FIELD OF THE INVENTION

This invention relates generally to the field of load drivers, and more particularly driver circuits for driving a load. BACKGROUND OF THE INVENTION

Drivers may be used to provide a voltage supply to a load, such as an LED for example. In such drivers, there is typically a primary converter adapted to receive and convert an input power supply in response to at least one desired supply signal from control circuitry. The primary converter and the output load are usually isolated from one another by a pair of magnetically coupled windings.

Isolating power supply designs offer benefits over non- isolating designs, most notably with regard to user safety. Both linear power supplies and switched mode power supplies can be designed in an isolating configuration. Isolating power supplies use an indirect mechanism to transfer power between a first part and a second part of a circuit. For example, a transformer allows power to be coupled between its primary and secondary winding magnetically, with no direct electrical connection. If a short circuit or power spike occurs at the input power source, there is only a risk of damage to the transformer; the electronics attached to the transformer output remains safe, due to the lack of a direct electrical connection.

In the field of device drivers, in particular lighting drivers, power supplies with high power factors and safety isolated outputs are typically preferred. These drivers can either employ power-factor-correction in the front-end, with an isolating output stage, or they can use an isolating input stage that operates at a high power factor and hence does not require isolation in the second power stage.

Systems designed to operate internationally may need to cater for more than one input power source. Although the input power source could be manually selected for different mains supplies using a switch, this is far less practical when switching to a backup power source of a different frequency. Thus, it is preferable to be able to transmit information relating to the attached input power source across the isolation barrier without violating the safety provided by the isolation barrier. For example, information passed across the isolation barrier may enable the correct switching frequency to be established for a switched mode power supply, regardless of the input power source, by using a feedback loop to provide control information.

DALI is a known protocol used for the control of lighting driver. DALI wiring may be run alongside the mains input and is not considered to be isolated from the mains wiring with regards to safety. Also, a DALI system will typically require knowledge of the input power source characteristics in order for appropriate control to be established. For example, a DALI will normally need to be informed if the input supply voltage is AC (e.g. as in normal situations) or DC (e.g. as in emergency situations). Using such information, the control may respond appropriately to an emergency condition.

Transmitting such information across the isolation barrier can lead to the undesirable addition of expensive isolating components, or the duplication of expensive components. Typically, one of four approaches is used to provide information regarding an input power supply to a driver control system (such as a DALI system).

The first approach involves using a microcontroller on the primary side of the isolation barrier to handle the DALI protocol and other functions, with only analogue control of any power stages on the secondary side of the isolation barrier.

The second approach uses a microcontroller on each of the primary and secondary sides of the isolation barrier. Here, the DALI protocol is handled on the primary side, and communication between primary and secondary controllers is achieved using isolating components, such as an opto-coupler (or opto -isolator).

The third approach again uses a microcontroller on each of the primary and secondary sides of the isolation barrier, but the DALI protocol is handled on the secondary side. Communication between the microcontrollers is performed using isolating components, such as an opto-coupler.

Finally, the fourth approach uses analogue control on the primary side of the isolation barrier along with a microcontroller on the secondary side of the isolation barrier. Communication between the primary and secondary side of the circuit is performed using isolating components, such as an opto-coupler.

In cases where analogue control can handle the required dynamic range of the output, the first approach may be used. For deep dimming applications however, this may not be suitable. The second and third approaches work in deep dimming applications.

However, they are less favourable from a cost perspective due to the presence of at least two microcontrollers. Where digital control is required on both sides of the isolation barrier, this is not a problem, but in other cases such extra cost should preferably be avoided. Also, the third approach is not suitable for DALI since it would require double-isolation in order to meet the safety specifications, thus resulting in additional costs.

Finally, although the fourth approach removes the second microcontroller of the second and third approaches, it has the drawback of requiring isolating components for transmission of mains information across the isolation barrier.

Thus, it remains desirable to implement an isolation means in a driver while allowing both power and information relating to the input power supply to be passed across the isolation barrier, preferably without increasing costs due to additional expensive or duplicated components for example. SUMMARY OF THE INVENTION

It would be advantageous to have a low cost driver and drive method.

Specifically, it would be advantageous to have a driver having an isolation barrier across which information regarding an input power supply may be transmitted without the need for additional expensive or duplicated components.

