to control the first DC voltage output by the voltage regulator by controlling a property of the voltage input to the voltage regulator.

The property of the input voltage to the voltage regulator may be a duty cycle of the input voltage.

The voltage regulator may be configured to allow the input voltage to the voltage regulator to be stepped down such that the first DC voltage is less than the input voltage.

The voltage regulator may be configured to allow the input voltage to the voltage regulator to be stepped up such that the first DC voltage is greater than the input voltage.

The control arrangement may be configured to adjust the first DC voltage supplied to the inductive heating circuit by the voltage regulator based on a determined value of the power previously supplied to the heating circuit.

The control arrangement may be configured to adjust the first DC voltage output by the voltage regulator based on a determined temperature of the susceptor arrangement.

The inductive heating circuit may comprise a resonant LC circuit.

The resonant LC circuit may be a parallel LC circuit comprising a capacitive element arranged in parallel with the inductive element.

The resonant LC circuit may be configured to operate at a resonant frequency of the LC resonant circuit to heat the susceptor arrangement. The switching arrangement may be configured to alternate between a first state and a second state to cause the varying current through the inductive element. The switching arrangement may be configured to alternate between the first state and the second state in response to voltage oscillations within the resonant circuit.

The voltage oscillations within the resonant circuit may act to cause the alternating of the switching arrangement between the first state and the second state to thereby cause the current through the inductive element to vary at the resonant frequency of the resonant circuit.

The apparatus may comprise a temperature determiner, wherein the temperature determiner is configured to determine a temperature of the susceptor arrangement based on the first DC voltage supplied by the voltage regulator to the heating circuit and one or more electrical properties of the heating circuit.

The one or more electrical properties of the heating circuit may comprise one or more of: a frequency at which the heating circuit is operating; a current drawn by the heating circuit; and an impedance of the heating circuit.

The voltage supply from which the voltage regulator receives the input voltage may be a DC voltage supply.

According to a second aspect of the present invention there is provided an aerosol generating device comprising the apparatus according to the first aspect.

According to a third aspect of the present invention there is provided an aerosol generating system comprising the aerosol generating device according to the second aspect and a susceptor arrangement arranged to be heated by the inductive element to thereby heat the aerosol generating material to generate a flow of aerosol.

The susceptor arrangement may be provided in a component separate from, and configured to releasably engage with, the aerosol provision device. According to a fourth aspect of the present invention there is provided a method of operating an aerosol generating system comprising an aerosol generating device and a susceptor arrangement arranged to be heated by the aerosol generating device to thereby heat an aerosol generating material to generate a flow of aerosol, the aerosol generating device comprising apparatus comprising: an inductive heating circuit configured to inductively heat the susceptor arrangement, the inductive heating circuit comprising: an inductive element configured to inductively heat the susceptor arrangement when a varying current flows through the inductive element; and a switching arrangement for causing the varying current to flow through the inductive element when a first DC voltage is supplied to the inductive heating circuit; a voltage regulator configured to receive an input voltage from a voltage supply and output the first DC voltage across the inductive heating circuit; and a control arrangement; wherein the method comprises: adjusting, by the control arrangement, the first DC voltage output by the voltage regulator, thereby to adjust a power supplied to the heating circuit for heating the susceptor arrangement.

Brief Description of the Drawings

The invention will now be described, by way of example only, with reference to the following figures, in which:

Figure 1 illustrates schematically an aerosol generating device according to an example;

Figure 2 illustrates schematically aspects of the circuitry of the aerosol generating device; Figure 3 illustrates aspects of the circuitry shown in Figure 2 in further detail, according to one example;

Figures 4A and 4B show schematic plots of voltages input to and output from the voltage regulator, according to examples;

Figure 5 illustrates schematically a first example heating circuit; Figure 6 illustrates schematically a second example heating circuit;

Figure 7 illustrates schematically a third example heating circuit; and

Figure 8 illustrates schematically a fourth example heating circuit. Detailed Description

Induction heating is a process of heating an electrically conducting object (or susceptor) by electromagnetic induction. An induction heater may comprise an inductive element, for example, an inductive coil and a device for passing a varying electric current, such as an alternating electric current, through the inductive element. The varying electric current in the inductive element produces a varying magnetic field. The varying magnetic field penetrates a susceptor suitably positioned with respect to the inductive element, generating eddy currents inside the susceptor. The susceptor has electrical resistance to the eddy currents, and hence the flow of the eddy currents against this resistance causes the susceptor to be heated by Joule heating. In cases where the susceptor comprises ferromagnetic material such as iron, nickel or cobalt, heat may also be generated by magnetic hysteresis losses in the susceptor, i.e. by the varying orientation of magnetic dipoles in the magnetic material as a result of their alignment with the varying magnetic field.

In inductive heating, as compared to heating by conduction for example, heat is generated inside the susceptor, allowing for rapid heating. Further, there need not be any physical contact between the inductive heater and the susceptor, allowing for enhanced freedom in construction and application.

An induction heater may comprise an LC circuit, having an inductance L provided by an induction element, for example the electromagnet which may be arranged to inductively heat a susceptor, and a capacitance C provided by a capacitor. The circuit may in some cases be represented as an RLC circuit, comprising a resistance R provided by a resistor. In some cases, resistance is provided by the ohmic resistance of parts of the circuit connecting the inductor and the capacitor, and hence the circuit need not necessarily include a resistor as such. Such a circuit may be referred to, for example as an LC circuit. Such circuits may exhibit electrical resonance, which occurs at a particular resonant frequency when the imaginary parts of impedances or admittances of circuit elements cancel each other. One example of a circuit exhibiting electrical resonance is an LC circuit, comprising an inductor, a capacitor, and optionally a resistor. One example of an LC circuit is a series circuit where the inductor and capacitor are connected in series. Another example of an LC circuit is a parallel LC circuit where the inductor and capacitor are connected in parallel. Resonance occurs in an LC circuit because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, while the discharging capacitor provides an electric current that builds the magnetic field in the inductor. The present disclosure focuses on parallel LC circuits. When a parallel LC circuit is driven at the resonant frequency, the dynamic impedance of the circuit is at a maximum (as the reactance of the inductor equals the reactance of the capacitor), and circuit current is at a minimum. However, for a parallel LC circuit, the parallel inductor and capacitor loop acts as a current multiplier (effectively multiplying the current within the loop and thus the current passing through the inductor). Driving the RLC or LC circuit at or near the resonant frequency may therefore provide for effective and/or efficient inductive heating by providing for the greatest value of the magnetic field penetrating the susceptor.

A transistor is a semiconductor device for switching electronic signals. A transistor typically comprises at least three terminals for connection to an electronic circuit. In some prior art examples, an alternating current may be supplied to a circuit using a transistor by supplying a drive signal which causes the transistor to switch at a predetermined frequency, for example at the resonant frequency of the circuit.

A field effect transistor (FET) is a transistor in which the effect of an applied electric field may be used to vary the effective conductance of the transistor. The field effect transistor may comprise a body B, a source terminal S, a drain terminal D, and a gate terminal G. The field effect transistor comprises an active channel comprising a semiconductor through which charge carriers, electrons or holes, may flow between the source S and the drain D. The conductivity of the channel, i.e. the conductivity between the drain D and the source S terminals, is a function of the potential difference between the gate G and source S terminals, for example generated by a potential applied to the gate terminal G. In enhancement mode FETs, the FET may be OFF (i.e. substantially prevent current from passing therethrough) when there is substantially zero gate G to source S voltage, and may be turned ON (i.e. substantially allow current to pass therethrough) when there is a substantially non-zero gate G-source S voltage.

An n-channel (or n-type) field effect transistor (n-FET) is a field effect transistor whose channel comprises an n-type semiconductor, where electrons are the majority carriers and holes are the minority carriers. For example, n-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with donor impurities (such as phosphorus for example). In n-channel FETs, the drain terminal D is placed at a higher potential than the source terminal S (i.e. there is a positive drain- source voltage, or in other words a negative source-drain voltage). In order to turn an n-channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is higher than the potential at the source terminal S.

A p-channel (or p-type) field effect transistor (p-FET) is a field effect transistor whose channel comprises a p-type semiconductor, where holes are the majority carriers and electrons are the minority carriers. For example, p-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with acceptor impurities (such as boron for example). In p-channel FETs, the source terminal S is placed at a higher potential than the drain terminal D (i.e. there is a negative drain- source voltage, or in other words a positive source-drain voltage). In order to turn a p- channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is lower than the potential at the source terminal S (and which may for example be higher than the potential at the drain terminal D).

