Technical Field

The invention relates to a microwave oven having a cooking chamber, a door _f a microwave generator, a driver circuit and at least one safety switch operata- ble by the door .

Background Art

It has been known to pro ide microwave ovens with safety switches that are opened when opening the door . These safety switches typically carry the supply device' s supply voltage, i.e. voltage that is used to power the microwave generator. Thus, opening the switch door renders it impossible to generate microwave power.

Security regulations typically request the provision of several safety switches arranged in series such that a safe switch-off is guaranteed even if one of them should fail and remain closed when the door is opened .

The power fed to the microwave generator is in the range of several 100 W or even more than 1 kW. Hence, the supply voltage fed through the safety switche: is typically a high voltage, i.e. it exceeds 100 V or even 200 V, and the currents are in the range of several Amperes . Looping such voltage and current levels through the microwave oven to the safety switches at the door is expensive and needs additional measures to provide electromagnetic compatibility and electrical insulation.

Also, the safety switches need to be able to reliably carry these voltages and currents without failing. Disclosure of the Invention

The problem to be solved by the present invention is to provide a microwave oven having a simple and inexpensive safety mechanism.

This problem is solved, in a f irst aspect , by the microwave oven of claim 1. Accordingly, the oven comprises

a control unit,

a cooking chamber,

a door closing said cooking chamber,

a microwave generator coupled to said cooking chamber and having at least one semiconductor switch, a driver circuit connected to feed control signals to said semiconductor switch,

a driver power supply feeding extra-low-voltage po er to said driver circuit ,

at least one safety switch mechanically op- eratable by opening and closing the door, wherein said safety switch is arranged in a supply line between said driver power supply and said driver circuit.

The control unit controls at least part of the operations of the microwave oven.

The cooking chamber is adapted and structured to receive the food to be cooked. The door provides user access to the cooking chamber.

The microwave generator is adapted and structured to generate microwave radiation, which is fed to the cooking chamber. Typically, the microwave generator comprises a magnetron, but it may also comprise a semiconductor-based high-frequency generator. In any case, it has one or more semiconductor switches .

The driver circuit generates the control signals for the at least one semiconductor switch. This can e.g. be the signal applied to the gate or base of the semiconductor switch . The driver power supply generates the supply power for the dri er circuit . This power is "extra-low- voltage" in the sense defined below.

The at least one safety switch is operatable by opening and closing the door . In other words , the movement of the door causes the safety switch to be opened or closed mechanically. The safety switch is arranged in the supply line through which power is supplied from the driver power supply to the driver circuit .

Hence, when opening the switch, the driver circuit loses its supply power, which renders it inoperable to generate control signals to the at least one semiconductor switch of the microwave generator.

The invention is based on the understanding that switching off the extra-low-voltage power supply to the driver circuit is a means to disable microwave generation that provides the same level of safety as the conventional designs where the high-voltage power supply is switched off . In particular, it also relies on mechanical means only and does not require any electronic processing or software (even though software can be used to provide additional safety or diagnosis features) .

If the microwave generator comprises a magnetron, it also comprises a magnetron power supply having a transformer and an alternating current generator feeding an alternating current to the transformer . In that case, the "at least one semiconductor switch" is part of the magnetron power supply . In particular, it arranged in a bridge or half-bridge of the alternating current genera- tor.

Alternatively, the microwave generator can comprise a semiconductor-based microwave-frequency generator, i.e. a microwave generator using semiconductor switches instead of a magnetron to generate the microwave radiation . In this case, the "at least one semiconductor switch" is part of the semiconductor-based microwave-frequency generator. Advantageously, the microwave ove comprises several of said safety switches. In particular, it comprising at least a first and a second safety switch arranged in series, or even a third safety switch arranged in series to the first and second safety switches in order to provide redundancy.

In that case, the control unit of the microwave oven may have at least one input connected to a location downstream of the first or second safety switch, thus allowing the control unit to monitor the state of one or more of the switches . The term "downstream" is defined below.