A very basic idea of the embodiments of the invention is detecting the switching frequency of the driver at the secondary side. In a general condition of constant output power in the driver, the switching frequency of the driver relates to the input power supply. Thus, by analysing the switching frequency, information related to the input power supply can be retrieved, directly at the secondary side. Thus, there is no need to use an opto- coupler between the primary side and the secondary side.

To address at least one of these concerns, the invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a driver circuit for driving a load, comprising: a transformer, having: a primary winding adapted to be coupled to an input power supply; and a secondary winding magnetically coupled to the primary winding and adapted to be coupled to a load, wherein the input power supply to the primary winding is switched by the driver circuit; a switching detection circuit connected to the secondary winding and adapted to detect a switching of the driver circuit based on voltage variations across the secondary winding; and a control circuit connected to the switching detection circuit and adapted to determine a switching frequency of the driver circuit based on the detected switching of the driver circuit; and to determine a characteristic of the input power supply based on the determined switching frequency of the driver circuit.

Proposed is a concept for passing information about the input power supply to the secondary side (e.g. across the isolation barrier) without using additional isolating components (such as an opto-coupler for example). In particular, it is proposed to detect a switching frequency of the driver circuit based on voltage variations across the secondary winding. Based on the detected switching frequency, a characteristic of the input power supply may be determined. A principle behind this solution is that the switching frequency of a driver normally depends on the amplitude of the input power supply. By analysing the switching frequency, information relating to the amplitude of the input power supply may be derived. Embodiments may therefore utilise the isolation provided by the windings of the transformer in order to transmit/communicate information regarding an input power supply coupled to the primary winding.

Embodiments may therefore eliminate a need for multiple micro-controllers to handle DALI via circuitry on the primary side (as otherwise needed in the second and third approaches detailed in the background section above). Embodiments may also eliminate a need for an additional opto-coupler (as otherwise need in the fourth approach detailed in the background section above). A number of components required to allow information to be passed across the isolation barrier may thus be reduced by embodiments, thereby providing for reduced complexity and/or cost.

There may be proposed a detection circuit that enables the switching frequency of the driver circuit to be obtained across the isolation barrier of the transformer without the need for additional isolation components. This may provide a means of transferring both power and switching information across the isolation barrier in a cost- effective (e.g. cheap) manner, for example by reducing a number of required circuit components. Such information can, for example, be particularly beneficial for power factor correction.

Also, by detecting switching using voltage variations across the secondary winding, and determining the frequency of the switching, a characteristic of the input power supply (such as whether it is AC or DC) may be determined even in cases where there is little (i.e. low or light) or no load coupled to the secondary winding. This is in contrast to conventional approaches which may be unable to determine such a characteristic of the input power supply when there is a small or zero load.

In an embodiment, the control circuit may comprise: an envelope detection circuit adapted to generate an envelope waveform of the voltage switching across the secondary winding. The control circuit may then be adapted to use the envelope waveform from the envelope detection circuit and to determine the characteristic of the input power supply based on the envelope waveform. A relatively simple envelope generation

arrangement may therefore be implemented that enables determination of a characteristic of the input power supply.

For example, the switching detection circuit may comprise: a high-pass filter adapted to filter the voltage switching across the secondary winding to generate a filtered signal; and the envelope detection circuit may comprise: a half- wave rectifier adapted to half- wave rectify the filtered signal so as to generate a rectified signal; and an RC circuit adapted to generate an envelope waveform of the rectified signal. Cheap components and relatively simple circuitry arrangements may be used, thereby reducing the associated complexity and/or cost of obtaining information about the input power supply across an isolation barrier.

The control circuit may be adapted to determine the peak-to-valley ratio of the envelope waveform and to determine whether the input power supply is AC or DC based on the determined peak-to-valley ratio. For example, the determined peak-to-valley ratio may be compared against a predetermined threshold value to determine if the peak-to-valley ratio is large or small. If the peak-to-valley ratio is found to exceed such a threshold value (e.g. found to be large), the input power supply may be determined to be AC. Conversely, If the peak-to-valley ratio is found not to exceed the threshold value (e.g. found to be small), the input power supply may be determined to be DC.