A metal-oxide- semiconductor field effect transistor (MOSFET) is a field effect transistor whose gate terminal G is electrically insulated from the semiconductor channel by an insulating layer. In some examples, the gate terminal G may be metal, and the insulating layer may be an oxide (such as silicon dioxide for example), hence “metal-oxide-semiconductor”. However, in other examples, the gate may be made from other materials than metal, such as polysilicon, and/or the insulating layer may be made from other materials than oxide, such as other dielectric materials. Such devices are nonetheless typically referred to as metal-oxide- semiconductor field effect transistors (MOSFETs), and it is to be understood that as used herein the term metal-oxide- semiconductor field effect transistors or MOSFETs is to be interpreted as including such devices.

A MOSFET may be an n-channel (or n-type) MOSFET where the semiconductor is n-type. The n-channel MOSFET (n-MOSFET) may be operated in the same way as described above for the n-channel FET. As another example, a MOSFET may be a p-channel (or p-type) MOSFET, where the semiconductor is p-type. The p- channel MOSFET (p-MOSFET) may be operated in the same way as described above for the p-channel FET. An n-MOSFET typically has a lower source-drain resistance than that of a p-MOSFET. Hence in an “on” state (i.e. where current is passing therethrough), n-MOSFETs generate less heat as compared to p-MOSFETs, and hence may waste less energy in operation than p-MOSFETs. Further, n-MOSFETs typically have shorter switching times (i.e. a characteristic response time from changing the switching potential provided to the gate terminal G to the MOSFET changing whether or not current passes therethrough) as compared to p-MOSFETs. This can allow for higher switching rates and improved switching control.

Figure 1 illustrates schematically an aerosol generating device 100, according to an example. The aerosol generating device 100 comprises a DC power source 104, in this example a battery 104, a susceptor arrangement 110, and circuitry 140 comprising an inductive element 158 for heating the susceptor arrangement 110. The susceptor arrangement 110 is arranged to heat an aerosol generating material 116 to thereby generate an aerosol, e.g., for inhalation by a user.

In the example of Figure 1, the susceptor arrangement 110 is located within a consumable 120 along with the aerosol generating material 116. The DC power source 104 is electrically connected to the circuitry 140 and is arranged to provide DC electrical power to the circuitry 140. The circuitry 140 comprises a control arrangement 106, a voltage regulator 154 and a heating circuit 150 (shown in Figure 2) which comprises the inductive element 158. The control arrangement 106 may comprise means for switching the device 100 on and off, for example in response to a user input. The control arrangement 106 may for example comprise a puff detector (not shown), as is known per se, and/or may take user input via at least one button or touch control (not shown). The control arrangement 106 may comprise means for monitoring the temperature of components of the device 100 or components of a consumable 120 inserted in the device.

The inductive element 158 may be, for example, a coil, which may, for example, be planar. The inductive element 158 may, for example, be formed from copper (which has a relatively low resistivity). The circuitry 140 is arranged to convert an input DC current from the DC power source 104 into a varying, e.g. alternating, current through the inductive element 158, as will be described below in more detail. The circuitry 140 is arranged to drive the varying current through the inductive element 158.

The susceptor arrangement 110 is arranged relative to the inductive element 158 for inductive energy transfer from the inductive element 158 to the susceptor arrangement 110. The susceptor arrangement 110 may be formed from any suitable material that can be inductively heated, for example a metal or metal alloy, e.g., steel. In some implementations, the susceptor arrangement 110 may comprise or be entirely formed from a ferromagnetic material, which may comprise one or a combination of example metals such as iron, nickel and cobalt. In some implementations, the susceptor arrangement 110 may comprise or be formed entirely from a non-ferromagnetic material, for example aluminium. The inductive element 158, having varying current driven therethrough, causes the susceptor arrangement 110 to heat up by Joule heating and/or by magnetic hysteresis heating, as described above. The susceptor arrangement 110 is arranged to heat the aerosol generating material 116, for example by conduction, convection, and/or radiation heating, to generate an aerosol in use. In some examples, the susceptor arrangement 110 and the aerosol generating material 116 form an integral unit that may be inserted and/or removed from the aerosol generating device 100, and may be disposable. In some examples, the inductive element 158 may be removable from the device 100, for example for replacement. The aerosol generating device 100 may be hand-held.

It is noted that, as used herein, the term “aerosol generating material” includes materials that provide volatilised components upon heating, typically in the form of vapour or an aerosol. Aerosol generating material may be a non-tobacco-containing material or a tobacco-containing material. For example, the aerosol generating material may be or comprise tobacco. Aerosol generating material may, for example, include one or more of tobacco per se, tobacco derivatives, expanded tobacco, reconstituted tobacco, tobacco extract, homogenised tobacco or tobacco substitutes. The aerosol generating material can be in the form of ground tobacco, cut rag tobacco, extruded tobacco, reconstituted tobacco, reconstituted material, liquid, gel, gelled sheet, powder, or agglomerates, or the like. Aerosol generating material also may include other, non tobacco, products, which, depending on the product, may or may not contain nicotine. Aerosol generating material may comprise one or more humectants, such as glycerol or propylene glycol.

The aerosol generating device 100 comprises an outer body 112 which houses the DC power supply 104, the control circuitry 106 and the circuitry 150 comprising the inductive element 158. The consumable 120 comprising the aerosol generating material 116, and in this example also comprising the susceptor arrangement 110, is inserted into the body 112 to configure the device 100 for use. The outer body 112 comprises a mouthpiece 114 to allow aerosol generated in use to exit the device 100.

In use, a user may activate, for example via a button (not shown) or a puff detector (not shown), the circuitry 140 to cause varying, e.g. alternating, current to be driven through the inductive element 158, thereby inductively heating the susceptor arrangement 110, which in turn heats the aerosol generating material 116, and causes the aerosol generating material 116 thereby to generate an aerosol. The aerosol is generated into air drawn into the device 100 from an air inlet (not shown), and is thereby carried to the mouthpiece 114, where the aerosol exits the device 100 for inhalation by a user. In other examples, the device 100 itself may not comprise a mouthpiece. For example, the consumable 120 may be configured to be engaged by the user to inhale the generated flow of aerosol.

The device 100 may be arranged to heat the aerosol generating material 116 to a range of temperatures to volatilise at least one component of the aerosol generating material 116 without combusting the aerosol generating material. For example, the temperature range may be about 50°C to about 350°C, such as between about 50°C and about 300°C, between about 100°C and about 300°C, between about 150°C and about 300°C, between about 100°C and about 200°C, between about 200°C and about 300°C, or between about 150°C and about 250°C. In some examples, the temperature range is between about 170°C and about 250°C. In some examples, the temperature range may be other than this range, and the upper limit of the temperature range may be greater than 300°C.

It will be appreciated that there may be a difference between the temperature of the susceptor arrangement 110 and the temperature of the aerosol generating material 116, for example during heating up of the susceptor arrangement 110, for example where the rate of heating is large. It will therefore be appreciated that in some examples the temperature at which the susceptor arrangement 110 is heated to may, for example, be higher than the temperature to which it is desired that the aerosol generating material 116 is heated. In this example, the susceptor arrangement 110 is a part of the consumable 120 which is insertable into the device 100 to configure the device 100 for use. However, in other examples, the susceptor arrangement 110 may form a part of the device 100. For example, the susceptor arrangement 110 may form a tube defining a heating chamber in which aerosol generating material 116 may be received to be heated.

Figure 2 shows a schematic representation of the circuitry 140 of the device 100 and the voltage supply 104, according to an example. The circuitry 140 comprises the control arrangement 106, the voltage regulator 154 and a heating circuit 150. The heating circuit 150 comprises the inductive element 158 and a switching arrangement 180 which is configured to cause a varying, e.g. alternating, current to flow through the inductive element 158 in order to inductively heat the susceptor arrangement 110.

The voltage regulator 154 allows inductive heating of the susceptor arrangement 110 by the resonant circuit 150 to be controlled by controlling a voltage supplied to the resonant circuit 150. Controlling the voltage supplied to the circuit 150 controls the current flowing in the resonant circuit 150, and hence may control the energy transferred to the susceptor arrangement 110 by the resonant circuit 150. This allows the degree to which the susceptor arrangement 110 is heated to be controlled. In some examples, the temperature of the susceptor arrangement 110 may be monitored and a determination made of whether the susceptor arrangement 110 is to be heated to a greater or lesser degree. A desired heating power may be determined accordingly. The desired heating power may then be supplied by employing the voltage regulator 154 to change the magnitude of the voltage supplied to the heating circuit 150. In some examples, the desired heating power at any time during a usage session of the device 100 may be predetermined. In another example, the desired power level may be set according to a determined temperature of the susceptor arrangement 110. For example, a target temperature for the susceptor arrangement 110 may be defined and the voltage regulator 154 may be used to adjust the heating power based on a comparison of a determined temperature of the susceptor arrangement 110 to the target temperature. The target temperature may in some examples change throughout a usage session, for example, according to a predefined heating profile.

The device 100 in some examples may comprise a temperature determiner for determining the temperature of the susceptor arrangement 110, e.g. by use of one or more temperature sensors, or via wireless means. For example, the control arrangement 106 may act as a temperature determiner which determines the temperature of the susceptor arrangement 110 based on one or more electrical properties of the heating circuit 150. As will be described below in more detail, in some examples the heating circuit is a resonant LC circuit which may operate at a resonant frequency. The resonant frequency may change based on the temperature of the susceptor arrangement 110. In such examples, the frequency at which the heating circuit 150 is operating and/or the current drawn by the heating circuit 150 and/or the impedance of the heating circuit 150 may be correlated with the temperature susceptor arrangement 110. Any one of these correlations may be used to determine the temperature of the susceptor arrangement 110. For example, the operating frequency and/or current draw and/or impedance and the temperature of the susceptor arrangement 110 may be measured while the heating circuit 150 is tested in order to obtain one or more calibration curves relating the frequency/current draw/impedance to the temperature of the susceptor arrangement 110. Then, for example, in operation, the frequency at which the heating circuit 150 is operating, the current drawn by the heating circuit 150 or the impedance of the heating circuit 150 may be measured and a look-up performed in a table to obtain a value for the temperature of the susceptor arrangement 110.

The control arrangement 106 may control the operation of the voltage regulator 154 to allow the voltage supplied by the voltage regulator 154 to the heating circuit 150 to be controlled. For example, the control arrangement 106 may control a duty cycle of an input voltage provided to the voltage regulator 154.

In some examples, the voltage regulator 154 is a buck regulator which is configured to step down the voltage received from the voltage supply 104 by a given amount. This may allow the heating power to be controlled, e.g. reduced, by controlling the voltage supplied by the voltage regulator to the heating circuit 150.

In some examples, the voltage regulator 154 may be used to ensure that a constant voltage is supplied to the heating circuit 150. This may allow for better control of the heating power provided by the circuit 150 to heat the susceptor arrangement 110. For example, in the absence of the voltage regulator, the voltage supplied to the heating circuit 150 may vary based on the loading provided by the heating circuit 150, which may vary based on the temperature of the susceptor arrangement 110, among other factors. The voltage regulator 154 may provide for a known, e.g. constant, voltage to be supplied to the heating circuit 150. This may allow for the power supplied by the heating circuit 150 to heat the aerosol generating material 116 to be controlled. In some examples, ensuring that the voltage supplied to the heating circuit 150 is known may provide for calculations performed by the control arrangement 106 which use the voltage supplied to the heating circuit 150 to be simplified. For example, the control arrangement 106 may be configured to monitor parameters such as the power provided by the heating circuit 150. Determining such parameters, in some examples, may require that the voltage provided to the heating circuit 150 is known. In some examples, parameters may be predetermined and stored in a look-up table. Ensuring that the voltage is a constant value may allow for the look-up table to be generated with fewer entries corresponding to fewer different values of the voltage supplied to the circuit 150. Therefore, if the voltage regulator 154 provides a fixed known voltage to the heating circuit 150 this may simplify the control of the device 100.

In some examples, the control arrangement 106 may be configured, in certain circumstances, to allow the output voltage from the voltage supply 104, e.g. a raw battery voltage, to be provided to the heating circuit 150 without that voltage being regulated by the voltage regulator 154. This may allow for lower energy losses by allowing the voltage regulator 154 to be bypassed when regulation of the voltage is not desired.

In some examples, a voltage may be supplied to drive the switching arrangement 180 which provides for the heating circuit 150 to generate a varying current through the inductive element 158. In the example shown in Figure 2, the switching arrangement 180 receives a drive voltage from the control arrangement 106. The control arrangement 106 may, for example, provide for a voltage signal to be supplied to the switching arrangement 180 to power and/or control various components of the switching arrangement 180. Examples of the switching arrangement 180 will be described in more detail below. The control arrangement 106 may in some examples comprise a micro controller unit (MCU) configured to take an input from the voltage supply 104 and to receive various other inputs from sensors and the like. The MCU may also be configured to supply various outputs to components of the device 100, such as one or more outputs to provide power to the heating circuit 150 and to provide a drive voltage to the switching arrangement 180.

Figure 3 shows the voltage regulator 154 and the heating circuit 150 according to an example. The voltage regulator 154 in Figure 3 is a buck regulator which is configured to receive an input voltage of V0 from the control arrangement 106 and to output a voltage VI across the heating circuit 150. The voltage regulator 154 is configured to allow the input voltage V0 to be stepped down to the output voltage VI, such that the output voltage VI has a lower magnitude than the input voltage V0. In this example, therefore, the voltage regulator 154 may be referred to as a buck regulator.

The voltage regulator 154 shown in Figure 3 comprises a first transistor 351, a second transistor 352, a gate driver 353, an output inductor 361 and an output capacitor 362. The first transistor 351 and second transistor 352 are both n-channel FETs. Each of the first transistor 351 and the second transistor 352 has a gate terminal G, a drain terminal D and a source terminal S. The gate terminals G of the first and second transistors 351, 352 are both connected to a gate driver 353. The gate driver 353 is configured to receive a gate drive signal Vg, in this example from the control arrangement 106, and to supply a gate voltage to operate the transistors 351, 352. The control arrangement 106 supplies a voltage V0 across a positive terminal and a negative terminal, which are labelled respectively + and - in Figure 3. The positive terminal + connects to the drain terminal D of the first transistor 351. The source terminal S of the first transistor 351 is connected to the drain terminal D of the second transistor 352 and both of these terminals are connected to a first side of the output inductor 361. The source terminal S of the second transistor 352 is connected to ground 151. The output capacitor 362 of the voltage regulator 154 is connected between a second side of the output inductor 361 and ground 151. The heating circuit 150 is connected in parallel with the output capacitor 362, i.e. between the second side of the output inductor 361 and ground 151.

In one example, the voltage VO across the positive terminal + and the negative terminal - is a fixed frequency voltage signal supplied from the control arrangement 106. A duty cycle of the fixed frequency voltage signal V0 may be variable by the control arrangement 106. Decreasing the duty cycle may allow the output voltage VI of the voltage regulator 154 to be reduced. The voltage regulator 154 thus may be arranged to supply a variable DC voltage VI to the heating circuit 150.

Figure 4A and Figure 4B show, schematically, an idealised representation of the resulting output voltage V 1 output by the voltage regulator 154 for an input voltage V0 having different duty cycles. Figures 4 A and 4B show schematic plots of voltage V on the vertical axis against time t on the horizontal axis. In Figure 4A a first example of the input voltage signal V0 is shown having a first duty cycle of around 50%. That is, the voltage signal V0 is in an “on” state for around half of the cycle and in an “off’ state for rest of the cycle. The voltage VI output by the voltage regulator 154 according to this first example is a steady DC voltage having a magnitude less than the magnitude of the voltage signal V0 in the “on” state. In the first example, shown in Figure 4A, the voltage VI output by the voltage regulator 154 may have a magnitude of around half that of the input signal V0 in the “on” state. In Figure 4B a second example is shown in which the input voltage signal V0 has the same magnitude in the “on” state as in the first example but a different duty cycle. Figure 4B shows the input voltage signal V0 having a duty cycle of around 30%. In Figure 4B the output voltage VI is again a steady DC output voltage with a magnitude less than that of the input voltage V0 in the “on” state. However, because of the lower duty cycle of the input signal V0 in Figure 4B, the magnitude of VI in Figure 4B is corresponding reduced compared to the magnitude of VI in Figure 4A. For example, in Figure 4B, VI may have a magnitude of substantially 30% that of the input voltage V0 in the “on” state.

In a variation of the circuit shown in Figure 3 (which is not shown in the figures), the output inductor 361, the output capacitor 362 are omitted. It has been found that in certain examples, the inductance provided by the output inductor 361 and the capacitance provided by the output capacitor 362 can be provided by the heating circuit 150 where the heating circuit 150 is as described herein. This allows the number of parts in the circuitry 140 to be reduced.

Turning now to Figure 5 further details of the heating circuit 150 are shown, according to a first example. In this example the heating circuit 150 is a resonant LC circuit, arranged for inductive heating of the susceptor arrangement 110. The heating circuit 150 accordingly may be referred to as the resonant circuit in the examples which follow.

The resonant circuit 150 comprises a switching arrangement 180 which, in this example, comprises a first transistor Ml and a second transistor M2. The first transistor Ml and the second transistor M2 each comprise a first terminal G, a second terminal D and a third terminal S. The inductive element 158 and the capacitor 156 are connected in parallel. The second terminals D of the first transistor Ml and the second transistor M2 are connected to either side of parallel combination of the inductive element 158 and the capacitor 156, as will be explained in more detail below. The third terminals S of the first transistor Ml and the second transistor M2 are each connected to ground 151. The first transistor Ml and the second transistor M2 are both MOSFETS having first, gate, terminals G; second, drain, terminals D; and third, source, terminals S. The transistors Ml, M2 are both n-channel MOSFETs in this example.

It will be appreciated that in alternative examples other types of transistors may be used in place of the MOSFETs described above.

The resonant circuit 150 has an inductance L and a capacitance C. The inductance L of the resonant circuit 150 is provided by the inductive element 158 and may also be affected by an inductance of the susceptor arrangement 110 which is arranged for inductive heating by the inductive element 158. The inductive heating of the susceptor arrangement 110 is via a varying magnetic field generated by the inductive element 158, which, in the manner described above, induces Joule heating and/or magnetic hysteresis losses in the susceptor arrangement 110. A portion of the inductance L of the resonant circuit 150 may be due to the magnetic permeability of the susceptor arrangement 110. The varying magnetic field generated by the inductive element 158 is generated by a varying, for example alternating, current flowing through the inductive element 158.

The inductive element 158 may, for example, be in the form of a coiled conductive element. For example, inductive element 158 may be a copper coil. The inductive element 158 may comprise, for example, multi-stranded wire, such as Litz wire, for example a wire comprising a number of individually insulated wires twisted together. The AC resistance of a multi-stranded wire is a function of frequency and the multi-stranded wire can be configured in such a way that the power absorption of the inductive element is reduced at a driving frequency. As another example, the inductive element 158 may be a coiled track on a printed circuit board, for example. Using a coiled track on a printed circuit board may be useful as it provides for a rigid and self- supporting track, with a cross section which obviates any requirement for multi- stranded wire (which may be expensive), which can be mass produced with a high reproducibility for low cost. Although one inductive element 158 is shown, it will be readily appreciated that there may be more than one inductive element 158 arranged for inductive heating of one or more susceptor arrangements 110.

The capacitance C of the resonant circuit 150 is provided by the capacitor 156. The capacitor 156 may be, for example, a Class 1 ceramic capacitor, for example a COG type capacitor. The total capacitance C may also comprise the stray capacitance of the resonant circuit 150; however, this is or can be made negligible compared with the capacitance provided by the capacitor 156.

The resistance of the resonant circuit 150 is not shown in Figure 5 but it should be appreciated that a resistance of the circuit may be provided by the resistance of the track or wire connecting the components of the resonant circuit 150, the resistance of the inductor 158, and/or the resistance to current flowing through the resonant circuit 150 provided by the susceptor arrangement 110 arranged for energy transfer with the inductor 158. In some examples, one or more dedicated resistors (not shown) may be included in the resonant circuit 150

The resonant circuit 150 is supplied with a DC supply voltage V 1 provided from the DC power source 104 via the voltage regulator 154 as has been described above. The voltage regulator 154 outputs a DC voltage VI across the resonant circuit 150. A first point 159 and at a second point 160 in the resonant circuit 150 are at voltage VI while the source terminals of both the MOSFETs Ml and M2 connect to ground 151.

The resonant circuit 150 may therefore be considered to be connected as an electrical bridge with the inductive element 158 and the capacitor 156 in parallel connected between the two arms of the bridge. The resonant circuit 150 acts to produce a switching effect, described below, which results in a varying, e.g. alternating, current being drawn through the inductive element 158, thus creating the alternating magnetic field and heating the susceptor arrangement 110.

The first point 159 is connected to a first node A located at a first side of the parallel combination of the inductive element 158 and the capacitor 156. The second point 160 is connected to a second node B, to a second side of the parallel combination of the inductive element 158 and the capacitor 156. A first choke inductor 161 is connected in series between the first point 159 and the first node A, and a second choke inductor 162 is connected in series between the second point 160 and the second node B. The first and second chokes 161 and 162 act to filter out AC frequencies from entering the circuit from the first point 159 and the second point 160 respectively but allow DC current to be drawn into and through the inductor 158. The chokes 161 and 162 allow the voltage at A and B to oscillate with little or no visible effects at the first point 159 or the second point 160.

In this particular example, the first MOSFET Ml and the second MOSFET M2 are n-channel enhancement mode MOSFETs. The drain terminal of the first MOSFET Ml is connected to the first node A via a conducting wire or the like, while the drain terminal of the second MOSFET M2 is connected to the second node B, via a conducting wire or the like. The source terminal of each MOSFET Ml, M2 is connected to ground 151.

The resonant circuit 150 comprises a second voltage source V2, gate voltage supply (or sometimes referred to herein as a control voltage), with its positive terminal connected at a third point 165 which is used for supplying a voltage to the gate terminals G of the first and second MOSFETs Ml and M2. The control voltage V2 supplied at the third point 165 in this example is independent of the voltage V 1 supplied at the first and second points 159, 160, which enables variation of voltage VI without impacting the control voltage V2. A first pull-up resistor 163 is connected between the third point 165 and the gate terminal G of the first MOSFET Ml. A second pull-up resistor 164 is connected between the third point 165 and the gate terminal G of the second MOSFET M2. The control voltage V2 in this example is output from the control arrangement 106, which receives power from the voltage supply 104.

In other examples, a different type of transistor may be used, such as a different type of FET. It will be appreciated that the switching effect described below can be equally achieved for a different type of transistor which is capable of switching from an “on” state to an “off’ state. The values and polarities of the supply voltages V 1 and V2 may be chosen in conjunction with the properties of the transistor used, and the other components in the circuit. For example, the supply voltages may be chosen in dependence on whether an n-channel or p-channel transistor is used, or in dependence on the configuration in which the transistor is connected, or the difference in the potential difference applied across terminals of the transistor which results in the transistor being either on or off.

The resonant circuit 150 further comprises a first diode dl and a second diode d2, which in this example are Schottky diodes, but in other examples any other suitable type of diode may be used. The gate terminal G of the first MOSFET Ml is connected to the drain terminal D of the second MOSFET M2 via the first diode dl, with the forward direction of the first diode dl being towards the drain D of the second MOSFET M2. The gate terminal G of the second MOSFET M2 is connected to the drain D of the first second MOSFET Ml via the second diode d2, with the forward direction of the second diode d2 being towards the drain D of the first MOSFET Ml. The first and second Schottky diodes dl and d2 may have a diode threshold voltage of around 0.3 V. In other examples, silicon diodes may be used having a diode threshold voltage of around 0.7V. In examples, the type of diode used is selected in conjunction with the gate threshold voltage, to allow desired switching of the MOSFETs Ml and M2. It will be appreciated that the type of diode and gate supply voltage V2 may also be chosen in conjunction with the values of pull-up resistors 163 and 164, as well as the other components of the resonant circuit 150.

The resonant circuit 150 supports a current through the inductive element 158 which is a varying current due to switching of the first and second MOSFETs Ml and M2. Since, in this example the MOSFETs Ml and M2 are enhancement mode

MOSFETS, when a voltage applied at the gate terminal G of one of the MOSFETs is such that a gate-source voltage is higher than a predetermined threshold for that MOSFET, the MOSFET is turned to the ON state. Current may then flow from the drain terminal D to the source terminal S which is connected to ground 151. The series resistance of the MOSFET in this ON state is negligible for the purposes of the operation of the circuit, and the drain terminal D can be considered to be at ground potential when the MOSFET is in the ON state. The gate-source threshold for the MOSFET may be any suitable value for the resonant circuit 150 and it will be appreciated that the magnitude of the voltage V2 and resistances of the resistors 163, 164 are chosen dependent on the gate-source threshold voltage of the MOSFETs Ml,

M2, essentially so that voltage V2 is greater than the gate threshold voltage(s).

The switching procedure of the resonant circuit 150 which results in varying current flowing through the inductive element 158 will now be described starting from a condition where the voltage at first node A is high and the voltage at the second node B is low. When the voltage at node A is high, the voltage at the drain terminal D of the first MOSFET Ml is also high because the drain terminal of Ml is connected, directly in this example, to the node A via a conducting wire. At the same time, the voltage at the node B is held low and the voltage at the drain terminal D of the second MOSFET M2 is correspondingly low (the drain terminal of M2 being, in this example, directly connected to the node B via a conducting wire).

