EP2854480A1 - Microwave oven with fluctuation controlled heating power - Google Patents

Abstract

The control circuit for a microwave oven has a push-pull output stage (T1, T2) for driving a heating transformer (15), with which the cathode heating of the magnetron (3) is operated. For the generation of the high voltage, a separate high-voltage transformer (14) is provided, which is fed by a bridge circuit (T3 - T6). The control unit (13) of the device is designed to determine fluctuations in a parameter dependent on the anode current of the magnetron (3). If these fluctuations are high, the heating power of the cathode heating is increased. In this way, the magnetron (3) can be operated with an optimal, low heating power.

Description

Field of the invention

The invention relates to a microwave oven with a magnetron comprising an anode, a cathode and a cathode heater and with a drive circuit for the magnetron. The invention also relates to a method for operating such a microwave oven.

background

Normally, a microwave oven has a transformer with two secondary windings. The one secondary winding is used to drive the cathode heater of the magnetron, while the other secondary winding is used to generate the high voltage between the cathode and anode. In such devices, separate control of the anode current and the heating current is not possible.

In US 4,742,442 a device is described in which the drive circuit has a controllable separately from the high voltage generator Heizstromgenerator. In particular, two separate transformers for the heating current and the high voltage are provided. After activating the device, an AC voltage is first applied to the heating current transformer to heat the cathode. After a certain time, for example five seconds, then an AC voltage for the high-voltage transformer is then generated, so that the high voltage is applied only after preheating the cathode to the magnetron.

The lifetime of a magnetron is directly dependent on the temperature of the filament of the cathode heater. The service life is extended if the heating power during operation is as low as possible. However, the cathode temperature must be so high that there are enough free electrons for the generation of the microwaves.

In the course of aging of the magnetron, this requires an increasingly greater heating power for stable operation. In conventional solutions, therefore, the heating power is chosen so large that an already aged magnetron can still be safely operated. It is accepted that this heat output is in itself too high for a new magnetron and that it therefore comes to a faster aging of the magnetron.

Presentation of the invention

It is therefore the object to provide a microwave oven and a method of the type mentioned, in which the magnetron has a long life. This object is achieved by the device or method according to the independent claims.

Accordingly, the drive circuit of the magnetron in a conventional manner has a high voltage generator for generating the high voltage between the anode and the cathode, and a Heizstromgenerator for generating the heating current for the cathode heater. In addition, a controller is provided which controls these components.

Furthermore, a measuring circuit is provided, which is designed to determine fluctuations in a parameter dependent on the anode current of the magnetron, and the controller is designed to control the Heating current generator depending on these fluctuations to control such that the heating current is increased with increasing fluctuations.

1. (A) Messen von Schwankungen in einem vom Anodenstrom des Magnetrons abhängigen Parameter. Bei diesem Parameter kann es sich z.B. um den Anodenstrom selbst oder um einem anderen vom Anodenstrom abhängigen Parameter handeln.
2. (B) Steuern des Heizstromgenerators abhängig von den Schwankungen derart, dass der Heizstrom bei zunehmenden Schwankungen erhöht wird. Mit anderen Worten wird bei einem Anstieg der Schwankungen, z.B. über einen Schwankungs-Schwellwert, der Heizstrom erhöht.

Accordingly, the invention also relates to a method for operating a microwave oven, wherein the microwave oven comprises a magnetron having a cathode, an anode and a cathode heater. As part of this process, at least the following steps are performed:

1. (A) Measuring variations in a parameter dependent on the magnetron anodic current. This parameter may, for example, be the anode current itself or another parameter dependent on the anode current.
2. (B) controlling the heating current generator depending on the fluctuations such that the heating current is increased with increasing fluctuations. In other words, with an increase in the fluctuations, for example over a fluctuation threshold, the heating current is increased.

The invention is based on the finding that fluctuations in the anode current are an early indicator that the cathode is too cold. The characteristics required make it possible to take this circumstance into account. In particular, the heating power can thus be increased until the fluctuations decline. As a result, the cathode can always be heated with the power just required for stable operation. This prolongs the life of the magnetron. With aging of the magnetron, the heating current is increased automatically and as needed. Tolerances of the device and in particular the parameters of the magnetron are automatically compensated, as well as fluctuations in the mains voltage.