Further, the control circuit may be adapted to determine the peak-to-valley ratio of the envelope waveform over a time period greater than the cycle time of a mains AC power supply, and the control circuit may be further adapted to determine a mean voltage value of the AC input power supply based on the determined peak-to-valley ratio. By way of example, the peak-to-valley ratio may be correlated to the RMS value of the input power supply, thus enabling a RMS value of an AC input power supply to be calculated based on a determined peak-to-valley ratio. For example, an 110V AC typically has a smaller peak-to- valley ratio than that of a 220V AC, and such a relationship between peak-to-valley ratio and RMS voltage value may be exploited in order to be able to determine a RMS voltage value. In an embodiment, the control circuit may comprise: a timing circuit adapted to determine either: a number of voltage pulses at the secondary winding in a predetermined sampling time period; or an interval between consecutive voltage pulses at the secondary winding in a predetermined sampling time period. The control circuit may then be adapted to use the determined number of voltage pulses or interval between consecutive voltage pulses from the timing circuit to determine the characteristic of the input power supply based on the received number or interval. For example, to determine if the input power supply is AC or DC, an embodiment may be arranged to determine a frequency of pulses (e.g. by timing the interval between pulses or counting the number of pulses in a predetermined time period). Such measurement of pulse frequency may be repeated a plurality of times throughout the duration of a typical mains cycle (e.g. 10ms in Europe). If the pulse frequency measurements are substantially similar, it may be determined that the input power supply is DC.

Conversely, if the pulse frequency measurements vary more than a predetermined amount, it may be determined that the input power supply is AC.

Thus, the sampling time period may be less than half of the cycle time of a mains AC power supply, and the timing circuit may be adapted to determine either: a number of voltage pulses for a plurality of sampling time periods; or an interval between consecutive pulses for a plurality of sampling time periods, the plurality of sampling time periods being contained within the cycle time of the mains AC power supply. The control circuit may also be adapted to receive the determined number of voltage pulses or interval between consecutive voltage pulses for each of the plurality of sampling time periods and to determine whether the input power supply is AC or DC based on a variation in the determined number or interval for the plurality of sampling time periods.

By way of example, the control circuit may be adapted to determine that the input power supply is AC if the variation in the determined number or the variation in the intervals for the plurality of sampling time periods exceeds a predetermined threshold value and to determine that the input power supply is DC if the variation in the determined number or the variation in intervals for the plurality of sampling time periods does not exceed the predetermined threshold value.

In some embodiments, the control circuit may be adapted to determine a mean voltage value of the AC input power supply based on the variation in the determined number or the variation in intervals for the plurality of sampling time periods. By way of example, a ratio between maximum and minimum detected voltage pulse frequency may be correlated to the RMS value of the input power supply, thus enabling a RMS value of an AC input power supply to be calculated based on detected variation in voltage pulse frequency.

The switching detection circuit may comprise: a first circuit component for sensing the variation of said voltage at the secondary winding and generating a current signal indicative of the voltage variation; a second circuit component for limiting said current signal, and the timing circuit may further comprise a micro controller for receiving said current signal limited by said second circuit component and for counting the number of current signal or calculating said interval between consecutive current signal. Cheap components and relatively simple circuitry arrangements may be used, thereby reducing the associated complexity and/or cost of obtaining information about the input power supply across an isolation barrier.

Also, the first circuit component may comprise a small value capacitor (for example, in the range of 200-5 OOnF) and said second circuit component may comprise a large value resistor (for example, in the range of 200-500kD). It should, however, be understood that other suitable circuit components of other values may be implemented.

According to an example, there may be provided a switch mode power supply comprising a driver circuit according to an embodiment. Thus, a switch mode power supply may be provided which may implement a DALI system with reduced complexity and/or cost compared to conventional versions that require microcontrollers and/or opto-couplers.

Also, the driver circuit may be a power factor correction stage adapted to provide power factor correction for the load.

The driver circuit may be a resonant LLC converter or an isolated flyback converter. Embodiments may be used in analogue-controlled LLC PFC architecture drivers which require DALI emergency, zero-crossing detect or intelligent voltage selection functionality. Proposed embodiments may therefore be of particular benefit in applications that employ multiple output drivers for lighting colour control for example. The proposed LLC PFC architecture may also be of benefit for drivers of reduced, since it may be more compact than conventional designs.

Embodiment may also be applicable to other types of isolating PFC circuits. Such applicability to multiple types of isolating PFC circuit may make proposed

embodiments useful for a wide range of applications.