Accordingly, at this time, the value of the drain voltage of Ml is high and is greater than the gate voltage of M2. The second diode d2 is therefore reverse-biased at this time. The gate voltage of M2 at this time is greater than the source terminal voltage of M2, and the voltage V2 is such that the gate-source voltage at M2 is greater than the ON threshold for the MOSFET M2. M2 is therefore ON at this time.

At the same time, the drain voltage of M2 is low, and the first diode dl is forward biased due to the gate voltage supply V2 to the gate terminal of Ml. The gate terminal of Ml is therefore connected via the forward biased first diode dl to the low voltage drain terminal of the second MOSFET M2, and the gate voltage of Ml is therefore also low. In other words, because M2 is on, it is acting as a ground clamp, which results in the first diode dl being forward biased, and the gate voltage of Ml being low. As such, the gate-source voltage of Ml is below the ON threshold and the first MOSFET Ml is OFF.

In summary, at this point the circuit 150 is in a first state, wherein: voltage at node A is high; voltage at node B is low; first diode dl is forward biased; second MOSFET M2 is ON; second diode d2 is reverse biased; and first MOSFET Ml is OFF. From this point, with the second MOSFET M2 being in the ON state, and the first MOSFET Ml being in the OFF state, current is drawn from the supply V 1 through the first choke 161 and through the inductive element 158. Due to the presence of inducting choke 161, the voltage at node A is free to oscillate. Since the inductive element 158 is in parallel with the capacitor 156, the observed voltage at node A follows that of a half sinusoidal voltage profile. The frequency of the observed voltage at node A is equal to the resonant frequency / ₀ of the circuit 150.

The voltage at node A reduces sinusoidally in time from its maximum value towards 0 as a result of the energy decay at node A. The voltage at node B is held low (because MOSFET M2 is on) and the inductor L is charged from the DC supply VI. The MOSFET M2 is switched off at a point in time when the voltage at node A is equal to or below the gate threshold voltage of M2 plus the forward bias voltage of d2. When the voltage at node A has finally reached zero, the MOSFET M2 will be fully off.

At the same time, or shortly after, the voltage at node B is taken high. This happens due to the resonant transfer of energy between the inductive element 158 and the capacitor 156. When the voltage at node B becomes high due to this resonant transfer of energy, the situation described above with respect to the nodes A and B and the MOSFETs Ml and M2 is reversed. That is, as the voltage at A reduces towards zero, the drain voltage of Ml is reduced. The drain voltage of Ml reduces to a point where the second diode d2 is no longer reverse biased and becomes forward biased. Similarly, the voltage at node B rises to its maximum and the first diode dl switches from being forward biased to being reverse biased. As this happens, the gate voltage of Ml is no longer coupled to the drain voltage of M2 and the gate voltage of Ml therefore becomes high, under the application of gate supply voltage V2. The first MOSFET Ml is therefore switched to the ON state, since its gate-source voltage is now above the threshold for switch-on. As the gate terminal of M2 is now connected via the forward biased second diode d2 to the low voltage drain terminal of Ml, the gate voltage of M2 is low. M2 is therefore switched to the OFF state.

In summary, at this point the circuit 150 is in a second state, wherein: voltage at node A is low; voltage at node B is high; first diode dl is reverse biased; second MOSFET M2 is OFF; second diode d2 is forward biased; and first MOSFET Ml is ON.

At this point, current is drawn through the inductive element 158 from the supply voltage VI through the second choke 162. The direction of the current has therefore reversed due to the switching operation of the resonant circuit 150. The resonant circuit 150 will continue to switch between the above-described first state in which the first MOSFET Ml is OFF and the second MOSFET M2 is ON, and the above- described second state in which the first MOSFET Ml is ON and the second MOSFET M2 is OFF.

In the steady state of operation, energy is transferred between the electrostatic domain (i.e., in the capacitor 156) and the magnetic domain (i.e., the inductor 158), and vice versa.

The net switching effect is in response to the voltage oscillations in the resonant circuit 150 where we have an energy transfer between the electrostatic domain (i.e., in the capacitor 156) and the magnetic domain (i.e., the inductor 158), thus creating a time- varying current in the parallel LC circuitry, which varies at the resonant frequency of the resonant circuit 150. This is advantageous for energy transfer between the inductive element 158 and the susceptor arrangement 110 since the resonant circuit 150 operates at its optimal efficiency level and therefore achieves more efficient heating of the aerosol generating material 116 compared to circuitry operating off resonance. The described switching arrangement is advantageous as it allows the resonant circuit 150 to drive itself at the resonant frequency under varying load conditions. What this means is that in the event that the properties of the resonant circuit 150 change (for example if the susceptor 110 is present or not, or if the temperature of the susceptor changes, or even physical movement of the susceptor element 110), the dynamic nature of the resonant circuit 150 continuously adapts its resonant point to transfer energy in an optimal fashion, thus meaning that the resonant circuit 150 is always driven at resonance. Moreover, the configuration of the resonant circuit 150 is such that no external controller or the like is required to apply the control voltage signals to the gates of the MOSFETS to effect the switching.

In examples described above, the gate terminals G are supplied with a gate voltage via a second power supply which is different to the power supply for the source voltage VI. However, in some examples, the gate terminals may be supplied with the same voltage supply as the source voltage VI. In such examples, the first point 159, second point 160 and third point 165 in the circuit 150 may, for example, be connected to the same power rail output from the voltage regulator 154. In such examples, it will be appreciated that the properties of the components of the circuit must be chosen to allow the described switching action to take place. For example, the gate supply voltage and diode threshold voltages should be chosen such that the oscillations of the circuit trigger switching of the MOSFETs at the appropriate level. The provision of separate voltage values for the gate supply voltage V2 and the source voltage VI allows for the source voltage VI to be varied independently of the gate supply voltage V2 without affecting the operation of the switching mechanism of the circuit.

The resonant frequency / ₀ of the circuit 150 may be in the MHz range, for example in the range 0.5 MHz to 4 MHz, for example in the range 2 MHz to 3 MHz. It will be appreciated that the resonant frequency / ₀ of the resonant circuit 150 is dependent on the inductance L and capacitance C of the circuit 150, as set out above, which in turn is dependent on the inductive element 158, capacitor 156 and additionally the susceptor arrangement 110. As such, the resonant frequency / ₀ of the circuit 150 can vary from implementation to implementation. For example, the frequency may be in the range 0.1 MHz to 4MHz, or in the range of 0.5 MHz to 2 MHz, or in the range 0.3 MHz to 1.2 MHz. In other examples, the resonant frequency may be in a range different from those described above. Generally, the resonant frequency will depend on the characteristics of the circuitry, such as the electrical and/or physical properties of the components used, including the susceptor arrangement 110.

It will also be appreciated that the properties of the resonant circuit 150 may be selected based on other factors for a given susceptor arrangement 110. For example, in order to improve the transfer of energy from the inductive element 158 to the susceptor arrangement 110, it may be useful to select the skin depth (i.e. the depth from the surface of the susceptor arrangement 110 within which the current density falls by a factor of 1/e, which is at least a function of frequency) based on the material properties of the susceptor arrangement 110. The skin depth differs for different materials of susceptor arrangements 110 and reduces with increasing drive frequency. On the other hand, for example, in order to reduce the proportion of power supplied to the resonant circuit 150 and/or driving element that is lost as heat within the electronics, it may be beneficial to have a circuit which drives itself at relatively lower frequencies. Since the drive frequency is equal to the resonant frequency in this example, the considerations here with respect to drive frequency are made with respect to obtaining the appropriate resonant frequency, for example by designing a susceptor arrangement 110 and/or using a capacitor 156 with a certain capacitance and an inductive element 158 with a certain inductance. In some examples, a compromise between these factors may therefore be chosen as appropriate and/or desired.

The resonant circuit 150 of Figure 5 has a resonant frequency fo at which the current I is minimised and the dynamic impedance is maximised. The resonant circuit 150 drives itself at this resonant frequency and therefore the oscillating magnetic field generated by the inductor 158 is maximum, and the inductive heating of the susceptor arrangement 110 by the inductive element 158 is maximised.

The voltage regulator 154 allows inductive heating of the susceptor arrangement 110 by the resonant circuit 150 to be controlled by controlling the supply voltage VI provided to the resonant circuit 150. Controlling the supply voltage VI in turn controls the current flowing in the resonant circuit 150, and hence may control the energy transferred to the susceptor arrangement 110 by the resonant circuit 150, and hence the degree to which the susceptor arrangement 110 is heated. In some examples, the temperature of the susceptor arrangement 110 may be monitored and a determination made of whether the susceptor arrangement 110 is to be heated to a greater or lesser degree. A desired heating power may be determined accordingly. The desired heating power may then be supplied by employing the voltage regulator 154 to change the magnitude of the DC voltage VI supplied to the resonant circuit 150. The actual power being supplied to the heating circuit 150 for heating the susceptor arrangement 110 may be determined, for example, based on the DC voltage V 1 and a current drawn by the resonant circuit 150, e.g. as determined by use of a current sense resistor.