In a second aspect, the invention relates to a microwave oven comprising

a cooking chamber,

a door closing said cooking chamber,

a microwave generator coupled to said cooking chamber and having at least one semiconductor switch, wherein said at least one semiconductor switch has a gate or base input,

a driver circuit connected to said gate or base input, wherein said driver ci cuit has a power supply line,

at least one safety switch mechanically op- eratable by opening and closing the door, wherein said safety switch is arranged in said supply line .

The microwave oven of the second aspect can be combined with any of the features of the microwave oven described above .

In a third aspect , the invention relates to a microwave oven comprising

a cooking chamber,

a dcor closing said cooking chamber,

a magnetron having an anode and a cathode, a high-voltage generator connected to said anode and cathode, said high-voltage generator comprising semiconductor switches and forming a half-bridge or a full-bridge and a transformer having a primary winding connected to said half-bridge- or full-bridge , wherein said semiconductor switches have control inputs for being switched on and off,

a drive circuit connected to said control inputs ,

a driver power supply feeding power to said driver circuit,

at least one safety switch mechanically operatable by opening and closing the door, wherein said safety switch is arranged in a supply line between said driver power supply and said driver circuit .

The microwave oven of the third aspect can be combined with any of the features of the microwave oven described above .

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof . This description makes refer- ence to the annexed drawings, wherein:

Fig . 1 shows a schematic, sectional view of a microwave oven,

Fig . 2 shows a circuit diagram of the relevant, parts of the microwave oven,

Fig. 3 shows a possible design of one of the safety switches and of a control switch,

Fig, 4 shows the states of switches while closing and opening the door, and

Fig. 5 shows a block circuit diagram of a second embodiment of a microwave oven . Modes for Carrying Out the Invention

Definitions :

The term "extra-low-voltage" designates a voltage of no more than 50 V AC or 120 V DC, in particular no more than 20 V.

The term "extra-low-voltage power" designates an electrical power of extra-low-voltage in the sense above .

A switch is defined to be "closed" when its contacts are touching and electricity can flow between them and "open" when the contacts are separated and the switch is non-conducting.

"Cooking" designates any process of supplying heat to a foodstuff, e.g. for thawing, for warming up, or for a classical cooking process,

"Microwave-frequency" is understood to designate a frequency between 300 MHz and 300 GHz . Typical frequencies used for microwave ovens are 0.915 GHz and 2.45 GHz.

A location "downstream" of a safety switch is a location along the power line between the driver power supply and the driver circui that is on the driver circuit side of said safety switch, i.e. the safety switch is arranged between the downstream location and the driver power supply.

A "semiconductor switch" is any type of solid-state semiconductor swi ch, in particular a transistor, such as an IGBT, FET or bipolar transistor.

Microwave oven;

The microwave oven of Fig. 1 comprises a housing 1 enclosing a cooking chamber 3. A door 2 is arranged at a front side of cooking chamber 3.

Door 2 can e.g. be a conventional oven door that is pivotal about a horizontal pivot axis along its bottom edge . Safety switches 4 are provided at door 2 or at its frame for detecting the state of door 2, i.e. for detecting if door 2 is closed or open.

The device of Fig. 1 is e.g. also equipped with a user interface 6 that e.g. comprises a display and input controls, an electronic unit 7 comprising control and driver circuitry, and a microwave generator 8.

Microwave generator 8 is coupled, e.g.

through conventional waveguides 9, to cooking chamber 3.

The oven can be a pure microwave oven, i.e. microwave generator 8 can form the sole means for heating the foodstuff in chamber 3. Alternatively, the oven can be a combined device that has also has other means for heating the foodstuff, such as a resistive heater, a hot- air generator or a steam generator.

Device circuitry:

Fig . 2 shows one possible embodiment of circuitry to be used in a microwave oven.

The shown circuitry basically corresponds to the one described in EP 2854480.

It comprises a control unit 10, which can e.g. be a microprocessor, microcontroller, FPGA or a combination of such devices.