For example, there may be provided a lighting device comprising a driver circuit according to an embodiment. According to examples in accordance with an aspect of the invention, there is provided a drive method for driving a load coupled to a secondary winding of a transformer, the secondary winding being magnetically coupled to a primary winding of the transformer, wherein the method comprises: detecting a switching frequency based on voltage variations across the secondary winding of the transformer; and determining a characteristic of an input power supply coupled to the primary winding of the transformer based on the detected switching frequency.

Embodiments may further comprise: generating an envelope waveform of the voltage variations across the secondary winding, and the step of determining a characteristic of an input power supply coupled to the primary winding of the transformer may comprise determining the characteristic of the input power supply based on the envelope waveform.

An embodiment may further comprise determining either: a number of voltage pulses in a predetermined sampling time period; or an interval between consecutive voltage pulses in a predetermined sampling time period, and the step of determining a characteristic of an input power supply coupled to the primary winding of the transformer may comprise determining the characteristic of the input power supply based on the received number or interval.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 is a schematic block diagram of a driver circuit according an

embodiment; Fig. 2 is schematic block diagram of the driver circuit of Figure 1, where an exemplary implementation of the switching detection circuit 20 and the control circuit 22 is shown in more detail;

Fig. 3 is a schematic block diagram of a driver circuit according to an embodiment;

Fig. 4 depicts simulated values of signals for the embodiment of Figure 3, wherein Figure 4A depicts a simulated output from the filter in response to the inverter switching frequency depicted in Figure 4B;

Fig. 5 depicts simulated values of signals for the embodiment of Figure 3, wherein Figure 5A depicts a simulated mains input voltage as the input power source, and wherein Figure 5B shows the corresponding rectified envelope signals at the output terminal OUT;

Fig. 6 depicts a modification to the driver circuit of Figure 1, wherein a switching element is connected between the input power supply and the primary winding;

Fig. 7 depicts a modification to the driver circuit of Figure 1, wherein an inductive element and a capacitive element are introduced between the input power supply and the primary winding to provide power factor correction;

Fig. 8 is a schematic diagram of a switched mode power supply comprising a driver circuit according to an embodiment;

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

Proposed is a concept of using changes in voltage across the secondary winding of an isolation transformer to obtain information regarding switching, determining the switching frequency, and using the switching frequency to derive information about the input power. For example, for an AC input power supply, the switching frequency will change over a mains cycle in order to modulate the input power and give a high power factor. Conversely, for a DC input power supply (e.g. in an emergency-mode), the switching frequency will be relatively constant once the steady-state has been achieved. By detecting and distinguishing the switching frequency, a characteristic of the input power supply, such as whether it is AC or DC, may be determined. Furthermore, for different operating voltages of drivers, the range of switching frequencies will be distinct, thus allowing the mean voltage value of the input power supply to be determined.

By way of example, switching frequency information may be obtained via a high-pass filter attached to the secondary winding of an isolation transformer.

A proposed embodiment of a driver circuit may comprise: a transformer with a primary winding, which can be coupled to an input power supply; and a secondary winding, electrically isolated from but magnetically coupled to the primary winding, which can be coupled to a load. Further, the driver circuit may comprise: a switching detection circuit connected to the secondary winding; and a control circuit connected to the switching detection circuit. The switching detection circuit may detect switching of the driver circuit based on the voltage variations detected across the secondary winding. The control circuit may then determine the switching frequency of the switching, and then determine a characteristic of the input power supply based on the determined switching frequency of the driver circuit.

Embodiments may therefore enable changes in voltage across the secondary winding of the transformer to be used to obtain information relating to an input power supply.

For instance, the switching frequency of the driver circuit may change over the mains cycle in order to modulate the input power supply. Modulating the input power supply may enable a high power factor to be realised, which can lead to a more efficient supply of power to the load. Such switching may be implemented using an electronic switch, such as a transistor or a relay, connected between the input power source and the primary winding.

Embodiments may thus propose detection of voltage variations across the secondary winding of an isolation transformer using a switching detection circuit. It is proposed that a switching detection circuit is connected to the secondary winding. The switching detection circuit may detect switching, the control circuit may then determine the switching frequency, and from the determined switching frequency, AC and DC input conditions may be determined even if the output load is small or absent. Also, zero crossing information of an input power supply may be extracted from a signal from the switching detection circuit.