As mentioned above, the inductance L of the resonant circuit 150 is provided by the inductive element 158 arranged for inductive heating of the susceptor arrangement 110. At least a portion of the inductance L of resonant circuit 150 is due to the magnetic permeability of the susceptor arrangement 110. The inductance L, and hence resonant frequency fo of the resonant circuit 150 may therefore depend on the specific susceptor(s) used and its positioning relative to the inductive element(s) 158, which may change from time to time. Further, the magnetic permeability of the susceptor arrangement 110 may vary with varying temperature of the susceptor 110.

Figure 6 shows a second example of a resonant circuit 250. The second resonant circuit 250 comprises many of the same components as the resonant circuit 150 and like components in each of the resonant circuits 150, 250 are provided with the same reference numerals and will not be described in detail again.

The second circuit 250 differs from the first circuit 150 in that the second circuit 250 does not comprise the diodes dl, d2, via which the gate terminals Gl, G2 of each of the transistors Ml, M2 are respectively connected to the drain terminals Dl, D2 of the other of the transistors Ml, M2. Instead of the diodes dl, d2 which are included in the first circuit 150, the second circuit 250 comprises a third MOSFET M3 and a fourth MOSFET M4. In the second circuit 250, the gate G1 of the first MOSFET Ml is connected to the drain D2 of the second MOSFET M2 via the third MOSFET M3. The gate G2 of the second MOSFET M2 is similarly connected to the drain D1 of the first MOSFET Ml via a fourth MOSFET M4. The control voltage V2 is supplied from the point 165 to gate terminals G3, G4 of both the third MOSFET M3 and the fourth MOSFET M4. In an example, such as the example represented by Figure 6, the gate terminals G3, G4 of the third MOSFET M3 and the fourth MOSFET M4 are connected to one another via an electrical conductor, for example an electrical track, and the voltage V2 supplied to a point on the electrical conductor. It will be appreciated that each of the third MOSFET M3 and the fourth MOSFET M4 has a gate threshold voltage such that when a voltage greater than the threshold voltage is applied to its gate terminal G3, G4, the respective MOSFET M3, M4 is turned “on” such that current may flow from its drain terminal to its source terminal. In examples, the voltage V2 is greater than the threshold voltages of the third and fourth MOSFETs M3, M4 such that applying the control voltage V2 turns the third and fourth MOSFETs M3, M4 to the ON state. In an example, the threshold voltage of the third MOSFET M3 is equal to the threshold voltage of the fourth MOSFET M4. In some examples, the second circuit 250 may comprise one of more pull-down resistors (not shown in Figure 6) connected between the gates Gl, G2 of the first and second MOSFETs Ml, M2 and ground.

The second circuit 250 operates as a self-oscillating circuit which causes a varying current to flow through the inductive element 158 in the manner described with reference to the first example circuit 150 with reference to Figure 5. Differences in the behaviour of the second circuit 250 from that of the first example circuit 150 due to the use of MOSFETs M3, M4 rather than diodes dl, d2, will become apparent from the following description.

The switching procedure of the second circuit 250 which results in a varying current flowing through the inductive element 158 will now be described.

When the voltage V2 is applied to the gates G3, G4 of the third and fourth MOSFETs M3, M4, the third and fourth MOSFETs are turned “on”. Providing that a voltage VI, at this point, each of the first, second, third and fourth MOSFETs M1-M4 is in the ON state. At this point, the voltages at nodes A and B start to fall. Certain imbalances may exist in the circuit 250, for example differences in resistance between the MOSFETs M1-M4, or the properties of the values of inductors present in the circuit. These imbalances act such that the voltage at one of the nodes A or B begins to fall faster than the voltage at the other of these nodes A, B. The MOSFET Ml, M2 corresponding to the node A, B at which the voltage falls fastest will remain in the ON state. The other of the MOSFETS Ml, M2, corresponding with the other of nodes A, B is switched to the OFF state. The following describes the situation wherein the voltage at node A begins oscillating and the voltage at the node B remains at zero. However, equally, it may be the case that it is the voltage at the node B which begins oscillating while the voltage at node A remains at zero volts.

When the voltage at node A rises, the voltage at the drain terminal D1 of the first MOSFET Ml also rises because the drain terminal D1 of first MOSFET Ml is connected to the node A via a conducting wire. At the same time, the voltage at the node B is held low and the voltage at the drain terminal D2 of the second MOSFET M2 is correspondingly low (the drain terminal D2 of the second MOSFET M2 being, in this example, directly connected to the node B via a conducting wire).

As the voltage at the node A and the drain D1 of the first MOSFET Ml rises, the voltage at the gate G2 of the second MOSFET M2 rises. This is due to the drain D1 being connected via the fourth MOSFET M4 to the gate G2 of the second MOSFET M2 and the fourth MOSFET M4 being “on” due to the voltage V2 being applied to its gate terminal G4.

As the voltage at the drain D1 of the first MOSFET Ml rises, the voltage at the gate G2 of the second MOSFET M2 continues to rise until it reaches a maximum voltage value V _m ax. The maximum voltage value V _m ax reached at the gate G2 of the second MOSFET M2 is dependent on the control voltage V2 and the gate-source voltage of the fourth MOSFET M4 (V _S M4). The maximum value V _max may be expressed as Vmax = V2 - V gsM4· After a half cycle of oscillation at the resonant frequency of the circuit 250, the voltage at the drain D1 of the first MOSFET Ml begins decreasing. The voltage at the drain D1 of the first MOSFET Ml decreases until it reaches 0V. At this point, the first MOSFET Ml turns from “off’ to “on” and the second MOSFET M2 turns from “on” to “off’.

The circuit then continues to oscillate in a similar manner as described above, except with the node A remaining at zero volts while the node B is free to oscillate. That is, the voltage at the drain D2 of the second MOSFET M2 and at the node B then begins rising, while the voltage at the drain D1 of the first MOSFET Ml and the node A remains at zero.

As the voltage at the node B and the drain D2 of the second MOSFET M2 rises, the voltage at the gate G1 of the first MOSFET Ml rises since the drain D2 is connected via the third MOSFET M3 to the gate G1 of the first MOSFET Ml and the third MOSFET M3 is “on” due to the voltage V2 being applied to its gate terminal G3.

As the voltage at the drain D2 of the second MOSFET M2 rises, the voltage at the gate G1 of the first MOSFET Ml continues to rise until it reaches a maximum voltage value V _m ax. The maximum voltage value V _m ax reached at the gate G1 is dependent on the control voltage V2 and the gate-source voltage of the third MOSFET M3 (VgsM3). The maximum value Vmax may be expressed as Vmax = V2 - V _S M3. In this example, the gate-source voltages of the third and fourth MOSFETs M3, M4 are equal to one another, i.e. V _gS M3 = V _S M4.

After a half cycle of oscillation at the resonant frequency of the second circuit 250, the voltage at the drain D2 of the second MOSFET M2 begins decreasing. The voltage at the drain D2 of the second MOSFET M2 decreases until it reaches 0V. At this point, the second MOSFET M2 turns from “off’ to “on” and the first MOSFET Ml turns from “on” to “off’. In the manner described with reference to the first example circuit 150, when the second MOSFET M2 is in the ON state, and the first MOSFET Ml is in the OFF state, current is drawn from the supply VI through the first choke 161 and through the inductive element 158. When the first MOSFET Ml is in the ON state, and the second MOSFET M2 is in the OFF state, current is drawn from the supply VI through the second choke 162 and through the inductive element 158. The second example circuit 250 therefore oscillates in the same manner as described for the first example circuit 150, with the direction of the current reversing with each switching operation of the circuit 250.