The invention also makes it possible to operate the magnetron in most cases at lower heat output than in a conventional operation, so that the efficiency of the device is increased.

Advantageously, a power regulator is provided, which is designed so that the power absorbed by the cathode heater power can be controlled to a desired value. The control is designed in this case to specify the desired value for the heating power depending on the fluctuations.

In a further advantageous embodiment, the high voltage generator has an inverter and a high voltage transformer. The inverter feeds current pulses into the primary winding of the high voltage transformer. The secondary winding of the high-voltage transformer generates the voltage between the anode and the cathode of the magnetron via a rectifier. The mentioned measuring circuit is designed to measure fluctuations in the current pulses through the primary winding. The heights of the current pulses as well as their slew rates are directly related to the anode current and, since they can be easily measured on the primary side, form a very suitable measurement parameter for the purpose described here.

In this case, the measuring circuit is preferably designed to measure rising rates of the current pulses and to determine fluctuations in the rising rates. This is based on the finding that for a practical operation, the current pulses compared to inductance of the high voltage transformer are so short that the current does not reach its maximum value, but that the rate of increase of the current at the beginning of the pulse is a measure of this maximum value and thus also for the anode current is.

Brief description of the drawings

• Fig. 1 einen Schnitt durch die im vorliegenden Zusammenhang wichtigsten Teile eines Mikrowellenofens,
• Fig. 2 ein vereinfachtes Schaltungsdiagramm des Mikrowellenofens,
• Fig. 3 ein Diagramm einiger Signale der Ansteuerschaltung für die Kathodenheizung,
• Fig. 4 ein Diagramm einiger Signale der Ansteuerschaltung für den Hochspannungsgenerator,
• Fig. 5 eine Detailansicht des Verlaufs des Spannungsabfalls Ur und
• Fig. 6 die Anstiegsgeschwindigkeit der Strompulse im stabilen Betrieb (a) und im instabilen Betrieb (b).

Further embodiments, advantages and applications of the invention will become apparent from the dependent claims and from the following description with reference to FIGS. Showing:

• Fig. 1 a section through the most important parts of a microwave oven in the present context,
• Fig. 2 a simplified circuit diagram of the microwave oven,
• Fig. 3 a diagram of some signals of the drive circuit for the cathode heating,
• Fig. 4 a diagram of some signals of the drive circuit for the high voltage generator,
• Fig. 5 a detailed view of the course of the voltage drop Ur and
• Fig. 6 the slew rate of the current pulses in stable operation (a) and in unstable operation (b).

Ways to carry out the invention definitions:

In the present context, high voltage is understood to mean a voltage which is required as anode-cathode voltage for operation of the magnetron. In practice, this voltage is in most cases at least 1 kV, usually several kilovolts.

A push-pull output stage is a series connection of two electronic components, which can be alternately switched continuously, so that at the center tap of the two components, a time-varying voltage.

A half-bridge circuit is a circuit with exactly one push-pull final stage.

A full-bridge circuit (H-circuit, H-bridge) is a circuit with two push-pull output stages connected in parallel, with the load between the center taps of the two push-pull final stages.

Basic structure:

The invention relates to a microwave oven, as exemplified in Fig. 1 is shown. The microwave oven has a cooking chamber 1 for receiving the food to be heated, which can be closed to the user by a user door 2. In the device, a magnetron 3 is also arranged, which is connected via a Holleiter 4 with the cooking chamber 1 in combination. A controller 5 controls the function of the device.

Fig. 2 shows the most important components of the controller 5 in the present context.

The mains voltage of e.g. 230 volts at 50 Hz is rectified in a rectifier 10. The first intermediate voltage Uz thus produced is then slightly filtered via a first capacitor C1, the capacitor C1, however, being dimensioned so that, under load, the value of the first intermediate voltage Uz varies by at least 50% with twice the mains frequency. The intermediate voltage Uz is also tapped via a diode D1 and further filtered via a second capacitor C2 to form a second intermediate voltage Uz '.

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

The operation of the high voltage generator 11 and the Heizstromgenerators 12 is controlled by a control unit 13, for example in the form of a microprocessor.

An analog-to-digital converter of the control unit 13 is supplied via a voltage divider R5, R6 a proportional to the intermediate voltage Uz value so that it can determine the intermediate voltage Uz.