It further comprises the microwave generator 8 containing the power components for generating the microwave radiation as well as a driver circuit 14 for driving several semiconductor switches Tl - T6 in microwave generator 8.

The device is operated using mains power (line power) at e.g. 110 or 230 V, which is rectified in a rectifier 16. The rectified mains power is fed as a high-voltage supply power to microwave generator 8.

A driver power supply 18 generates an extra- low-voltage of e.g. 15 V for operating driver circuit 14. In more detail, microwave generator 8 comprises a magnetron 20. A first transformer 22 and a rectifier D2 , D3 generate a high DC voltage to be applied over the magnetron' s anode and cathode . A second trans- former 24 generates a heating current for the magnetron' s cathode .

A pair of transistors Tl, T2 are provided for generating an alternating current in the primary winding of transformer 24 in order to heat the magnetron's cath- ode . The transistors Tl, T2 form a half-bridge coupled to one terminal of the primary winding of transformer 24 while a capacitive voltage divider C5, C6 is connected to its other terminal .

A full bridge formed by the transistors T3, T , T5, and T6 generates the alternating current in the primary winding of transformer 22.

The transistors Tl - T6 form the semiconductor switches of driving circuit 14. They receive their control signals, i.e. their gate or base voltages, from driver circuit 14.

In the embodiment shown, driver circuit 14 consists of three driver devices 14a, 14b, 14c, each of which is capable of generating the control signals of the transistors of a single half-bridge .

Each such driver device can e.g. be an integrated high and low side driver as sold under device number IRS2181 by International Rectifier, California (USA) .

The skilled person will understand that driver circui 14 can also use other components , based either on specialized integrated driver circuits or on general-purpose components .

Driver circuit 14 may comprise transistor output stages, at least one for each semiconductor switch of microwave generator 8. It may further comprise voltage level shifters, filters and other circuitry for smoothly operating the transistors Tl - T6. Driver circuit 14 requires an operating power, such as the Vcc and Vb inputs of the IRS2181 mentioned above, to drive their transistor output stages and/or other parts of its circuitry.

This operating power for driver circuit 14 is provided on a line 26, and it is used to implement the safety mechanism as described in the next section.

Control unit 10 is connected to driver circuit 14 to send it the signals controlling the switching of the semiconductors Tl - T6.

The circuitry of Fig. 2 is operated e.g. as described in EP 2854480.

In particular, control unit 10 alternatingly switches the transistors Tl and T2 on and off in order to generate the alternating current in the primary winding of transformer 24 and thereby the cathode heating current of magnetron 20.

Further, control unit 10 operates the full- bridge of the transistors T3 - T6 for generating the al- ternating current in the primary winding of transformer 22 and thereby the high-voltage applied over the anode and cathode of magnetron 20.

Microwave generator 8 generates a number of feedback signals , such as the voltages Uih and Ur indica- tive of the currents in the two transformers 24 , 22 , or a voltage generated by an opto-coupler 28 indicative of the anode-cathode voltage of magnetron 20.

Control unit 10 uses these feedback signals for controlling the operation of microwave generator 8 as e.g. described in EP 2854480.

Safety mechanism:

As mentioned, the microwave oven is provided with safety switches 4 (Fig. 1) or, in more general terms , with a safety mechanism, whose purpose is to ensure that microwave generation is safely disabled when door 2 is open. In the present embodiment, there are several safety switches, namely a first safety switch SI, a second safety switch S2, and a third safety switch S3. These safety switches SI, S2 , S3 are arranged in series along supply line 26 between driver power supply 18 and driver circuit 1 . In addition, there may be a control switch Sc and a thermal overload switch St. The roles of these switches will be explained in more detail now.

As can be seen from Fig . 2, the supply volt- age f om driver power supply 18 is first fed through a fuse 30. From there it passes the safety switches SI, S2, S3 as well as the thermal overload switch St, and it finally arrives through line 26 at driver circuit 14.