The use of third and fourth MOSFETs M3, M4, in some examples, may be advantageous because it may allow for lower energy losses. That is, the first example circuit 150 may result in resistive losses due to some current draw through the pull-up resistors 163, 164 to ground 151. For example, when the first MOSFET Ml is in the ON state, the second diode d2 is forward biased and thus a small current may be drawn through the second pull-up resistor 164, resulting in resistive losses. Similarly, when the second MOSFET M2 is in the ON state, there may be resistive losses due to current drawn through the first pull-up resistor 163. The second example circuit in examples may omit the resistors 163, 164. The second example circuit 250 may reduce such losses by substituting the pull-up resistors 163, 164 and the diodes dl, d2 for third and fourth MOSFETs M3, M4. For example, in the second example circuit 250, when the first MOSFET Ml is in the OFF state the current drawn through the third MOSFET M3 may be essentially zero. Similarly, in the second example circuit 250, when the second MOSFET M2 is in the OFF state the current drawn through the fourth MOSFET M4 may be essentially zero. Thus, resistive losses may be reduced by use of the arrangement shown in the second circuit 250. Further, energy may be required to charge and discharge the gates Gl, G2 of first MOSFET Ml and second MOSFET M2. The second circuit 250 may provide for this energy to be effectively provided from the nodes A and B.

Example circuits above have been described comprising two choke inductors 161, 162. In another example, an example inductive heating circuit may comprise only one choke inductor. In such an example circuit, the inductor coil 158 may be “centre- tapped”.

Figure 7 shows a third example circuit 350 which is a variation on the first example circuit 150 and in which the coil 158 is a centre-tapped coil and a single choke inductor 461 replaces the first and second choke inductors 161, 162. Again, components that are the same as those in the circuit 150 illustrated in Figure 5 are given the same reference numerals in Figure 7.

In the third circuit 350, voltage VI is applied via the choke inductor 461 to a centre of the inductor coil 158, at a single point 459 as opposed to at first and second points 159, 160 in the first example circuit 150. Rather than, as in the first and second example circuits 150, 250, current being drawn alternately through the first choke 161 and the second choke 162 as the current in the circuit changes direction due to the resonant oscillations of the circuit, current is drawn through the single choke inductor 461 and alternately drawn through a first part 158a of the inductor 158 and through a second part 158b of the inductor 158 as the current oscillations in the circuit 350 change direction due to the switching operation of the MOSFETs Ml, M2. The third circuit 350 operates in an equivalent manner to the first circuit 150 in other respects.

A fourth example circuit is shown in Figure 8. Again, components that are the same as those in the circuit 150 illustrated in Figure 5 are given the same reference numerals in Figure 8. The fourth circuit 450 differs from the third circuit 350 in that, rather than comprising the single capacitor 156 of the third circuit 350, the fourth circuit 450 is provided with a first capacitor 156a and a second capacitor 156b. The fourth circuit 450, similarly to the third circuit 350 comprises a centre-tapped arrangement with the inductor comprising a first part 158a and a second part 158b. The voltage VI is applied via the choke inductor 461 to a centre of the inductor coil 158 (as in the arrangement of Figure 7) and, further, the centre of the inductor coil 158 is electrically connected to a point between the first capacitor 156a and the second capacitor 156b. Two adjacent circuit loops are therefore provided, one comprising the first inductor part 158a and the first capacitor 156a and the other comprising the second inductor part 158b and the second capacitor 156b. The fourth circuit 450 operates in an equivalent manner to the third circuit 350 in other respects.

The centre-tapped arrangement described with reference to Figure 7 and Figure 8 can equally be applied in an arrangement which uses third and fourth MOSFETs instead of diodes, in the manner described with reference to Figure 6. The use of a centre-tapped arrangement may be advantageous since the number of parts required to assemble the circuit may be reduced. For example, the number of choke inductors may be reduced from two to one.

In examples described herein the susceptor arrangement 110 is contained within a consumable and is therefore replaceable. For example, the susceptor arrangement 110 may be disposable and for example integrated with the aerosol generating material 116 that it is arranged to heat. The resonant circuit 150 allows for the circuit to be driven at the resonance frequency, automatically accounting for differences in construction and/or material type between different susceptor arrangements 110, and/or differences in the placement of the susceptor arrangements 110 relative to the inductive element 158, as and when the susceptor arrangement 110 is replaced. Furthermore, the resonant circuit 150 is configured to drive itself at resonance regardless of the specific inductive element 158, or indeed any component of the resonant circuit 150 used. This is particularly useful to accommodate for variations in manufacturing both in terms of the susceptor arrangement 110 but also with regards to the other components of the circuit 150. For example, the resonant circuit 150 allows the circuit to remain driving itself at the resonant frequency regardless of the use of different inductive elements 158 with different values of inductance, and/or differences in the placement of the inductive element 158 relative to the susceptor arrangement 110. The circuit 150 is also able to drive itself at resonance even if the components are replaced over the lifetime of the device. In some examples, the aerosol generating device 100 is configured to be usable with a plurality of different types of consumables each of which consumables comprises a different type of susceptor arrangement to the other consumables. The different susceptor arrangements may be formed, for example, of different materials or be of different shapes or different sizes or different combinations of different materials or shapes or sizes.

In use, the resonant frequency of the circuit 150 is dependent upon the particular susceptor arrangement of whichever type of consumable is coupled to, for example inserted into, the device 100. However, the alternating frequency through the inductive element 158 of the resonant circuit, due to the self-oscillating arrangement of the circuit 150, is configured to self-adjust to match changes in the resonant frequency caused by the coupling of a different susceptor/consumable to the inductive element. Accordingly, the circuit is configured to heat a given susceptor arrangement at the resonant frequency of the circuit 150 when that consumable is coupled to the device 100, regardless of the properties of the susceptor arrangement or consumable.

In some examples, the aerosol generating device 100 is configured to receive a first consumable having a first susceptor arrangement and the device is also configured to receive a second consumable having a second susceptor arrangement that is different to the first susceptor arrangement.

For example, the device 100 may be configured to receive a first consumable comprising an aluminium susceptor of a particular size and also be configured to receive a second consumable comprising a steel susceptor, which may be of a different shape and/or size to the aluminium susceptor.

The varying current in the circuit 150 is maintained at a first resonant frequency of the resonant circuit 150 when the first consumable is coupled to the device and is maintained at a second resonant frequency of the resonant circuit when the second consumable is coupled to the device 100.

The aerosol generating device 100 in examples comprises a receiving portion for receiving a consumable. The receiving portion may be configured to receive a plurality of types of consumables, such as the first consumable or the second consumable. Figure 1 shows the aerosol generating device 100 in receipt of a consumable 120, which is schematically shown to be received in a receiving portion 130 of the aerosol generating device 100. The receiving portion 130 may be a cavity or chamber in the body 112 of the device. When the consumable 120 is in the receiving portion 130, the susceptor arrangement 110 of the consumable 120 is arranged in proximity for inductive coupling and heating by the inductive element 158.

The device 100 may be configured to receive a plurality of different consumables of different shapes.

In examples, as mentioned above, the inductive element 158 is an electrically conductive coil. In such examples, at least a part of the susceptor arrangement of a consumable may be configured to be received within the coil. This may provide efficient inductive coupling between the susceptor arrangement and the inductive element and as such provide for efficient heating of the susceptor arrangement.

Operation of the aerosol generating device 100 comprising resonant circuit 150, will now be described, according to an example. Before the device 100 is turned on, the device 100 may be in an ‘off’ state, i.e. no current flows in the resonant circuit 150. The device 100 is switched to an ‘on’ state, for example by a user turning the device 100 on. Upon switching on of the device 100 the circuitry 140 begins drawing current from the voltage supply 104, such that a voltage is supplied to the heating circuit 150 via the voltage regulator 154 and the current through the inductive element 158 varies at the resonant frequency fo _. The device 100 may remain in the on state until a further input is received by the control arrangement 106, for example until the user no longer pushes the button (not shown), or the puff detector (not shown) is no longer activated, or until a maximum heating duration has elapsed. The resonant circuit 150 being driven at the resonant frequency fo causes an alternating current I to flow in the resonant circuit 150 and the inductive element 158, and hence for the susceptor arrangement 110 to be inductively heated, for a given voltage. As the susceptor arrangement 110 is inductively heated, its temperature (and hence the temperature of the aerosol generating material 116) increases. In this example, the susceptor arrangement 110 (and aerosol generating material 116) is heated such that it reaches a steady temperature TMAX. The temperature TMAX may be a temperature which is substantially at or above a temperature at which a substantial amount of aerosol is generated by the aerosol generating material 116. The temperature TMAX may be between around 200 and around 300°C for example (although of course may be a different temperature depending on the material 116, susceptor arrangement 110, the arrangement of the overall device 100, and/or other requirements and/or conditions). The device 100 is therefore in a ‘heating’ state or mode, wherein the aerosol generating material 116 reaches a temperature at which aerosol is substantially being produced, or a substantial amount of aerosol is being produced. It should be appreciated that in most, if not all cases, as the temperature of the susceptor arrangement 110 changes, so too does the resonant frequency fo of the resonant circuit 150. This is because magnetic permeability of the susceptor arrangement 110 is a function of temperature and, as described above, the magnetic permeability of the susceptor arrangement 110 influences the coupling between the inductive element 158 and the susceptor arrangement 110, and hence the resonant frequency fo of the resonant circuit 150.