By way of example, it is proposed that the switching detection circuit may be adapted to detect voltage variations across the secondary winding, and the control circuit may use an envelope detection circuit to obtain an envelope waveform of the voltage switching across the secondary winding. Also, it may be preferable to filter the signal from the secondary winding using a low-pass, high-pass, band-pass or band-stop filter, before passing the signal to the envelope detector. Thus, the switching detection circuit may comprise a filter arrangement, wherein the filter arrangement is adapted to remove residual traces of an AC input power supply, reject excessive noise and/or reject frequencies outside of a

predetermined range of anticipated switching frequencies.

In some embodiments, output of the switching detection circuit may be indicative of each switching, and the output of the switching detection circuit over time may then be indicative of the frequency of switching. The presence of an AC or DC input power supply may be determined by the variations in the output of the switching detection circuit over time. The variations over time may, for example, be representative of the change in switching frequency or variations the input power supply. For instance, for a DC input, the switching frequency may be relatively constant once a steady-state has been achieved. It may therefore be determined that an input power supply is DC if the detected switching frequency remains relatively constant or stable for a predetermined length of time. A detected DC input may, for example, indicate the use of an emergency or a backup battery.

Also, for different input power supply voltages, the switching frequencies may be distinct from each other. Thus, the voltage of the input power supply may be determined from a switching frequency. Furthermore, the frequency of the input power supply and the zero crossing of the input power supply may be determined from the switching frequency.

Embodiments may employ a control circuit which is adapted to determine a characteristic of the input power source based upon a determined switching frequency of the driver circuit. The control circuit may for example determine a frequency of voltage pulses and use the determined frequency to determine the presence of an AC or a DC input power source and/or to find mains zero crossing information. The control circuit may, for example, be a timing circuit, such as a microcontroller. Also, a signal may be provided to the control circuit via a resistor and a capacitor arrangement which is configured to shape the voltage pulses. The resistor may also provide input protection for the control circuit.

It should be understood the switching frequency may not necessary be the value of how many switched in a one second time period. Any information directly or indirectly related to how often the switching occurs should be construed as the switching frequency. By way of example, the control circuit may determine the frequency of voltage pulses either by timing an interval between pulses in a fixed/predetermined window of time or counting the number of pulses in a fixed/predetermined window of time. The length of the interval between each pulse or the number of pulses in one window of time may be indicative of how often the switching occurs. Such measurements may be performed more than once throughout a mains cycle and, if the plurality of measurements are found to be similar (e.g. exhibit a variation within a predetermined allowable range) the input power source may be determined to be DC. Conversely, if frequency measurements are found to vary more than predetermined threshold amount, the input power source may be determined to be AC. Also, a ratio between a minimum and maximum measured voltage pulse frequency may be used to determine a value (e.g. a mean or RMS value) for the voltage of the input power source.

Figure 1 depicts a schematic representation of a driver circuit according to an embodiment of the invention. An input power source 10 is connected to a primary winding 12 of a transformer 14 and is adapted to provide power to the primary winding 12. The secondary winding 16 of the transformer 14 is galvanically isolated from the primary winding 12 and connected to the load 18. Thus, the ground potential of the input power source 10 and the load 18 may be dissimilar. The secondary winding 16 is additionally connected to a switching detection circuit 20. The switching detection circuit 20 is further connected to a control circuit 22.

In the depicted example, the switching detection circuit 20 is adapted to detect the switching of the switched mode power supply based on voltage variations across at least a part of the secondary winding 16. By detecting the switching in the isolated secondary winding 16, no further switching information or other input power source information is required to traverse the isolation barrier, thus removing the need for additional isolating components, such as optocouplers.

In the depicted example, the detected switching are subsequently transmitted to the control circuit 22, which is adapted to synthesise the detected switching with time information so as to determine frequency of switching and determine a characteristic of the input power source 10 based upon the detected switching frequency. By way of example, the control circuit 22 may detect the frequency of incoming voltage/current (more generally, signal) pulses (wherein each pulse is indicative of a switching) from the switching detection circuit 20, and use such frequency of the voltage pulse detection to determine a frequency of input power source 10 and/or zero crossing information.

By way of example, Figure 2 is schematic block diagram of the driver circuit of Figure 1, where an exemplary implementation of the switching detection circuit 20 and the control circuit 22 is shown in more detail.