High voltage generator:

The high voltage generator 11 comprises a full bridge circuit with four electronic switching elements T3 - T6, in particular in the form of IGBT transistors, each with a freewheeling diode. The switching elements T3 - T6 are arranged in a known manner in two branches T3 and T4 or T5 and T6, wherein the switching elements of each branch are respectively arranged in series between the first intermediate voltage Uz and ground. Between the switching elements of each branch, a center tap is provided in each case, wherein the two center taps are connected to the two terminals of the primary winding of a high-voltage transformer 14. Thus, the switching elements T3 - T6 form an inverter, which feeds an alternating voltage into the primary winding of the high-voltage transformer.

The high voltage transformer 14 has a secondary winding with a much higher number of turns 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 thus generated is applied between the cathode K and the anode A of the magnetron 3.

For driving the switching elements T3 - T6, a drive circuit 16 is provided, which is controlled by the control unit 13. The drive circuit 16 generates the control voltages (gate or base voltages) UG3 - UG6 for the switching elements T3 - T6. The control unit 13 is designed to switch the two branches of the full bridge circuit T3 - T6 alternately. The control is done so that during a switching cycle, the primary winding of high voltage transformer 14 is not permanently between the first intermediate voltage Uz and ground, but that the primary winding is decoupled during a time to be selected by the control unit 13 from the intermediate voltage Uz, ie the circuit is with Pulse width modulation clocked so that the value of the high voltage Uh can be controlled.

To monitor the high voltage Uh this can be divided by a voltage divider R10 - R13 and R14 and fed to an optocoupler 17 whose output signal is forwarded to the control unit 13. For example, a lack or non-ignition of the magnetron can be detected in this way.

Furthermore, a resistor R20 is provided between the two branches T3, T4 or T5, T6 and a fixed reference potential, in particular ground. The initial increase in the voltage drop Ur across this resistor at the beginning of a current pulse is a measure of the anode current of the magnetron 3 and is supplied to the control unit 13 via an amplifier 18 for measurement purposes. This will be described in detail below.

heating current:

The Heizstromgenerator 12 is formed in the present embodiment of a half-bridge with two operated as push-pull final stage switching elements T1 and T2. The switching elements T1 and T2, which in turn are e.g. can be configured as IGBT transistors and which are each equipped with a freewheeling diode, are arranged in series between the second intermediate voltage Uz 'and ground.

The center tap between the two switching elements T1, T2 is connected to one terminal of the primary winding of a heating transformer 15. The second connection 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 connected in series between the second intermediate voltage Uz 'and ground.

The diode D1 prevents current from being discharged from the capacitors C5, C6 when the high voltage generator 11 connected to the intermediate voltage Uz draws current.

The secondary winding of the heating transformer 15 is connected to the cathode heater, i. connected to the filament, the magnetron 3 and supplies them with electricity.

For driving the switching elements T1 and T2, a drive circuit 20 is provided, which is controlled by the control unit 13. The drive circuit 20 generates the control voltages (gate or base voltages) UG1, UG2 for the switching elements T1 and T2. The type of control will be described in detail below.

Between the push-pull output stage, formed by the switching elements T1, T2, and the ground (or another fixed reference potential), a resistor R21 is arranged, through which the current from the push-pull output stage T1, T2 through the heating transformer to ground (or. the reference potential). The voltage drop across this resistor is a measure of the current flowing from the second intermediate voltage Uz 'through the primary coil of the high voltage transformer 15 to ground (or reference potential). It is tapped by an amplifier 21 and fed to an analog-to-digital converter of the control unit 13.

Control of the heating current generator:

The following is based on Fig. 3 describes how the control unit 13 controls the switching elements of the heating current generator 12. The figure shows the course of the voltages UG1 and UG2, which are applied to the control inputs of the switching elements T1 and T2, as well as the course of the voltage Uih, which drops across the resistor R21.

The control unit 13 is designed to switch the two switching elements T1 and T2 cyclically alternately. A typical cycle period Tz is advantageously in the range of 10 - 50 μs.

The periods in which one of the switching elements T1 or T2 is turned on are referred to below as heating phases H1 and H2, respectively, and are shown in FIG Fig. 3 drawn, wherein in the heating phase H1, the first switching element T1 and H2 in the heating phase, the second switching element T2 is turned on. Between the heating phases H1 and H2 or H2 and H1 both switching elements T1, T2 are turned off. The phases in which both switching elements T1 and T2 are turned off are referred to as resting phases R1 and R2 and are in Fig. 3 also marked. The heating phases have a duration th, the rest periods a duration tr.

