DK2852250T3

DK2852250T3 - Microwave with load equalization in the high voltage generator

Info

Publication number: DK2852250T3
Application number: DK14001471.3T
Authority: DK
Inventors: Alfred Betschart
Original assignee: V-Zug Ag
Priority date: 2014-04-24
Filing date: 2014-04-24
Publication date: 2017-07-10
Also published as: EP2852250A1; PL2852250T3; EP2852250B1; SI2852250T1

Description

Field of the Invention

The invention relates to a microwave oven with a magnetron and with a controlling circuit for the magnetron. The invention also relates to a method for operating such a microwave oven.

Background

Normally a microwave oven has a high voltage transformer which is driven by a half bridge or full bridge circuit. US 4 742 442 and DE 3842910 describe devices with the high voltage transformer driven by a full bridge.

In case of driving it in this way, substantial power loss may occur in the switching elements of the full bridge, which can lead to thermal problems.

Description of the Invention

It is the object of the invention to provide a microwave oven and a method of the type mentioned at the beginning, in case of which thermal problems at the switching elements can be combated.

According to this, the driver circuit of the magnetron has, as known, a high voltage generator for generating the high voltage for the operation of the magnetron. Additionally, a controller is provided, which drives the high voltage generator.

The high voltage generator has a high voltage transformer and a full bridge. The full bridge has a first and a second push-pull amplifier. The push-pull amplifiers are connected in parallel, and each push-pull amplifier has a first switching element in series with a second switching element. The primary winding of the high voltage transformer is arranged between the two push-pull amplifiers .

The controller is adapted in a conventional manner, alternatingly, - to switch on the first switching element of the first push-pull amplifier and the second switching element of the second push-pull amplifier, and - to switch on the second switching element of the first push-pull amplifier (T 3, T4) and the first switching element of the second push-pull amplifier.

In this way, it is possible to generate an alternating current in the primary winding of the high voltage transformer.

According to the claim, the controller is furthermore adapted - during an operating mode A to switch off the first switching element of the first push-pull amplifier, while the second switching element of the second push-pull amplifier is still switched on, and - during an operating mode A' to switch off the second switching element of the second push-pull amplifier, while the first switching element of the first push-pull amplifier is still switched on.

Furthermore, the controller is adapted to switch back and forth between the operating modes A and A'.

This embodiment is based on the recognition that the main losses occur in the switching element that is first switched off, and that it is possible to switch between said operating modes A and A' in order to balance the losses between the first switching element of the first push-pull amplifier and the second switching element of the second push-pull amplifier.

In order to also balance the losses between the second switching element of the first push-pull amplifier, and the first switching element of the second push-pull amplifier, the controller may further be adapted - during an operating mode B to switch off the first switching element of the second push-pull amplifier, while the second switching element of the first push-pull amplifier is still switched on, and - during an operating mode Bf to switch off the second switching element of the first push-pull amplifier while the first switching element of the second push-pull amplifier is still switched on.

Furthermore, the controller is adapted to switch back and forth between the operating modes B and B'.

The back and forth switching between the operating modes is done preferably periodically.

In a particularly simple embodiment, said operating modes A, A' , B and B' can be combined in such a way that the controller is adapted to combine the operating mode A with the operating mode B' and to combine the operating mode A' with the operating mode B. In this case, the controller has actually only two different operating modes: in one of them the switching elements are operated according to A and B and in the other according to A' and B'.

In order to achieve a complete thermal balance, the controller is advantageously adapted to use the operating modes A and A' alternatingly, each one during the same time interval.

The invention also relates to a method for operating such a microwave oven, wherein - during an operating mode A the first switching element of the first push-pull amplifier is switched off while the second switching element of the second push-pull amplifier is still switched on, and during an operating mode A' the second switching element of the second push-pull amplifier is switched off while the first switching element of the first push-pull amplifier is still switched on, wherein it is switched back and forth between the operating modes A and A' .

In order to also achieve a thermal balance between the second switching element of the first push-pull amplifier and the first switching element of the second push-pull amplifier, the method may further be characterized in that - during an operating mode B the first switching element of the second push-pull amplifier is switched off, while the second switching element of the first push-pull amplifier is still switched on, and during an operating mode B' the second switching element of the first push-pull amplifier is switched off, while the first switching element of the second push-pull amplifier is still switched on, wherein it is switched back and forth between the operating modes B and B' .