The switching detection circuit 20 may be connected to the control circuit 22, and the control circuit may comprise a timing circuit such as a microcontroller 21. The secondary winding 16 may be connected to the timing pin of microcontroller 21 via a switching detection circuit 20. In the depicted example of Figure 2, the output of the secondary winding of the transformer (before diode Dl) is connected in series to a small value capacitor CI (for example, in the range of 200-500nF) , then to a large value resistor Rl (for example, in the range of 200-500kD), then directly into a timing pin of the microcontroller 21. The small capacitor CI is for providing a dv/dt function for the voltage at the secondary winding 16, and the capacitor CI would generate a current in case a switching happens since the switching causes a significant voltage change at the secondary winding 16. Input protection diodes of the microcontroller 21 may limit the input voltages, while the large series resistance Rl may keep the currents well below the safe levels for the microcontroller chip 21. Of course, it should, however, be understood that other suitable circuit components of other values may be implemented. . In such an arrangement, the microcontroller 21 of the control circuit 22 can be adapted to determine a characteristic of the input power source 10 based on either the temporal interval between voltage pulses that occur in a predetermined time period or the number of voltage pulses that occur in a predetermined time period. Such measurements may be performed for multiple times throughout a cycle of the input power source 10. If the measurements are substantially similar for each measurement time, the input power source 10 may be determined to be DC. Conversely, if the measurements in each time fluctuate beyond a predetermined range, the input power source 10 may be determined to be AC.

Further, a ratio between a minimum and maximum measured voltage pulse frequency may allow the voltage value (e.g. mean or RMS value) of the input power source to be determined.

Using a detected switching frequency to determine characteristics of the input power source 10 may be advantageous over alternative approaches, such as measuring the current passing through the load. For example, it may enable detection of characteristics of the input power source 10 even in the case of a low current draw, or absent load.

The switching frequency of the switched mode power supply may be greater than the frequency of the input power source 10. The switching frequency of the switched mode power supply may therefore be monitored continuously, or it may be sampled a plurality of times in a time period less than the anticipated cycle time for one, three or five periods of the input power source 10, for example.

Figure 3 depicts a schematic representation of a driver circuit according to an embodiment, wherein the switching detection 20 circuit comprises a capacitor 32 and a resistor 34 for forming a high pass filter which provides a dv/dt function to the voltage on the secondary winding so as to detect its switching, and wherein the control circuit 22 comprises an envelope detector comprising a transistor Ql and a buffer circuit 26/28.

The capacitor 32 and resistor 34 form a high-pass filter in order to capture the high-frequency pulses corresponding to the switching action of the MOSFETs of the half- bridge 46 on the primary side. This high-frequency signal is then input into an envelope- detector circuit 20.

The envelope detector is connected to the switching detection circuit 20. In this instance, it is advantageous to establish a connection across one end, and the centre of the secondary winding 15. This provides a signal to the envelope detector 20 with only a positive component and hence enables simpler analysis circuitry at the control circuit 22. In the depicted example, the envelope detector comprises a transistor Ql, a diode 24, a resistive element 26 and a capacitive element 28. More specifically, the base of the transistor Ql is connected to the output of switching detection circuit 20, and the diode 24 is connected between the base of the transistor Ql and an output terminal OUT. The purpose of the diode 24 is to prevent excessive reverse bias across the emitter-base junction of the transistor Ql and could be omitted in case this excessive reverse bias is prevented via some other means. The resistive element 26 and the capacitive element 28 are connected in parallel with each other and connected between the output terminal OUT and a tap 15 at least part of the way along the secondary winding 16 (wherein the tap 15 may be at the centre of the secondary winding 15, for example). The local supply, Vcc, is referenced to the secondary ground GND.

The capacitive element 28 of the control circuit 22 stores charge on the rising edge of the voltage on the secondary winding 16, and then discharges stored charge through the resistive element 26 on the falling edge of the input signal. The higher the switching frequency, the higher voltage the capacitive element 28 would be charged. Since the switching frequency is in relation to the instantaneous amplitude of the input power, the voltage on the capacitive element would follow the instantaneous amplitude of the input power.

If, for example, both the switching detection circuit 20 and the control circuit 22 are passive circuits, a buffering mechanism, such as a transistor Ql, should preferably be implemented in the control circuit 22. In an embodiment using a buffering mechanism, the buffer would be referenced to the ground of the secondary winding to avoid breaching the isolation barrier. The components of the filter 20 may be selected to prevent excessive voltages reaching the buffer, both to prevent damage and to prevent any loss of switching information. If the filter is implemented using an active design, for example using an op-amp, buffering is typically inherent in the active filter and hence additional buffering components may not be required.