All of these switches are arranged in series, If any one of them is open, no supply po er is received by driver circuit 14.

Driver circuit 14 is designed to be in its off state in the absence of supply power, i.e. it generates no signals for closing the semiconductor switches Tl - T6 if it receives no supply power .

As can be seen from Fig. 2 , control switch Sc is arranged between a downstream location LI of first safety switch SI and zero voltage potential .

All safety switches SI - S3 as well as con- trol switch Sc are operated by the movement of door 2.

Fig. 3 shows one possible mechanical implementation for implementing switches ope ated by the door's movement . The shown embodiment shows an assembly comprising the first safety switch SI as well as control switch Sc .

In the embodiment shown, both these switches SI, Sc are micro switches ha ing a nub-shaped actuator 32. When actuator 32 is in its extended position, the switch is open, i.e. non-conducting . When it is de- pressed, the switch is closed, i.e. conducting .

The switches SI, Sc are mounted to a common frame 34. A slider 36 is held in frame 34 and displace- able along a direction of displacement 38. It has a first position (shown in solid lines) as well as a second position (shown in dashed lines) . A bias spring 40 can e.g. be provided for urging slider 36 into its first position .

When door 2 is open, it does not touch slider 36 and slider 36 is in its first position as shown in solid lines in Fig. 3.

When door 2 is being closed, it comes into contact with a door-facing end 42 of slider 36 and pushes slider 36 into its second position, against the force of bias spring 40.

Slider 36 has fi st and second lateral guide surfaces 44, 46 positioned to interact with the actuators 32 of switch SI and switch Sc, respectively .

When door 2 is open, actuator 32 of safety switch SI does not touch lateral first guide surface 44, and therefore safety switch SI is open, i.e. non-conducting. At the same time, second guide surface 46 pushes against actuator 32 of control switch Sc, and therefore control switch Sc is closed, i.e. conducting .

When door 2 is being closed, first lateral guide surface 44 comes into contact with actuator 32 of first safety switch SI and the eby closes first safety switch SI. On the other hand, second lateral guide surface 46 loses its contact with actuator 32 of control switch Sc and control switch Si opens .

When opening door 2, first safety switch SI opens at a "first position" of door 2 , while control switch Sc closes at a "second position" of door 2.

The first position of door 2 is very close to the door' s closed position, advantageously such that, the microwave leakage at this position is below safety limits. It is e.g. reached whe door has moved away by 1 ° from its closed position.

The second position of door 2 is not as close to the door ' s closed position . Its angle (measured from the door's closed position) is larger than the one of the first position but such that microwave leakage is still low . It is e.g. reached when door has moved away by 3° from its closed position .

In other words, upon opening door 2, the first position occurs before the second position, i.e. first safety switch SI opens before control switch Sc closes . In other words, switch SI operates as a monitored safety switch .

Similarly, when closing door 2, control switch S2 opens before first safety switch SI closes .

The assembly of Fig . 3 and therefore first safety switch SI as well as control switch Sc are advantageously arranged at an upper edge region of door 2, in particular at a first one of its upper corners.

Second safety switch S2 can be arranged e.g. close to the door' s bottom edge.

Third safety switch S3 can be arranged, similarly to first safety switch SI, at the door' s upper edge region, e.g. at the second one of its upper corners.

Advantageously, second safety switch S2 is located to open at a third position of door 2 that is e e further away from its closed position that the second position, but advantageous ly it is still small enough to prevent a user from manipulating the first safety switch. It is e.g. at 5° from the door' s closed position.

Third safety switch S3 can e.g. be opened at the door' s first position .

Fig . 4 shows the state of the various

switches SI, S2 , S3, and Sc during closing and opening door 2. Any line in its upper position indicates the respective switch to be closed (conducting) , while an open (non-conducting) switch is represented by the line being in its lower position.

As can be seen, when door 2 is open, all the safety switches SI, S2 , S3 are open, while control switch Sc is closed. Upon closing door 2 , second safety switch S2 will be the first to close . Then, control switch Sc opens, and, finally, the first and third control switches close .