Brief Description of the Drawings

Further embodiments, advantages and applications of the devices and the methods will be apparent from the dependent claims and the following description of exemplary embodiments based on the figures. It is shown in FIG. 1 a section through those parts of the microwave oven, which are the most important in the present context, FIG. 2 a simplified circuit diagram of the microwave oven, FIG. 3 a diagram of certain signals of the driver circuit for the cathode heating,

Fig. 4 a diagram of certain signals of the driver circuit for the high voltage generator for the operating modes A, B,

Fig. 5 the diagram of Fig. 4 for the operating modes A', B',

Fig. 6 the diagram of Fig. 4 for the operating modes A, B', and

Fig. 7 the diagram of Fig. 4 for the operating modes A', B.

Ways of implementing the invention

Definitions:

High voltage in the present context is understood as a voltage which is necessary for the operation of the magnetron as anode-cathode-voltage. In practice, this voltage is in most cases 1 kV, normally several kilovolts . A push-pull amplifier is a series circuit of two electronic components which can be alternatingly switched to conduct, such that a time-varying voltage is generated at the center tap of the two components. A half bridge circuit is a circuit with precisely one push-pull amplifier. A full bridge circuit (H-circuit, H-bridge) is a circuit with two parallel push-pull amplifiers, wherein the load is located between the center taps of the two push-pull amplifiers.

Basic structure:

The invention relates to a microwave oven as exemplarily shown in Fig. 1. The microwave oven has a cooking space 1 for receiving the foodstuff to be heated up, which can be closed towards the user by a user door 2. A magnetron 3 is additionally arranged inside the device, which is connected to the cooking space 1 via a hollow conductor 4. A controller 5 controls the function of the device.

Fig. 2 shows the most important components of the controller 5 with respect to the present context.

The mains voltage of e.g. 230 volts at 50 Hz is rectified in a rectifier 10. The first intermediary voltage Uz generated in this way is subseguently slightly filtered via a first capacitor Cl, wherein the capacitor Cl is however dimensioned such that in the presence of load the value of the first intermediary voltage Uz oscillates by at least 50% with the double mains frequency. The intermediary voltage Uz is additionally tapped via a diode D1 and is further filtered via a second capacitor C2, in order to generate a second intermediary voltage Uz'.

The first intermediary voltage Uz is supplied to a high voltage generator 11, by means of which the high voltage is generated for driving the magnetron 3, as described below. The second intermediary voltage Uz' is supplied to a heating current generator 12, by means of which the heating current for the cathode heating of the magnetron 3 is generated as described below.

The operation of the high voltage generator 11 and of the heating current generator 12 is controlled by a controller unit 13, e.g. a microprocessor. A proportional value to the intermediary voltage Uz is supplied to an analog-digital-converter of the controller circuit 13 via a voltage divider R5, R6, such that it can determine the intermediary voltage Uz.

High voltage generator:

The high voltage generator 11 comprises a full bridge circuit with four electronic switching elements T3 - T6, particularly as IGBT-transistors. A free-wheeling diode 30 is arranged above each switching element T3 -T6. The free-wheeling diode 30 is connected in parallel with the respective switching element and is oriented such that it only allows current passage when the latter flows in an opposite direction to the normal flow direction of the switching element.

The switching elements T3 - T6 are, as known, arranged in two branches, that is: push-pull amplifiers T3 and T4 or T5 and T6, respectively, wherein the switching elements of each branch are each arranged in series between the first intermediary Uz and ground. A center tap is provided between the switching elements of each branch, wherein the two center taps are connected to the two connectors of the primary winding of a high voltage transformer 14. Thus, the switching elements T3 - T6 form an inverter which supplies an alternating voltage into the primary winding of the high voltage transformer.

The high voltage transformer 14 has a secondary winding with a substantially higher winding number than the primary winding for generating the high voltage. The high voltage is rectified via two diodes D2 and D3, doubled and filtered by means of two capacitors C3 and C4. The high voltage Uh generated in this way is applied between the cathode K and the anode A of the magnetron 3. A driver circuit 16 is provided for driving the switching elements T3 - T6, which is controlled by the control unit 13. The driver circuit 16 generates the controlling voltages (gate or base voltages) UG3 - UG6 for the switching elements T3 - T6. The controller unit 13 is adapted to switch the two branches of the full bridge circuit T3 - T6 alternatingly. The driving is done in such a way that during a switching cycle the primary winding of high voltage transformer 14 is not all the time between the first intermediary voltage Uz and ground, but that the primary winding is decoupled from the intermediary voltage Uz during a time interval to be chosen by the controller unit 13, i.e. the circuit is clocked with pulse width modulation, such that the value of the high voltage Uh can be controlled.

For monitoring the high voltage Uh, it can be divided by a voltage divider RIO - R13 and R14 and is fed to an optocoupler 17, the output signal of which is forwarded to the controller unit 13. For example, in this way absence or non-sparking of the magnetron can be detected.

Furthermore, a resistor R20 is provided between the two branches T3, T4 or T5, Τβ, respectively, and a fixed reference potential, particularly ground. The initial increase of the voltage drop Ur across this resistor at the beginning of a current pulse is a measure for the anode current of the magnetron 3 and is supplied to the controller unit 13 via an amplifier 18 for measurement purposes. In this way, the anode current can be monitored.

Heating current generator:

In the present embodiment, the heating current generator 12 is formed by a half bridge with two switching elements T1 and T2 operated as push-pull amplifier. The switching elements T1 and T2, which can e.g. be formed as IGBT-transistors and which are each equipped with a free-wheeling diode 30, are arranged in series between the second intermediary voltage Uz' and ground.

The center tap between the two switching elements TI, T2 is connected to the one connector of the primary winding of a heating transformer 15. The second connector of the primary winding of the heating transformer 15 is connected to the center tap of a capacitive voltage divider of two capacitors C5 and C6. The two capacitors C5 and C6 are in series between the second intermediary voltage Uz' and ground.

The diode D1 avoids that current is taken away from the capacitors C5, C6, when the high voltage generator 11 connected to the intermediary voltage Uz draws current.

The secondary winding of the heating transformer 15 is connected to the cathode heating, i.e. the filament, of the magnetron 3 and supplies it with current. A driver circuit 20 is provided for driving the switching elements T1 and T2, which is controlled by the controller unit 13. The driver circuit 20 generates the driver voltages UGI, UG2 for the switching elements T1 or T2, respectively. The driver circuit 20 generates the driver voltages (gate or base voltages) UGI, UG2 for the switching elements T1 or T2, respectively. The type of the driving is described in detail further down. A resistor R21 is arranged between the push-pull amplifier, formed by the switching elements TI, T2 and ground (or another fixed reference potential), through which the current from the push-pull amplifier TI, T2 flows away through the heating transformer towards ground (or the reference potential). The voltage drop across this resistor is a measure for the current flowing from the second intermediary voltage Uz' through the primary coil of the high voltage transformer 15 towards ground (or reference potential). It is tapped by an amplifier 21 and fed into an analog-digital-converter of the controller unit 13.

Driving the heating current generator:

In the following, it is described by Fig. 3 how the controller unit 13 drives the switching elements of the heating current generator 12. The figure shows the course of the voltages UG1 and UG2, which are present at the control inputs of the switching elements T1 and T2, as well as the course of the voltage Uih dropping across the resistor R21.

The controller unit 13 is adapted to switch on the two switching elements T1 and T2 cyclically alternating. A typical cycle period Tz is preferably in the range between 10 - 50 ps .

In the following, the time intervals during which one of the switching elements T1 or T2 is switched on, are called heating phases HI or H2, respectively, and are illustrated in Fig. 3, wherein the first switching element 11 is switched on in heating phase HI and the second switching element T2 is switched on in heating phase H2. Both switching elements TI, T2 are switched off between the heating phases HI and H2, or H2 and HI, respectively. The phases during which both switching elements TI, T2 are switched off, are called idle phases R1 and R2 and are also illustrated in Fig. 3. The heating phases have the duration th, the idle phases a duration tr .

In a simple embodiment, the time th for both switching elements T1 and T2 can be chosen identically and in the same way tr.

In this way, an alternating current is generated in the primary winding of the heating transformer 15, which is supplied to the cathode unit of the magnetron 3 as heating power (except losses in the components, particularly in the heating transformer 15). The averaged parameter of the heating power is a function of the sample ratio, i.e. the quotient th/Tz.

As can be seen in Fig. 3, after switching on one of the switching elements TI, T2, the current through the primary winding of the heating transformer 15 and therefore also the voltage drop Uih across resistor R21 increase and can be measured by the controller unit 13 via the amplifier 21.

The voltage drop Uih forms a parameter depending on the resistance of the cathode heating of the magnetron 3. When assuming that no loss occurs in the heating transformer 15, Uih is inversely proportional to the resistance of the cathode heating towards the end of the heating pulse. Thus, the resistor R21 forms together with the amplifier 21 a measurement circuit which is adapted to determine a parameter depending on the resistance of the cathode heating.

In Fig. 3, an instant tm is indicated, when the controller 13 measures the voltage drop Uih. This instant tm is preferably shortly before the end tx of the respective heating phase HI or H2, respectively, e.g. at most 1 ys before the end tx of the heating phase. Advantageously, a measurement is performed in every heating phase.

The measurement unit 13 is adapted to keep the product P = Uz'· Uih (tm) · th constant by varying the duration th of the heating phases depending on the values of Uih (tm) and Uz'. The product P is at least approximately proportional to the power supplied to the cathode heating.

The value of the intermediary voltage Uz, in the way it is determined by the controller unit across the voltage divider R5, R6, can be used as approximation for the value of the intermediary voltage Uz' . As long as (during the pre-heating phase) the high voltage generator 11 is not in operation, Uz' corresponds to the value of Uz, except for the voltage drop across Dl. Thereafter, Uz' is sometimes slightly larger than Uz, however the difference remains small when the components are dimensioned in a suitable way. If Uz' shall be determined precisely, a second voltage divider may be provided additionally or alternatively, to R5, R6, which supplies the second intermediary voltage Uz' for measurement to the controller unit 13.

Preferably, P is averaged over a filter time which amounts to at least half a clock period of the mains voltage, i.e. at least 10 ms. An adjustment of the pulse width th occurs only after termination of the filter time. P is a direct measure for the power provided by the push-pull amplifier TI, T2, and therefore (by ignoring the power loss, particularly in the heating transformer 15) also a measure for the heating power of the cathode heating of the magnetron 3. Consequently, the controller unit 13 also forms a power regulator by means of which the power received by the cathode heating can be regulated to a target value.

Driving the high voltage generator:

In the following, it is described by means of Fig. 4 how the controller unit 13 drives the switching elements T3 - T6 of the high voltage generator 11. The figure shows the course of the voltages UG3 - UG6, which are present at the drive inputs of the switching elements T3 - T6, as well as the course of the current Ip in the primary winding of the high voltage transformer and the voltage Ur which drops across the resistor R20.

The controller unit 13 is adapted to operate the four switching elements T3 - T6 in a cyclic way. A typical cycle period tc is preferably in the range between 10 - 50 ps .

Each cycle period comprises four phases PI - P4. In the variant according to Fig. 4, these four phases are as follows: - In phase PI, the switching elements T3 and T6 are switched on and the switching elements T4 and T5 are switched off, such that a positive current Ip is generated by the intermediary voltage Uz through the bridge circuit towards ground. - In phase P2, the switching element T6 remains switched on. The switching element T3 is switched off and thereafter the switching element T4 is switched on. The current through the high voltage transformer 14 fades out by flowing through the switching element T6 and the freewheeling diode of the switching element T4.

In phase P3, the switching element T6 is switched off and the switching element T5 is switched on. A negative current Ip is now generated by the intermediary voltage Uz through the bridge circuit and the primary winding towards ground. - In phase P4, the switching element T4 remains switched on. The switching element T5 is switched off and thereafter the switching element T6 is switched on. The current through the high voltage transformer 14 fades out again by flowing through the switching element T4 and the free-wheeling diode of the switching element T6.

In operation, phases PI and P3 are preferably equally long. In the same way, phases P2 and P4 are preferably equally long. Phases PI and P3 are normally however shorter than or at most as long as the phases P2 and P4. The power to be supplied by the magnetron can be adjusted by the ratio between the sum of the durations of phases Pi and P3 and the cycle time tc. This ratio is adjusted by the controller 13 e.g. depending on the user settings .

Operation:

When the user activates the microwave oven, i.e. has given the command to supply energy to the foodstuff inside the cooking space, the controller 13 first initiates a preheating phase. During this preheating phase the switching elements T3 - T6 remain all switched off, such that no high voltage is present at the magnetron 3. Subsequently, an operating phase follows the preheating phase, during which the switching elements T3 - T6 are operated alternatingly in order to provide the high voltage to the magnetron and to generate the desired microwave radiation. In the following, the operating phase is described in more detail.

Firstly, the controller unit 13 operates the device e.g. with the pulse sequence shown in Fig. 4. According to this, during the first two phases PI and P2, firstly (in phase PI) the first switching element T3 of the first push-pull amplifier T3, T4 is switched on, as well as also the second switching element T6 of the second push-pull amplifier T5, T6. Thus, the current flows from the intermediary voltage Uz through T3, the primary winding and T6 towards ground. Now, the first switching element T3 of the first push-pull amplifier T3, T4 is switched off, while the second switching element T6 of the second push-pull amplifier remains switched on. This occurs in the presence of load, such that a relatively high thermal loss results in the switching element T3 during the switching transient.

Now, the first switching element T5 of the second push-pull amplifier T5, T6 is switched on during the two phases P3 and P4, while the second switching element T4 of the first push-pull amplifier T3, T4 is already switched on. At the end of the phase P3, the switching element T5 is switched off, while the switching element T4 still remains switched on. This switching process occurs again in the presence of load, such that a relatively high thermal loss occurs in the switching element T5.

Now the cycle restarts.

Evidently, the switching elements T3 and T5 are each switched under load, such that switching losses are generated therein. This leads to an undesired asymmetric heat up of the switching elements T3 and T5, while the switching elements T4 and T6 only heat up a little.

For this reason, the controller unit 13 is adapted to drive the full rectifier not only with the pulse seguence shown in Fig. 4.

Fig. 5 shows an alternative impulse sequence. During this impulse sequence, the switching elements T4 or T6, respectively, are switched off, while the switching elements T3 or T5, respectively, are still switched on. In this way, the primary power losses occur in the switching elements T4 and T6, while heating up of the switching elements T3 and T5 remains low.

Fig. 6 and 7 show two further variants:

In the sequence of Fig. 6, the switching elements T3 and T4 are switched off, while the switching elements T6 or T5, respectively, are still switched on. Thus, the highest power loss occurs in the switching elements T3 and T4.

In the sequence of Fig. 7, the switching elements T5 and T6 are switched off, while the switching elements T4 or T3, respectively, are still switched on. Thus, the highest power loss occurs in the switching elements T5 and T6.

In order to compensate for the thermal losses in the switching elements, the controller unit 13 is now adapted to use at least two sequences according to Fig. 4 - 7.

As an analysis of the figures shows, it is possible to differentiate between the switching process during phases PI and P2 and the ones during phases P3 and P4 .

In phases PI and P2, the current flowing through the switching elements T3 and T6 is switched off. Depending on the pulse sequence, the corresponding switching loss can be supplied either to T3 or to T6. Accordingly, it is possible to differentiate between two operating modes A and A': - During the operating mode A (Fig. 4, Fig. 6), T3 is switched off, while T6 is still on. In this case, the power loss occurs primarily in T3. - During the operating mode A' , T6 is switched off, while T3 is still on. In this case, the power loss occurs primarily in T6.

In the same way, the current flowing through the switching elements T4 and T5 is switched off in phases P3 and P4. Also in this case it is possible to differentiate between two operating modes B and B': - During the operating mode B (Fig. 4, Fig. 7), T5 is switched off, while T4 is still on. In this case, the power loss occurs primarily in T5. - During the operating mode B' , T4 is switched off, while T5 is still on. In this case, the power loss occurs primarily in T4.

In order to compensate for the power between the switching elements T3 and T6, the controller unit 13 should consequently switch between the operating modes A and A'. In order to compensate for the power between the switching elements T4 and T5, it should switch between the operating modes B and B'.

The switching between different operating modes can basically be carried out e.g. after each cycle tc. Advantageously, the controller unit 13 is however adapted to switch the operating modes synchronously with the double mains frequency. In other words, the switching between operating modes occurs therefore every 10 ms, or every 20 ms, 30 ms, etc. In this way, the power loss is averaged over half a period of the mains frequency -because the intermediary voltage Uz has a high residual ripple, it is possible in this way to average over the power variation occurring during half a period.

Preferably, the controller unit 13 is adapted to combine the operating mode A with the operating mode B and the operating mode A' with the operating mode B'. In other words, it is switched simultaneously from A to A' when it is switched from B to B', and it is switched back from B' to B when it is switched from A' to A, such that operation between the two similar pulse sequences is switched back and forth according to Fig. 4 and 5. In this way the switching complexity is reduced.

In a second preferred embodiment, the controller unit 13 is adapted to combine the operating modes A and B', as well as the operating modes A' and B, i.e. to switch back and forth between the two similar pulse sequences according to Fig. 6 and 7, particularly periodically. Again, in this way the circuit complexity can be reduced.

Notes :

In the embodiment described above, two separate transformers 14 and 15 are provided for high voltage and heating power. The described pulse sequence for the full bridge may however also be used in one device for which only one transformer with two secondary windings (for heating and for high voltage) is provided, the primary winding of which is fed by the full bridge.

To summarize, a controller circuit for a microwave oven is described, which has a full bridge T3 -T6 for feeding the high voltage transformer 14 with alternating current pulses. The controller unit 13 of the device is adapted to vary the sequence of the controlling pulses for the switching elements of the full bridge T3 -T6, in order to distribute the switching losses as uniformly as possible to the different switching elements .

The sequence control of the described method steps may be implemented as hardware and/or software in the controller unit 13.

Wile preferred embodiments of the invention are described in this application, it is clearly noted that the invention is not limited thereto and may be carried out in other ways within the scope of the now following claims .

Claims

A microwave oven with a magnetron (3) and with a control circuit for the magnetron (3), wherein the control circuit comprises: a high voltage generator (11) for generating a high voltage to the magnetron (3), wherein the high voltage generator (11) comprises a a high voltage transformer (14) and a full bridge (T3 - T6) having a first and a second receive end stage, wherein the receive end stages are connected in parallel with each receive end stage comprises a first switching element (T3, T5) in series with a second switching element (T4, T6), and wherein a primary winding of the high voltage transformer (14) is arranged between the two receiving end stages, and a control unit (13), wherein the control unit (13) is configured to alternate in switching cycles (tc) - first receiving end stage (T3, T4) first coupling element (T3) and second receiving end stage (T5, T6) second coupling element (T6) and - coupling the second receiving end stage (T3, T4) second coupling element (T4) and the second receive end step (T5, T6) first coupling element (T5), characterized in that the control unit (13) is designed to - in a operating mode A, disconnect the first coupling element (T3) of the first receive end stage, while the second switching end stage second coupling element (T6) ) is still switched on and - in a mode of operation A ', to disengage the second switching end stage second switching element (T6), while the first switching end stage first switching element (T3) is still switched on, where the control unit is designed to switch back and forth. between operating modes A and A '.

The microwave oven of claim 1, wherein the controller (13) is configured to - in a mode of operation B - disengage the first switching element (T5) of the second receiver end stage, while the second switching end stage second switching element (T4) is still engaged, and - switching off the second switching element (T4) of the first receive end stage (T4) in an operating mode B ', while the second switching end stage first switching element (T5) is still switched on, the control unit being designed to switch back and forth between operating modes B and B'.

Microwave according to claim 2, wherein the control unit is designed to combine operating mode A with operating mode B and combining operating mode A 'with operating mode B'.

Microwave according to claim 2, wherein the control unit is designed to combine operating mode A with operating mode B 'and combining operating mode A' with operating mode B.

Microwave according to one of claims 2 to 4, wherein the control unit is designed to use operating modes B and B 'alternately for each of the same time periods.

Microwave according to one of the preceding claims, wherein the control unit is designed to use operating modes A and A 'alternately for each of the same time periods.

Microwave according to one of the preceding claims, wherein the full bridge is provided with a unidirectional mains voltage which varies with a double mains frequency and wherein the control unit is designed to switch the operating modes synchronously with the double mains frequency.

A microwave oven according to any one of the preceding claims, wherein a free-running diode (30) is arranged over each coupling element.

Microwave according to one of the preceding claims, wherein the first switching end stage first switching element (T3) connects a unidirectional intermediate voltage (Uz) to a center outlet of the first receiving end stage, the second receiving end stage first switching element (T5) connects it unidirectional intermediate voltage (Uz) with a center outlet of the second receive end stage, the second coupling element (T4) of the first counter-clock end stage connects the middle outlet of the first receive end stage with a reference potential, and the second receive end stage second switching element (T6) connecting the center outlet of the second receiving end stage to the reference potential, and where a primary winding of the high voltage transformer (14) is arranged between the center outlets.

Method for operating a microwave oven according to one of the preceding claims, wherein the first switching end stage first switching element (T3) is switched off in an operating mode A, while the second switching end stage second switching element (T6) is still switched on and the second the second switch element (T6) of the receive end stage is switched off in an operating mode A ', while the first receive end stage first switch element (T3) is still switched on, which switches back and forth between operating modes A and A'.

The method according to claim 10, wherein - the first switching end stage first switching element (T5) is switched off in an operating mode B, while the second switching end stage second switching element (T4) is still switched on, and - the second switching end stage second switching element (T4) T4) is switched off in one operating mode B ', while the second switching end stage first switching element (T5) is still switched on, switching back and forth between operating modes B and B'.