Based on the above description of Figure 3, it will be understood that an embodiment may comprise: a switching detection circuit 20 comprising a filter arrangement (such as that in Figure 3) and a control circuit having an envelope detection circuit (as in Figure 3). It may be summarised that such an embodiment employs: a switching detection circuit comprising: a high-pass filter (32, 34) adapted to filter the voltage switching across the secondary winding 16 to generate a filtered signal; and a control circuit comprising: a half- wave rectifier (24) adapted to half- wave rectify the filtered signal so as to generate a rectified signal; and an RC circuit (26,28) adapted to generate an envelope waveform of the rectified signal.

Figures 4 and 5 depict simulated values of signals for the embodiment of Figure 3, wherein Figure 4 A depicts a simulated output from the filter (i.e. the voltage at the base of the transistor Ql) in response to the inverter switching frequency depicted in Figure 4B. More specifically, Figure 4 A depicts the simulated output from the filter 30 in response to the switching frequency input switching frequency depicted in Figure 4B.

The X-axis of figure 4A depicts time (t) in milliseconds (ms), whereas the Y- axis shows voltage (V) in volts. High-frequency voltage spikes 52 are shown in the response of the filter 30 attached to the secondary winding 16, which correspond to the switching of the half-bridge 46 attached to the primary winding 12.

The X-axis of Figure 4B depicts time (t) in milliseconds (ms), whereas the Y- axis shows voltage (V) in volts. Here, the input power supply comprises an inverter formed from two transistors. Switching of the first transistor of the half-bridge 46 is shown by a dashed line 54 in Figure 4B and switching of the second transistor of the half-bridge 46 is shown by a solid line 56 in Figure 4B.

Figures 4A and 4B thus concurrently show the switching of the input power supply and the output from the filter 30. The switching detection circuit 20 is adapted to detect the switching frequency of the input power supply by detecting the voltage spikes 52 shown in Figure 4A. In this example, the voltage spikes 52 correspond to the switching of the inverter. However, similar spikes are observed for other switching means, such as a switching element. The control circuit 22 subsequently determines the switching frequency based upon the detected switching behaviour.

Figure 5A shows the mains input voltage as the input power source. The X- axis of Figure 5 A depicts time (t) in milliseconds (ms), and the Y-axis shows voltage (V) in volts. A solid line shows an AC input voltage 58 provided as the input power source 10, while a dashed line corresponds with a DC input voltage 59 (277v) provided as the input power source 10.

Figure 5B shows the corresponding rectified envelope signals at the output terminal OUT. The envelope waveform of the DC input voltage 60 of Figure 5 A is shown in black, and the rectified envelope waveform of the AC input voltage 62 is shown in grey. The X-axis of figure 5B depicts time (t) in milliseconds (ms), and the Y-axis shows voltage (V) in volts. Figures 5 A and 5B concurrently show the power provided as an input power source 10 and the signal provided by the output OUT. The voltage of the signal provided at the output terminal OUT is representative of the switching frequency of the switched mode power supply. The rectified envelope of the DC waveform 60 shows that the switching frequency of the driver circuit is relatively constant for a DC input power source 10. The rectified envelope of the AC input voltage 62 in grey shows that the switching frequency varies with time, in this embodiment the variation is of a sinusoidal nature. This signal could then be polled by a microprocessor 100, used as one part of the control circuit 22, to determine the peak-to-valley ratio of this signal over some time interval longer than a mains cycle.

For example, the control circuit 22 may detect the presence of an AC or DC input by determining the peak-to-valley ratio of the signal at the output terminal OUT over a predetermined time interval (that may relate to an estimated mains cycle for example). By way of example, such a measurement time interval may be quarter of an expected mains cycle, half of a mains cycle or three quarters of a mains cycle. Alternatively, the

measurement time interval may be greater than one mains cycle, greater than three mains cycles, greater than five mains cycles or a fixed time period such as 10, 20 or 30

milliseconds. In another example, the peak-to-valley ratio of the signal from the switching detection circuit 20 may be determined continuously.

A large peak-to-valley ratio is indicative of an AC input power source 10 and a small peak-to-valley ratio is indicative of a DC input power source 10.

Figure 6 depicts a schematic representation of an implementation to the embodiment of Figure 1, wherein a switching element 36 is connected between the input power source 10 and the primary winding 12. The switching element 36 is adapted to control the current in the primary winding 12. This topology may be a fly-back converter in essence. The two windings 12 and 16 may be reversely dotted.

The switching element 36 may be an electronic switch, for example, a transistor. The switching element 36 may operate at the switching speed of the driver circuit. The use of a switching element is essential in realising an efficient power supply design as the switch dissipates very little power during operation. The switching element 36 may operate at different frequencies to maintain the desired voltage across the load 18 regardless of the input power source 10.

Figure 7 depicts a schematic representation of another modification to the embodiment of Figure 3, wherein an inductive element 38 and a capacitive element 40 are introduced between the input power source 10 and the primary winding 12 to provide power factor correction to the load 18. At the secondary side there is only one secondary winding 16, compared with the two secondary windings in Figure 3.

The resulting inductor, inductor, capacitor (LLC) power factor correction stage offers several advantages over the flyback power factor correction stage which is often used in switched mode power supply topologies. Advantages offered by the LLC power factor correction stage are improved power density, efficiency, common-mode impedance, surge immunity, and the option to act as a semi-bridgeless topology. The option to act as a semi- bridgeless topology is particularly beneficial as it renders two rectifier diodes superfluous and puts paid to their associated losses and costs. Furthermore, embodiments of the present invention may provide advantages when applied to the known flyback power factor correction stage. In other embodiments, the inductive element 38 may be a leakage inductance, and the transformer 14 and the leakage inductance 38 may be considered as a single magnetic component, such as a transformer. Furthermore, the transformer 14 may be coupled directly to a preceding circuit element such as an inverter or input power source with the capacitive element 40 connected across the inductive element 38.

Figure 8 depicts an embodiment of a switched mode power supply comprising the driver circuit of Figure 1. Figure 8 introduces a rectifier 42, an input power filter 44 and an inverter 46. The input power source 10 is connected to the rectifier 42, wherein the rectifier may, for example, comprise a full-bridge rectifier, a half-bridge rectifier or an active rectifier. The rectifier 42 is subsequently connected to the input power filter 44, for filtering the input power source 10. The input power filter 44 in this particular example uses a pair of capacitive elements 48 combined with an inductive element 50 to remove electromagnetic interference and electrical noise from the input power source 10. The input power filter 44 is subsequently connected to an inverter 46. In this example, the inverter 46 is comprised of two electronic switches, or more specifically transistors. In order to obtain the highest efficiency, it is preferable to operate the inverter at a 50% duty cycle. The switching frequency may be either directly or indirectly controlled. The switching frequency determines the power delivered to the load 18 and hence the power factor. In order to realise a high power factor, the switching frequency will vary over the mains cycle in an LLC or flyback power factor correction stage. The inverter 46 is subsequently connected to the primary winding 12 of the transformer 14 and is adapted to provide power to the primary winding 12. The secondary winding 16 of the transformer 14 is galvanically isolated from the primary winding 12 and connected to the load 18; hence, the ground potential of the input power source 10 and the load 18 may be dissimilar. The secondary winding 16 is additionally connected to the switching detection circuit 20. The switching detection circuit 20 is further connected to the control circuit 22.

In this example, the switching detection circuit 20 is adapted to detect the switching of the inverter 46 based on voltage variations across at least a part of the secondary winding 16. As the switching of the inverter 46 is detected in the isolated secondary winding 16 there is no further requirement for information relating to the input power source 10 to traverse the isolation barrier, hence removing the need for additional isolating components, such as additional transformer windings or optocouplers. The control circuit 22 receives the voltage variations detected by the switching detection circuit determines a switching frequency of the driver circuit based on the detected switching. The control circuit 22 then determines a characteristic of the input power source 10 based upon the determined switching frequency of the inverter 46. The switching frequency of the inverter may be the same as or greater than the frequency of the input power source 10.

The control circuit 22 may be used to interrogate the signal from the switching detection circuit 20 in order to determine the peak-to-valley ratio of this signal over a time interval greater than a mains cycle. A large peak-to-valley ratio is indicative that the input power source 10 has been provided with an AC input, whereas a small peak-to-valley ratio is indicative that the input power source 10 has been provided with a DC input.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.