The time th can be selected identically for both switching elements T1 and T2 in a simple embodiment, as well tr.

In this way, an alternating current is generated in the primary winding of the heating transformer 15, which is supplied (except for losses in the components, in particular in the heating transformer 15) as heating power of the cathode heater of the magnetron 3. The average magnitude of the heating power is a function of the duty cycle, i. of the quotient th / Tz.

How out Fig. 3 can be seen, after switching on one of the switching elements T1, T2, the current through the primary winding of the heating transformer 15 and thus the voltage drop Uih via resistor R21 and can be measured via the amplifier 21 from the control unit 13.

The voltage drop Uih forms a parameter that depends on the resistance of the cathode heater of the magnetron 3. Assuming that no losses in the Heating transformer 15 occur, Uih is opposite to the end of the heating pulse inversely proportional to the resistance of the cathode heater. Thus, resistor R21 together with amplifier 21 form a measuring circuit which is designed to determine a parameter dependent on the resistance of the cathode heating.

In Fig. 3 is a time tm plotted, to which the controller 13 measures the voltage drop Uih. This time tm is preferably just before the end tx of the respective heating phase H1 or H2, for example at most 1 μs before the end tx of the heating phase. Advantageously, a measurement takes place in each heating phase.

The control unit 13 is designed 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 heater.

For the value of the intermediate voltage Uz 'approximately the value of the intermediate voltage Uz can be used, as it is determined by the control unit via the voltage divider R5, R6. As long as (in the preheating phase) the high-voltage generator 11 is not in operation, Uz 'corresponds to the value of Uz except for the voltage drop across D1. Although Uz 'is sometimes somewhat larger than Uz, the difference remains small if the components are dimensioned appropriately. If Uz 'is to be determined exactly, in addition or as an alternative to R5, R6, a second voltage divider may be provided, which supplies the second intermediate voltage Uz' to the measurement of the control unit 13.

Preferably, P is averaged over a filter time which is at least half a clock period of the mains voltage, ie at least 10 ms. An adaptation of the pulse width th occurs only after the filter time has expired.

P is a direct measure of the power that the push-pull output stage T1, T2 delivers, and thus (neglecting the power losses, especially in the heating transformer 15) and a measure of the heating power of the cathode heater of the magnetron 3. Thus, the control unit 13 forms a power regulator, with which the power absorbed by the cathode heater power can be controlled to a desired value.

Control of the high voltage generator:

The following is based on Fig. 4 describes how the control unit 13 controls the switching elements T3 - T6 of the high voltage generator 11. The figure shows the course of the voltages UG3 - UG6, which are applied to the control inputs of the switching elements T3 - T6, and 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 control unit 13 is configured to cyclically operate the four switching elements T3-T6. A typical cycle period tc is advantageously in the range of 10 - 50 μs.

• In der Phase A sind die Schaltelemente T3 und T6 eingeschaltet und die Schaltelemente T4 und T5 ausgeschaltet, so dass sich ein positiver Storm Ip von der Zwischenspannung Uz durch die Brückenschaltung gegen Masse aufbaut. Dieser Strom führt zu einem ansteigenden Spannungsabfall Ur über R20. (Da die Induktivität des Hochspannungstransformators 14 wesentlich höher ist als jene des Heiztransformators 15, geht der Strom im Gegensatz zur Situation nach Fig. 3 nicht in Sättigung sondern steigt über die Phase A praktisch linear an.)
• In der Phase B bleibt Schaltelement T6 eingeschaltet. Schaltelement T3 wird ausgeschaltet und sodann Schaltelement T4 eingeschaltet. Der Strom durch den Hochspannungstransformator 14 baut sich wieder ab, indem er das Schaltelement T6 und die Freilaufdiode des Schaltelements T4 durchfliesst.

• In der Phase C wird das Schaltelement T6 abgeschaltet und das Schaltelement T5 eingeschaltet. Es baut sich nun ein negativer Strom Ip von der Zwischenspannung Uz durch die Brückenschaltung und die Primärwicklung gegen Masse auf. Dieser Strom führt wieder zu einem ansteigenden Spannungsabfall Ur über R20.
• In der Phase D bleibt Schaltelement T4 eingeschaltet. Schaltelement T5 wird ausgeschaltet und sodann Schaltelement T6 eingeschaltet. Der Strom durch den Hochspannungstransformator 14 baut sich wieder ab, indem er das Schaltelement T4 und die Freilaufdiode des Schaltelements T6 durchfliesst.

Each cycle period comprises four phases A - D:

• In phase A, the switching elements T3 and T6 are turned on and the switching elements T4 and T5 are turned off, so that a positive Storm Ip of the intermediate voltage Uz builds up by the bridge circuit to ground. This current leads to an increasing voltage drop Ur via R20. (Since the inductance of the high voltage transformer 14 is much higher than that of the heating transformer 15, the current goes against the situation Fig. 3 not in saturation but increases almost linearly over phase A.)
• In phase B, switching element T6 remains switched on. Switching element T3 is turned off and then switching element T4 is turned on. The current through the high voltage transformer 14 degrades again by he the switching element T6 and the freewheeling diode of the switching element T4 flows through.

• In phase C, the switching element T6 is turned off and the switching element T5 is turned on. Now, a negative current Ip builds up from the intermediate voltage Uz through the bridge circuit and the primary winding to ground. This current again leads to an increasing voltage drop Ur via R20.
• In phase D switching element T4 remains switched on. Switching element T5 is turned off and then switching element T6 is turned on. The current through the high voltage transformer 14 degrades again by flowing through the switching element T4 and the freewheeling diode of the switching element T6.

In operation, phases A and C are preferably the same length, i. the corresponding time periods tA and tC are identical. Similarly, phases B and D are preferably the same length, i. the corresponding time periods tB and tD are identical. The phases A and C are, however, usually shorter or at most the same length as the phases B and D. By the ratio of tA + tC to the cycle time tc, the power to be delivered by the magnetron can be adjusted. This ratio is determined by the controller 13 e.g. set according to the user's specifications.

Business:

When the user activates the microwave oven, ie has given the command to supply energy to the food in the oven, the controller 13 first starts a preheating phase. In this preheating phase, the switching elements T3 - T6 all remain switched off, so that no high voltage is applied to the magnetron 3. The preheating phase is then followed by an operating phase in which the switching elements T3-T6 are also put into operation alternately in order to apply the high voltage to the magnetron and to supply the desired microwave radiation produce. The operating phase is described in more detail below.

As mentioned above, the power of the cathode heater is kept as low as possible in the operating phase, and so deep that just a stable operation of the magnetron 3 is possible.

In order to recognize the limit of this stable operation, as already mentioned above, fluctuations in a parameter dependent on the anode current are investigated. In the embodiment described so far, this parameter is the rate of rise of the current pulses through the primary coil of the high voltage transformer 14. For this, the increase of the voltage drop Ur across R20 is measured.

Fig. 5 shows the course of the voltage drop Ur in detail. The control unit 13 performs two measurements per current pulse at the times t1 and t2 and calculates therefrom the corresponding difference Δ = Ur (t2) -Ur (t1), which is a measure of the rate of increase of the current pulse. Fig. 5 shows two corresponding measurements Δ _i and Δ _{i + 1} , where i and i + 1 represent the indices of two successive current pulses.

Instead of measuring two values at the times t1 and t2, only one value can be measured at a known time interval from the pulse start. However, the measurement of two values has the advantage that the pulse shape can be determined more accurately, which in particular allows a more reliable detection of false states. Thus, e.g. detected at the time t1 negative currents or voltages, if there is a short circuit at the output of the high voltage transformer 14.

1. 1) Eine erste Ursache liegt darin, dass die Zwischenspannung Uz nur unvollständig geglättet ist. Sie variiert mit einer Frequenz, welche dem Doppelten der Netzfrequenz entspricht, also mit ca. 100 Hz. Je höher die Zwischenspannung Uz ist, desto grösser ist die Anstiegsgeschwindigkeit der Strompulse. Die mit Uz skalierte Anstiegsgeschwindigkeit Δ/Uz ist jedoch deutlich weniger abhängig von der Zwischenspannung Uz. Dies ist in Fig. 6a illustriert, welche die Zwischenspannung Uz, den Wert der Anstiegsgeschwindigkeit Δ_i und der skalierten Anstiegsgeschwindigkeit Δ_i/Uz darstellt.
2. 2) Eine zweite Ursache liegt in der Instabilität des Magnetrons 3. Wenn die Heizleistung zu tief ist, beginnen die Anstiegsgeschwindigkeiten zu schwanken, vgl. Fig. 6b. Die Schwankungen sind auch in der skalierten Anstiegsgeschwindigkeit Δ/Uz zu beobachten.

Fluctuations in slew rates, as in Fig. 5 dashed lines can have two causes:

1. 1) A first cause is that the intermediate voltage Uz is only incompletely smoothed. It varies with a frequency which is twice the 100 Hz. The higher the intermediate voltage Uz, the greater the rate of increase of the current pulses. However, the slew rate Δ / Uz scaled by Uz is significantly less dependent on the intermediate voltage Uz. This is in Fig. 6a illustrating the intermediate voltage Uz, the value of the slew rate Δ _i and the scaled slew rate Δ _i / Uz.
2. 2) A second cause lies in the instability of the magnetron 3. If the heating power is too low, the rate of increase starts to fluctuate, cf. Fig. 6b , The fluctuations can also be observed in the scaled slew rate Δ / Uz.

Once the control unit 13 fluctuations in Fig. 6b As shown, it increases the setpoint for the heating power of the cathode heater.

Hierbei bezeichnet Uz_i die Zwischenspannung Uz zum Zeitpunkt des Pulses i. Die Summe erstreckt vorzugsweise über mindestens eine halbe Netzperiode, d.h. 10 ms. Liegt der Wert S über einem oberen Schwellwert S1, so erhöht die Steuereinheit 13 den Sollwert für die Heizleistung. Liegt der Wert S über einem tieferen, unteren Schwellwert S2, so reduziert die Steuereinheit 13 den Sollwert für die Heizleistung.In order to numerically detect the fluctuations, the control unit in the present embodiment calculates the following variable S for the fluctuations: $$\mathrm{S}={\mathrm{\Sigma }}_i\left|\frac{{\mathrm{\Delta }}_{i+1}}{{\mathit{Uz}}_{i+1}}-\frac{{\mathrm{\Delta }}_i}{{\mathit{Uz}}_i}\right|\mathrm{,}$$

Here, Uz _i denotes the intermediate voltage Uz at the time of the pulse i. The sum preferably extends over at least half a network period, ie 10 ms. If the value S is above an upper threshold value S1, then the control unit 13 increases the setpoint value for the heating power. If the value S is above a lower, lower threshold value S2, the control unit 13 reduces the setpoint value for the heating power.

wobei F die genannte Funktion ist, und N die Zahl der in der Funktion F berücksichtigten Paare von Strompulsen. F kann z.B. die Summe der Quadrate der Differenzen der skalierten Stromanstiegsgeschwindigkeiten sein.Instead of the formula (1), which determines the variations from the sum of the absolute values of the differences of the scaled current rise velocities, another function may be used to determine the fluctuations, for example, which of the absolute Differences in the scaled current slew rates of several pairs of successive current pulses i, i + 1 depends $$\mathrm{S}=F\left({\left|\frac{{\mathrm{\Delta }}_{i+1}}{{\mathit{Uz}}_{i+1}}-\frac{{\mathrm{\Delta }}_i}{{\mathit{Uz}}_i}\right|}_{i=\mathrm{1..}N}\right).$$

where F is said function, and N is the number of pairs of current pulses considered in function F. For example, F may be the sum of the squares of the differences in the scaled current slew rates.

It turns out that at the beginning of an instability, the first fluctuations between each successive current pulses become visible, as the current pulse increases faster and the next current pulse rises more slowly, as in Fig. 6b is illustrated. Therefore, a function of the kind of equation (1) or (2) is a particularly good indicator of the onset of instabilities.

However, as an alternative to a formula of the type shown in equation (2), another variable describing the variation in scaled current slew rates or unscaled current slew rates may be used. For example, the non-scaled current slew rates can be filtered at a high pass, and then their statistical variance calculated. The high pass has a cutoff frequency which is higher than twice the line frequency but lower than the switching frequency of the inverter.

Remarks :

In the above embodiment, the voltage drop Ur across R20 is used as a parameter for the fluctuations of the anode current of the magnetron 3. Alternatively, however, it is also possible, for example, to use a different value which determines the current in the primary or secondary circuit of the High voltage transformer 14 describes. For example, a measuring winding can be integrated in the high-voltage transformer 14, the voltage of which is monitored. Or the anode current can also be measured directly and transmitted to the control unit 13 via an optocoupler, for example.

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

In summary, therefore, a control circuit for a microwave oven is described. This has a push-pull output stage T1, T2 for driving a heating transformer 15, with which the cathode heater of the magnetron 3 is operated. For the generation of the high voltage, a separate high-voltage transformer 14 is provided, which is fed by a bridge circuit T3 - T6. The control unit 13 of the device is designed to determine fluctuations in a parameter dependent on the anode current of the magnetron 3. If these fluctuations are high, the heating power of the cathode heating is increased. In this way, the magnetron 3 can be operated with an optimal, low heating power.

While preferred embodiments of the invention are described in the present application, it is to be understood that the invention is not limited thereto and may be embodied otherwise within the scope of the following claims.

Claims (14)

Microwave oven with a magnetron (3) comprising a cathode (K), an anode (A) and a cathode heater and with a drive circuit for the magnetron (3), the drive circuit comprising: a high voltage generator (11) for generating a high voltage between the anode (A) and the cathode (K), a Heizstromgenerator (12) for generating a heating current for the cathode heating and a controller (13), characterized by a measuring circuit (R20, 18) adapted to determine variations in a parameter dependent on an anode current of the magnetron, the controller (13) being arranged to control the heating current generator (12) in dependence on the variations in that the heating current is increased with increasing fluctuations. Microwave oven according to claim 1 with a power regulator (13, 21, R21), with which a heating power absorbed by the cathode heater can be regulated, wherein the controller (13) is configured to specify a desired value for the heating power depending on the fluctuations. A microwave oven as claimed in any one of the preceding claims, wherein the high voltage generator (11) comprises an inverter (T3-T6) and a high voltage transformer (14), the inverter (T3-T6) feeding current pulses into a primary winding of the high voltage transformer (14), a secondary winding of the high-voltage transformer via a rectifier (D2, D3) generates a voltage across the anode (A) and the cathode (K) of the magnetron (3), and wherein the measuring circuit (R20, 18) is designed for measuring fluctuations in the current pulses. A microwave oven according to claim 3, wherein the inverter is a bridge circuit having four switching elements (T3-T6). Microwave oven according to one of claims 3 or 4, wherein between the inverter and a reference potential, in particular ground, a resistor (R20) is arranged, wherein the measuring circuit (R20, 18) for measuring a voltage drop across the resistor (R20) is configured. A microwave oven according to any one of claims 3 to 5, wherein the measuring circuit (R20, 18) is adapted to measure current pulse slew rates and to detect variations in slew rates. A microwave oven according to claim 6, wherein the measuring circuit (R20, 18) is adapted to at least two times (t1, t2) to be measured in each current pulse i is a current level and from this to determine the rate of rise Δ _i of the current in the current pulse. A microwave oven according to claim 7, wherein the controller (13) is configured to
calculate a scaled slew rate Δ _i / Uz _i , where Uz _{i is} an intermediate voltage present across the inverter (T3 - T6) at the time of the pulse i, and
determine the variations from the scaled slew rate. A microwave oven according to claim 8, wherein the controller (13) is adapted to calculate a function which depends on the absolute differences of the scaled current slew rates of a plurality of pairs of successive current pulses i, i + 1. A microwave oven as claimed in any one of the preceding claims, wherein the controller (13) is adapted to
calculate a characteristic value (S) for the fluctuations,
to increase the heating current when the characteristic value (S) rises above an upper threshold value (S1). An apparatus according to claim 10, wherein the controller (13) is further configured to reduce the heating current when the characteristic value (S) falls below a lower threshold value (S2). Method for operating a microwave oven according to one of the preceding claims, the method being characterized by the following steps: Measuring variations in a parameter dependent on an anode current of the magnetron (3) and Controlling the heating current generator (12) depending on the fluctuations such that the heating current is increased with increasing fluctuations. A method according to claim 12 wherein for generating the anode current a high voltage transformer (14) is connected to the magnetron via a rectifier (D2, D3), current pulses being applied to a primary winding of the high voltage transformer (14) and the fluctuations being fluctuations in the current pulses be measured. A method according to claim 13, wherein rise rates of the current pulses are measured and the variations are measured as variations in slew rates.

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