The present disclosure predominantly describes an arrangement in which an LC parallel circuit arrangement is powered to heat the susceptor arrangement 110. As mentioned above, for an LC parallel circuit at resonance, the impedance is maximum and the current is minimum. Note that the current being minimum generally refers to the current observed outside of the parallel LC loop, e.g., to the left of choke 161 or to the right of choke 162. Conversely, in a series LC circuit, current is at maximum and, generally speaking, a resistor may be inserted to limit the current to a safe value to prevent damage to certain electrical components within the circuit. This may reduce the efficiency of the circuit because energy is lost through the resistor. A parallel circuit operating at resonance may not have such restrictions.

In some examples, the susceptor arrangement 110 comprises or consists of aluminium. Aluminium is an example of a non-ferrous material and as such has a relative magnetic permeability close to one. What this means is that aluminium has a generally low degree of magnetisation in response to an applied magnetic field. Hence, it has generally been considered difficult to inductively heat aluminium, particularly at low voltages such as those used in aerosol provision systems. It has also generally been found that driving circuitry at resonance frequency is advantageous as this provides optimum coupling between the inductive element 158 and susceptor arrangement 110. For aluminium, it is observed that a slight deviation from the resonant frequency causes a noticeable reduction in the inductive coupling between the susceptor arrangement 110 and the inductive element 158, and thus a noticeable reduction in the heating efficiency (in some cases to the extent where heating is no longer observed). As mentioned above, as the temperature of the susceptor arrangement 110 changes, so too does the resonant frequency of the circuit 150. Therefore, in the case where the susceptor arrangement 110 comprises or consists of a non-ferrous susceptor, such as aluminium, the resonant circuit 150 of the present disclosure is advantageous in that the circuitry is always driven at the resonant frequency (independent of any external control mechanism). This means that maximum inductive coupling and thus maximum heating efficiency is achieved at all times enabling aluminium to be efficiently heated. It has been found that a consumable including an aluminium susceptor can be heated efficiently when the consumable includes an aluminium wrap forming a closed electrical circuit and/or having a thickness of less than 50 microns.

In examples where the susceptor arrangement 110 forms part of a consumable, the consumable may take the form of that described in PCT/EP2016/070178, the entirety of which is incorporated herein by reference.

While some examples described above include a circuit which is configured to self-drive at its resonant frequency, the above described concepts are also applicable to an induction heating circuit which is not configured to be driven at the resonant frequency. For example, the above described principles may be employed in an induction heating circuit which is driven at a predetermined frequency, which may not be the resonant frequency of the circuit. In one such example, the induction heating circuit may be driven via an H-Bridge, comprising a switching mechanism such as a plurality of MOSFETs. The H-Bridge may be controlled, via a microcontroller or the like to use a DC voltage to supply an alternating current to the inductor coil at a switching frequency of the H-Bridge, set by the microcontroller.

Although in the examples described above the voltage regulator is a buck regulator configured to step down the voltage from the DC voltage supply, in other examples, the voltage regulator may be a boost regulator which is configured to output a voltage greater than the output voltage of the DC voltage supply, i.e. to step up the voltage. This type of voltage regulator may be referred to as a boost regulator. In yet other examples, the voltage regulator may be configured to provide functionality for both stepping up the voltage and for stepping down the voltage, i.e. the voltage regulator may be controllable to output a range of voltages less than and greater than the input voltage from the voltage supply. This type of voltage regulator may be referred to as buck/boost regulator, or a buck/boost converter.

Certain examples described herein may form a “delivery system” or part of a delivery system. In certain examples a delivery system may be a “non-combustible” aerosol provision system, which also may be referred to as a non-combustible aerosol generating system. According to the present disclosure, a non-combustible aerosol provision system is one where a constituent aerosol-generating material of the aerosol provision system (or component thereof) is not combusted or burned in order to facilitate delivery of at least one substance to a user.

In some examples, the non-combustible aerosol provision system is an electronic cigarette, also known as a vaping device or electronic nicotine delivery system (END), although it is noted that the presence of nicotine in the aerosol generating material is not a requirement.

In some examples, the non-combustible aerosol provision system is an aerosol generating material heating system, also known as a heat-not-burn system. An example of such a system is a tobacco heating system. In some examples, the non-combustible aerosol provision system is a hybrid system to generate aerosol using a combination of aerosol-generating materials, one or a plurality of which may be heated. Each of the aerosol-generating materials may be, for example, in the form of a solid, liquid or gel and may or may not contain nicotine. In some examples, the hybrid system comprises a liquid or gel aerosol-generating material and a solid aerosol-generating material. The solid aerosol-generating material may comprise, for example, tobacco or a non-tobacco product.

Typically, the non-combustible aerosol provision system may comprise a non combustible aerosol provision device and a consumable for use with the non combustible aerosol provision device.

As above, an aerosol provision system may be referred to herein as an aerosol generating system and, further, an aerosol provision device may be referred to herein as an aerosol generating device.

In some examples, the disclosure relates to consumables comprising aerosol generating material and configured to be used with non-combustible aerosol provision devices. These consumables are sometimes referred to as articles or aerosol generating articles throughout the disclosure.

In some examples, the non-combustible aerosol provision system may comprise an area for receiving the consumable, an aerosol generator, an aerosol generation area, a housing, a mouthpiece, a filter and/or an aerosol-modifying agent.

In some examples, the consumable for use with the non-combustible aerosol provision device may comprise aerosol-generating material, an aerosol-generating material storage area, an aerosol-generating material transfer component, an aerosol generator, an aerosol generation area, a housing, a wrapper, a filter, a mouthpiece, and/or an aerosol-modifying agent. In some examples, the substance to be delivered may be an aerosol-generating material or a material that is not intended to be aerosolised. As appropriate, either material may comprise one or more active constituents, one or more flavours, one or more aerosol-former materials, and/or one or more other functional materials.

Aerosol-generating material is a material that is capable of generating aerosol, for example when heated, irradiated or energized in any other way. Aerosol-generating material may, for example, be in the form of a solid, liquid or gel which may or may not contain an active substance and/or flavourants. In some examples, the aerosol generating material may comprise an “amorphous solid”, which may alternatively be referred to as a “monolithic solid” (i.e. non-fibrous). In some examples, the amorphous solid may be a dried gel. The amorphous solid is a solid material that may retain some fluid, such as liquid, within it. In some examples, the aerosol-generating material may for example comprise from about 50wt%, 60wt% or 70wt% of amorphous solid, to about 90wt%, 95wt% or 100wt% of amorphous solid.

The aerosol-generating material may comprise one or more active substances and/or flavours, one or more aerosol-former materials, and optionally one or more other functional material.

The material may be present on or in a support, to form a substrate. The support may, for example, be or comprise paper, card, paperboard, cardboard, reconstituted material, a plastics material, a ceramic material, a composite material, glass, a metal, or a metal alloy. In some examples, the support comprises a susceptor. In some examples, the susceptor is embedded within the material. In some alternative examples, the susceptor is on one or either side of the material.

A consumable is an article comprising or consisting of aerosol-generating material, part or all of which is intended to be consumed during use by a user. A consumable may comprise one or more other components, such as an aerosol generating material storage area, an aerosol-generating material transfer component, an aerosol generation area, a housing, a wrapper, a mouthpiece, a filter and/or an aerosol- modifying agent. A consumable may also comprise an aerosol generator, such as a heater, that emits heat to cause the aerosol-generating material to generate aerosol in use. The heater may, for example, comprise combustible material, a material heatable by electrical conduction, or a susceptor.

A susceptor is a material that is heatable by penetration with a varying magnetic field, such as an alternating magnetic field. The susceptor may be an electrically- conductive material, so that penetration thereof with a varying magnetic field causes induction heating of the heating material. The heating material may be magnetic material, so that penetration thereof with a varying magnetic field causes magnetic hysteresis heating of the heating material. The susceptor may be both electrically- conductive and magnetic, so that the susceptor is heatable by both heating mechanisms. The device that is configured to generate the varying magnetic field is referred to as an inductive element herein but may also be referred to as a magnetic field generator.

An aerosol generator is an apparatus configured to cause aerosol to be generated from the aerosol-generating material. In examples of the present disclosure, the aerosol generator is configured to subject the aerosol-generating material to heat energy, so as to release one or more volatiles from the aerosol-generating material to form an aerosol.

The above examples are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the examples, or any combination of any other of the other examples. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.