Once all safety switches are closed and control switch Sc is open, supply power Vcc is established on line 26 and, upon receipt of signals from control unit 10, driver circuit 14 is able to operate the semiconductor switches Tl - T6 of microwave generator 8.

When opening door 2 , the first and third safety switches SI, S3 are the first to open, thereby disabling driver circuit 14 safely. Next, control switch Sc is closed, and finally second safety switch 14 is opened.

If, for some reason, first safety switch SI would fail to open upon opening door 2, control switch Sc would close while first safety switch SI is still closed. In that case, a strong current would flow through first safety switch Si into control switch Sc and a PTC resis- tor R. This current is sufficient to blow fuse 30, thus safely preventing an activation of microwave generator 8.

Thermal overload switch St is located in thermal contact with magnetron 30 (or any other part of microwave generator 8 that is likely to overheat) . It opens at the presence of an excess temperature in microwave generator 8.

Since overload switch St is in series with the safety switches SI - S3 , it will interrupt the power supply to driver circuit 14 in the present of such excess temperature .

As can be seen in Fig. 2 , control unit 10 comprises several inputs INI, IN2, IN3, IN4, which allow to monitor the state of the safety switches Si - S3, the thermal overload switch St as well as the control switch SC.

In particular, a first input INI is connected to a ] ocation LI downstream of first safety switch SI, namely a location LI between first safety switch SI and second safety switch S2. A second input IN2 is connected to a location L2 downstream of second safety switch S2, namely a location L2 between second safety switch S2 and third safety switch S3. A third input IN3 is connected to a location L3 downstream of third safety switch S3, namely a location L3 between third safety switch S3 and thermal overload switch St . Finally, a fourth input IN4 is connected to a location L4 downstream of thermal overload switch St, namely between thermal overload switch St and driver circuit 14.

If control unit 10 detects that all inputs INI - IN4 are in their high state (i.e. the supply voltage is present at all locations LI - L , it concludes that door 2 is closed and operation of the microwave oven can begin .

If control unit 10 detects that all inputs INI -IN4 are in their low state (i.e. no supply voltage is present at any of the locations LI - L4 it concludes that door 2 is open .

Any other combinations indicate either (if transitional ) that the door is being closed or opened or that there is a malfunction .

If fuse 30 is blown, in the doors closed state the voltages at the inputs INI - IN4 will be lower than if the fuse 30 is not blown. This allows to distinguish between states where t e fuse is blown and where the fuse is not blown, provided that the inputs INI - IN4 are able to distinguish between at least three different voltage levels . Advantageously, the inputs INI - IN4 are analog-digital-converting inputs .

Notes :

It must be noted that the embodiment shown in

Fig. 2 is only one of many possible implementations of the present invention . For example, the full-bridge formed by the transistors T3 - T6 could be replaced by a half-bridge, or the half-bridge formed by the transistors Tl , T2 could be a full-bridge. Alternatively, one single half- or full-bridge can be used for driving a transformer having separate secondary windings for generating the heating current and the anode-cathode-voltage of magnetron 20.

In another example, driver device 14a can be fixedly connected to driver power supply 18, and only the supply power for the driver devices 14b, 14c can be fed through the switches SI, S2, S3, and St , or (even though less advantageous ) vice versa.

Also, instead of a magnetron 30 , a solid state microwave generator 8 ' , i.e. semiconductor-based microwave-frequency generator , can be used, such as e.g. described in US 2015136760, US3557333, or US4504718, where one or more semiconductor switches T are operated at high f equency to generate electromagnetic waves by means of suitable antennae 40.

In this case, driver circuit 14 controls the semiconductor switches T in generator 8 ' , and the supply power to driver circuit 14 can be looped through the safety switches SI - S3.

Driver circuit 14 can e.g. generate the high frequency signals used to drive the semiconductor switches .

While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims .