JP3197749B2 - Magnetron drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter type magnetron drive circuit used for, for example, a microwave oven.

[0002]

2. Description of the Related Art Conventionally, input or output control of an inverter type magnetron driving circuit used in a microwave oven or the like is performed so that an input or output detection signal matches an input or output setting signal which is an output signal from a microcomputer. In this method, a feedback circuit composed of an analog circuit operates. FIG. 11 shows a conventional magnetron drive circuit of this type. In the figure, 1 is a commercial power supply,
After the AC voltage from the commercial power supply 1 is rectified by the rectifier 2,
The DC voltage is obtained by smoothing with the choke coil 3 and the capacitor 4. Reference numeral 5 denotes a switching element made of an IGBT. The switching element 5 is connected to a resonance capacitor 6 in series, and a freewheeling diode 7 is connected in parallel between its collector and emitter. In some cases, the resonance capacitor 6 is connected to the switching element 5 in parallel, similarly to the freewheeling diode 7. Reference numeral 8 denotes a high-frequency transformer, which includes a primary winding, a secondary winding, and a filament winding.
A DC voltage is supplied to the collector of the switching element 5 via the primary winding of the high-frequency transformer 8. Driver 1
The switching element 5 is turned on / off by the drive signal from the DC / DC converter 4, and the DC voltage is periodically switched and converted to a high frequency. This high frequency is supplied to the primary winding of the high frequency transformer 8. Further, a voltage doubler rectifier circuit composed of a voltage doubler capacitor 9 and diodes 10 and 11 is connected to the secondary winding. With this voltage doubling rectifier circuit, the high frequency high voltage generated in the secondary winding of the high frequency transformer 8 is voltage double rectified to obtain a DC high voltage for driving the magnetron 12.

[0003] A control system for controlling the power supplied to the magnetron 12 to a required set value is configured as follows. An output setting signal from the microcomputer 15 is sent as a digital signal to the output control circuit 16 via signal lines A, B, and C via photocouplers a, b, and c, respectively. The magnetron current is detected by 13 (referred to as T) and is taken into the output control circuit 16. The signal lines A, B, and C are signal lines for a data signal, a clock signal synchronized with the data signal, and a data signal end signal, respectively.
Lines are required. Further, in order to input a digital signal to the output control circuit 16 which is an analog feedback circuit,
A D / A converter 17 is provided at a stage preceding the output control circuit 16. Since the microcomputer 15 drives a panel switch and a display, the inverter circuit for generating a high voltage for driving the magnetron and the power supply of the microcomputer 15 need to be separated for safety. Therefore, to insulate the microcomputer 15 from the inverter circuit, Photocouplers a, b,
c is needed. The microcomputer 15 is mounted on the back of the panel to drive the panel switches and the display as described above, is separated from the inverter circuit, and requires four signal lines including the ground line, which is very costly. are doing. In this method, since an error in the variation of the output setting signal from the microcomputer 15 directly causes a variation in the output of the inverter, the output setting signal needs to be transmitted to the output control circuit 16 as an accurate digital signal. 1 signal line
A PWM signal transmission method, an FM signal transmission method, or the like can be considered as a method of omitting the arrangement of the D / A converter by adopting a linear method. Output fluctuation is large and not practical. Further, in this method, when an abnormality such as an excessive rise in the temperature of the switching element (IGBT) 5 or a voltage drop in the switching element drive circuit 18 occurs, the microcomputer 15 cannot directly detect the abnormality. Therefore, a signal line D for detecting an abnormality from the abnormality detection circuit 19 is connected to the microcomputer 15 via the photocoupler d.

[0004]

In the conventional magnetron drive circuit, the microcomputer and the output control circuit of the inverter are separated from each other and are electrically insulated. Therefore, in the conventional method, many signal lines and photocouplers for transmitting an output setting signal and the like from the microcomputer to the output control circuit are required, resulting in an increase in cost. In addition, since there are many signal lines, reliability is likely to be reduced due to disconnection of the signal line portion or poor contact of the connector.

On the other hand, in order to reduce the cost and improve the reliability, the input / output current of the inverter circuit is detected by the A / D converter of the microcomputer, and the switching element is turned on so that the inverter circuit becomes the setting input. A magnetron driving circuit of a type in which time is calculated and transmitted to a switching element driving unit by a PWM signal or an FM signal has been considered. That is, the input / output control of the inverter circuit is controlled by the microcomputer which directly insulates the output current of the inverter circuit from the C.I. Since it is detected by T and the on-time of the switching element is calculated, there is no problem between the microcomputer and the switching element drive circuit due to element variation and the like. Therefore, since a one-wire system such as a PWM system or an FM system is sufficient, only two signal lines including a ground line are required, and the reliability is improved at low cost. Further, data transmitted by the PWM method or the FM method can be converted into a DC voltage by using a resistor and a capacitor, so that a D / A converter is not required and the cost is reduced. Furthermore, since the microcomputer monitors the output current of the inverter circuit, abnormalities on the inverter side are known, and it is not necessary to transmit an abnormal signal from the inverter side to the microcomputer side using a photocoupler or the like. In addition, the number of signal lines and photocouplers can be reduced, and the cost can be reduced.

However, the magnetron drive circuit of this type has the following problems. That is, before the magnetron oscillates, it is necessary to perform a warm-run operation of applying a voltage between the anode and the cathode of the magnetron (secondary side of the high-frequency transformer) and a heater. At this time, almost no current flows between the cathode and the anode. The time required for the magnetron to start oscillating becomes shorter as the secondary voltage of the high-frequency transformer during warm running is higher. Therefore, the higher the secondary voltage is, the better the food preparation time is to be shortened, and the primary side switching element collector voltage proportional to this secondary voltage needs to be 560 V or more. On the other hand, since the commercial power supply fluctuates by about 90 V to 110 V, the ON time of the switching element requires 18 μsec to satisfy the collector voltage of 560 V at the lowest 90 V. However, as shown by the characteristic line a in FIG. 9, the secondary voltage increases in proportion to the commercial voltage, and when the power supply voltage increases, the secondary voltage becomes 8 kV or more, exceeding the rated 8 kV of the magnetron, and the magnetron is destroyed. I do. Therefore,
It is necessary to perform feedback by the collector voltage and to limit the collector voltage so that the switching-on time becomes shorter than 18 μsec. The b-characteristic line in FIG. 9 shows the case where the collector voltage is limited. Even if the power supply voltage fluctuates, the collector voltage becomes 575 V or more and 575 V at the maximum, so that the secondary voltage of the high-frequency transformer becomes 6.55 kV or less. . Therefore, the time until oscillation of the magnetron is shortened, so that the food can be prepared quickly, and the magnetron does not break down. Thereafter, when the magnetron oscillates, the current between the cathode and the anode shifts to normal output control. However, since this method is a method for calculating the ON time of the switching element, it does not immediately enter the output control state at the start of oscillation. That is, in order to increase the output, it is necessary to remove the feedback at the collector voltage. At this time, as shown in FIG.
At V, despite the maximum rated input of 1.2kW
It rises to 1.8 kW, destroying the switching element. In other words, when the power supply voltage is 110 V, the collector voltage is set to 580 V, so that the switching on time is 13 μsec due to the collector voltage feedback. However, the setting signal of PWM etc. from the microcomputer etc. is 18
Because of the μsec, the input becomes 1.8 kW when the collector voltage feedback is removed, and then the on-time corresponding to the output set value is output from the microcomputer. When the power supply voltage is 110 V and the input is 1.2 kW, the switching on time is 14 μsec. In addition, power supply voltage 100V
In the case of, there is a problem that the input similarly rises to 1.5 kW.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetron drive circuit capable of preventing a switching element from being destroyed in a low-cost and highly-reliable on-time operation method.

[0008]

SUMMARY OF THE INVENTION In order to solve the above problems, the present invention firstly converts a DC voltage obtained by rectifying an AC voltage from a commercial power supply into a high frequency by periodically switching the DC voltage by a switching element. An inverter circuit that drives the magnetron with the rectified DC output after boosting the high frequency, a current transformer that detects the input or output current of the inverter circuit, and an input of the inverter circuit based on the detected value of the current transformer. Or, in a magnetron drive circuit having a calculation unit that calculates the ON time of the switching element so that the output current becomes a required set value, the maximum voltage applied to the switching element during a warm run of the magnetron is a predetermined first time. 1 and a maximum value applied to the switching element after the magnetron oscillates. A control means for operating the switching element with an on-time output from the arithmetic unit in a limited state in which the voltage is switched to a second voltage value set corresponding to the maximum fluctuation value of the AC voltage. And

Second, an inverter that converts a DC voltage obtained by rectifying and converting an AC voltage from a commercial power supply into a high frequency by periodically switching with a switching element, boosts this high frequency, and drives a magnetron with the rectified DC output. Circuit, a current transformer for detecting an input or output current of the inverter circuit, and an on-time of the switching element such that an input or output current of the inverter circuit becomes a required set value based on a detected value of the current transformer. In the magnetron drive circuit having a calculation unit that calculates the maximum voltage applied to the switching element during a warm run of the magnetron to a predetermined voltage value, after the magnetron oscillates, the ON time of the switching element. After decreasing to a value set corresponding to the maximum fluctuation value of the AC voltage, And summarized in that a control means for operating said switching element in ON time output from the serial arithmetic unit.

Third, an inverter that drives a magnetron with a rectified DC output after converting a DC voltage obtained by rectifying and converting an AC voltage from a commercial power supply into a high frequency by periodically switching with a switching element, and boosting the high frequency. Circuit, a current transformer for detecting an input or output current of the inverter circuit, and an on-time of the switching element such that an input or output current of the inverter circuit becomes a required set value based on a detected value of the current transformer. In the magnetron drive circuit having a calculation unit for calculating the AC voltage value, the AC voltage value from the commercial power supply is detected, and the switching element is operated in an on-time corresponding to the AC voltage value to perform the warm run of the magnetron. After the oscillation of the magnetron, the switching element is turned on by the on-time output from the arithmetic unit. And summarized in that a control means for operating.

Fourthly, in the above-mentioned third configuration, the control means may change a maximum voltage applied to the switching element after the magnetron oscillates to a voltage value set in accordance with a maximum fluctuation value of the AC voltage. The gist is that the switching element is operated in the on-time output from the arithmetic unit in the state limited to the above.

[0012]

In the above configuration, first, the time until the start of oscillation of the magnetron can be shortened by limiting the maximum voltage applied to the switching element during the warm run to a predetermined first voltage value. Becomes When the magnetron starts to oscillate, it does not immediately enter the control state in which the switching element is operated with the on-time calculated by the calculation unit. Depending on the value fluctuation, the input to the inverter circuit may become excessive and the switching element may be destroyed. However, after the start of the oscillation of the magnetron, the maximum voltage applied to the switching element is changed to the second voltage set in accordance with the maximum fluctuation value of the AC voltage.
By switching to the above voltage value, even if the AC voltage fluctuates to the maximum value, the input becomes equal to or less than the predetermined value, and the destruction of the switching element is reliably prevented.

Second, after the start of oscillation of the magnetron, the on-time of the switching element is reduced to the on-time value set in accordance with the maximum fluctuation value of the AC voltage, so that the AC voltage fluctuates to the maximum value. Also, the input is suppressed to a predetermined value or less, so that destruction of the switching element is reliably prevented.

Third, during a warm run, an AC voltage value from a commercial power supply is detected, and the switching element is operated with an ON time corresponding to the AC voltage value, so that the AC voltage fluctuates to a maximum value. The input is suppressed to a predetermined value or less, and the destruction of the switching element is prevented.

Fourth, in the third control operation, the maximum voltage applied to the switching element after the oscillation of the magnetron is further limited to a voltage value set in accordance with the maximum fluctuation value of the AC voltage. For example, even if the AC voltage is 90 V during a warm run and the AC voltage fluctuates to 110 V after the magnetron oscillates, the input becomes less than a predetermined value and the switching element is more reliably prevented from being broken.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2, FIG.
The same or equivalent components as those described in the above are denoted by the same reference numerals as those described above, and redundant description is omitted.

First, the basic configuration and operation of this embodiment will be described with reference to FIG. In the figure, reference numeral 20 denotes a switching element drive unit, and reference numeral 30 denotes a microcomputer as control means. Then, when the warm run of the magnetron 12 ends and a current starts flowing through the magnetron 12, the output current of the inverter circuit becomes C.I. The detected value is directly read by the microcomputer 30 at T13. The microcomputer 30
The set value of the inverter circuit output previously stored in the memory or the set value by the user's setting key operation and the C.I. The ON time of the switching element 5 is determined by performing a comparison operation with the T detection value. The microcomputer 30 transmits a signal corresponding to the switching element on-time to the switching element driving unit 20 via a photocoupler 40 via a one-wire signal line, and the switching element driving unit 20 outputs an on-time pulse corresponding to the on-time. To drive the switching element 5.

In the system of this embodiment, C.I. Because T13 is electrically insulated from the inverter circuit,
If the microcomputer 30 has a C.I. The inverter output current can be detected via T13. Accordingly, since the on-time control of the switching element 5, that is, the control of the inverter output can be performed based on the detection information, the signal from the microcomputer 30 to the switching element driving unit 20 may be an analog signal, and there is no problem due to an element variation or the like. .

Next, the configuration and operation of this embodiment will be described in more detail with reference to FIG. Inverter circuit is quasi-E
The switching element 5 is turned on when the signal (SVCE) corresponding to the voltage applied to the switching element 5 falls below the signal (SVAC) corresponding to the DC voltage (VDC). The circuit 22 generates an on-timing pulse. The ON time of the switching element 5 is determined as follows. The microcomputer 30 determines the ON time of the switching element 5 by comparing the A / D converter 31 that reads the detected value of the current transformer 13 via the rectifier 37 with the output of the A / D converter 31 and the set value. An arithmetic unit 32 and a PWM output unit 33 that outputs a PWM signal corresponding to the ON time calculated by the arithmetic unit 32 are provided. Reference numeral 34 denotes a setting key for setting a set value. The PWM signal corresponding to the ON time has, for example, a frequency of 5 kHz and an ON time of 0 μsec and 0%
DUTY, ON time: 10%, 50% DUTY,
The on-time is set to 100% DUTY in 20 μsec, and such a PWM signal is sent to the photocoupler 40. The PWM signal passed through the photocoupler 40 is a resistor R
₁ , R ₂ , R ₃ , and the capacitor C ₁ are converted to a DC voltage. The switching element drive unit 20 converts the DC voltage into an on-time pulse by comparing the DC voltage with the sawtooth wave generated by the sawtooth wave generation circuit 23 by the first comparator 24, thereby generating a gate signal. The circuit 21 is driven by giving an on-time pulse to the switching element 5.

As described above, since the microcomputer 30 monitors the output current of the inverter circuit, it is possible to detect an abnormality in the output current of the inverter circuit. And C. If the inverter output current detected at T13 is abnormal, the duty of the PWM signal output from the PWM output unit 33 via the PWM setting unit 36 is set to 0% by the abnormality detection signal from the output current detection circuit 35, Drive unit 20
The on-time pulse output from the switching device 5 is stopped, and the driving of the switching element 5 is stopped.

Reference numeral 41 denotes a photocoupler, and reference numeral 25 in the drive unit 20 denotes a second comparator. The threshold voltage of the transistor 26 and the resistors R ₅ , R ₆ , and R ₇ are variable. The output terminal of the second comparator 25 is connected to the connection point of the resistors R ₂ and R ₃ via the transistor 42. Reference numeral 43 denotes a transformer for taking the power supply voltage signal of the commercial power supply 1 into the microcomputer 30.

Next, the control operation of the microcomputer 30 according to the claimed invention will be described in order.

First, the control operation according to the first aspect of the present invention will be described with reference to FIG. In the pre-oscillation warm-run state of the magnetron 12, the collector voltage of the switching element 5 is limited at a predetermined first voltage value of 580V. This is because the microcomputer 30 outputs a PWM signal corresponding to the on-time of the switching element 5 of 18 μsec, and the voltage is reduced by the second comparator 25 and the transistor 42 to reduce the DC voltage corresponding to the on-time. At this time, since the secondary current of the high-frequency transformer 8 is not flowing, a current is passed from the microcomputer 30 to the LED of the photocoupler 41 to turn on the transistor 26,
Threshold voltage corresponding to 80V is made of R ₅ and R ₆ ‖R _7. Next, when the microcomputer 30 detects that the magnetron 12 has oscillated and the current has started to flow, the flow of current to the LED of the photocoupler 41 is stopped, the transistor 26 is turned off, and the transistor 26 is turned off at a predetermined second voltage value. threshold voltage corresponding to a 750V is made by R ₅ and R _6. This situation is illustrated in FIG. That is, immediately after the warm run is completed, the input is 1.V at 110 VAC from the commercial power supply 1.
It is suppressed to 3 kW, and the switching element 5 can sufficiently withstand destruction.

The control operation according to the second aspect of the present invention will be described with reference to the flowcharts of FIGS. In the pre-oscillation warm-run state of the magnetron 12, the collector voltage limit of the switching element 5 is a predetermined voltage value 58.
It is applied at 0 V (step 51). This means that the on time of the switching element 5 is 18 μsec from the microcomputer 30.
, A PWM signal of 82% DUTY corresponding to the second comparator 2 is output.
5 and the transistor 42 reduce the DC voltage corresponding to the on-time. At this time, since the secondary current of the high-frequency transformer 8 is not flowing, by applying a current from the microcomputer 30 to the LED of the photo coupler 41 turns on the transistor 26, the threshold voltage corresponding to 580V is, R ₅ and R ₆
Made in ‖R _7. Next, when the microcomputer 30 detects that the magnetron 12 has oscillated and the current has started flowing, the PWM signal DUTY from the microcomputer 30 is lowered (steps 53 and 54), and the ON time of the switching element 5 of the IGBT is determined by the microcomputer 30. It is lowered until the detected secondary current decreases (step 55). Thereafter, the flow of the current to the LED of the photocoupler 41 is stopped, the transistor 26 is turned off, and the threshold voltage corresponding to, for example, 750 V becomes R ₅
And made of R ₆ (step 56). At this time, as shown in FIG.
In the range, the input / output control is started from 700 W by the on-time calculation method (step 57), so that the switching element 5 can sufficiently withstand destruction.

The control operation according to the third aspect of the present invention will be described with reference to the flowcharts of FIGS. The power supply voltage from the commercial power supply 1 is detected via the transformer 43 during the warm run before the magnetron 12 oscillates, and the switching on time is varied according to the power supply voltage value. That is, when the detected power supply voltage is 90 V, the ON time is 18 μsec.
c (82% DUTY) to perform a warm run (steps 61 and 62), and after the microcomputer 30 detects that the magnetron 12 oscillates and current starts flowing (step 6).
3), input / output control is performed by an on-time calculation method (step 70). ON time 15μ when power supply voltage is 100V
sec (68% DUTY) (step 65), 110V
In the case of (1), the warm-run is executed with the ON time of 13 μsec (59% DUTY) (step 67). In this control method, as shown in FIG.
At V, the input is suppressed to 1.1 kW, and the switching element 5 can sufficiently withstand breakdown.

The control operation according to the fourth aspect of the present invention will be described with reference to the flowcharts of FIGS. During a warm run, the power supply voltage of the commercial power supply 1 is, for example, 90 V, and after that, the magnetron 12 oscillates.
When the voltage becomes V, as shown in FIG. 7, the input rises to 1.8 kW, and the switching element 5 may be destroyed. Then, after the magnetron 12 oscillates, the collector voltage limit value is changed by applying feedback to, for example, 750 V (step 69), and the input / output control is executed by the on-time calculation method (step 70). This prevents the switching element 5 from being destroyed.

[0027]

As described above, according to the inventions described in the claims, the following effects can be obtained.

According to the first aspect of the present invention, the maximum voltage applied to the switching element during warm running of the magnetron is limited to a predetermined first voltage value, and after the magnetron oscillates, the maximum voltage is applied to the switching element. The second voltage set in accordance with the maximum fluctuation value of the AC voltage
Because the switching element is operated with the on-time output from the arithmetic unit in the limited state where the voltage value is switched to the voltage value, in the low-cost and highly-reliable on-time arithmetic method,
Even if the AC voltage fluctuates to the maximum value when the magnetron starts to oscillate, the input is suppressed to a predetermined value or less, and the switching element can be reliably prevented from being broken.

According to the second aspect of the present invention, the maximum voltage applied to the switching element during warm running of the magnetron is limited to a predetermined voltage value, and after the magnetron oscillates, the on-time of the switching element is reduced to an AC voltage. Since the switching element is operated during the on-time output from the arithmetic unit after the value is reduced to a value set corresponding to the maximum fluctuation value of the above, the same effect as the invention according to claim 1 is obtained. Can be

According to the third aspect of the present invention, an AC voltage value from a commercial power supply is detected, and a switching element is operated in an on-time corresponding to the AC voltage value to perform a warm run of the magnetron. After oscillating, the switching element is operated with the on-time output from the arithmetic unit, so that in the low-cost and highly-reliable on-time arithmetic method, even if the AC voltage fluctuates to the maximum value, the input is not changed. When the value is equal to or less than the predetermined value, destruction of the switching element can be reliably prevented.

According to a fourth aspect of the present invention, in the third aspect of the invention, the control means sets the maximum voltage applied to the switching element after the magnetron oscillates in accordance with the maximum fluctuation value of the AC voltage. The switching element is operated during the on-time output from the calculation unit in a state where the voltage is restricted to the voltage value, so that even if the AC voltage fluctuates to the maximum value after the magnetron oscillates, the input is suppressed to a predetermined value or less. Thus, the destruction of the switching element can be more reliably prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a magnetron drive circuit according to the present invention.

FIG. 2 is a circuit diagram showing in more detail an internal configuration and the like of a microcomputer and a driving unit in FIG. 1;

FIG. 3 is a characteristic diagram illustrating a relationship between a switching-on time and a collector voltage for explaining a first control operation example of the embodiment.

FIG. 4 is a characteristic diagram illustrating a relationship between a switching-on time and a collector voltage for describing a second control operation example of the embodiment.

FIG. 5 is a flowchart for explaining the second control operation example.

FIG. 6 is a characteristic diagram illustrating a relationship between a switching-on time and a collector voltage for explaining a third control operation example of the embodiment.

FIG. 7 is a characteristic diagram illustrating a relationship between a switching-on time and a collector voltage for describing a fourth control operation example of the embodiment.

FIG. 8 is a flowchart for explaining the third and fourth control operation examples.

FIG. 9 is a diagram illustrating a relationship between an input power supply voltage and a switching element collector voltage for explaining a problem of a conventional control operation.

FIG. 10 is a diagram illustrating a relationship between a switching-on time and a collector voltage for explaining a problem of a conventional control operation.

FIG. 11 is a circuit diagram of a conventional magnetron drive circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Commercial power supply 2 Rectifier 5 Switching element 8 High frequency transformer 12 Magnetron 13 Current transformer 20 Switching element drive part 24, 25 First and second comparator 30 Microcomputer (control means) 32 Operation part

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-313885 (JP, A) JP-A-3-102742 (JP, A) JP-A-4-215287 (JP, A) JP-A-63- 271884 (JP, A) JP-A-4-95386 (JP, A) JP-A-1-1599992 (JP, A) JP-A-4-62787 (JP, A) JP-A-3-269992 (JP, A) JP-A-62-69486 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H05B 6/66-6/68

Claims (4)

(57) [Claims] An inverter circuit for driving a magnetron with a rectified DC output after converting a DC voltage obtained by rectifying and converting an AC voltage from a commercial power supply into a high frequency by periodically switching with a switching element, and boosting the high frequency. ,
A current transformer for detecting an input or output current of the inverter circuit, and an on-time of the switching element is calculated based on a detected value of the current transformer such that the input or output current of the inverter circuit has a required set value. A maximum voltage applied to the switching element during warm running of the magnetron is limited to a predetermined first voltage value, and is applied to the switching element after the magnetron oscillates. A control unit that operates the switching element during an on-time output from the arithmetic unit in a limited state in which a maximum voltage is switched to a second voltage value set corresponding to a maximum variation value of the AC voltage. Characteristic magnetron drive circuit. 2. An inverter circuit for periodically switching a DC voltage obtained by rectifying and converting an AC voltage from a commercial power supply with a switching element to convert the DC voltage into a high frequency, boosting this high frequency, and driving a magnetron with the rectified DC output. ,
A current transformer for detecting an input or output current of the inverter circuit, and an on-time of the switching element is calculated based on a detected value of the current transformer such that the input or output current of the inverter circuit has a required set value. A magnetron drive circuit having an arithmetic unit, wherein the maximum voltage applied to the switching element is limited to a predetermined voltage value during a warm run of the magnetron, and after the magnetron oscillates, the on-time of the switching element is set to the AC voltage. A magnetron drive circuit comprising: control means for operating the switching element during an on-time output from the arithmetic unit after reducing the value to a value set in accordance with the maximum variation value of the above. 3. An inverter circuit for periodically switching a DC voltage obtained by rectifying and converting an AC voltage from a commercial power supply into a high frequency by switching with a switching element, boosting the high frequency, and driving a magnetron with a rectified DC output. ,
A current transformer for detecting an input or output current of the inverter circuit, and an on-time of the switching element is calculated based on a detected value of the current transformer such that the input or output current of the inverter circuit has a required set value. A magnetron drive circuit having an arithmetic unit, detecting an AC voltage value from the commercial power supply, operating the switching element in an on-time corresponding to the AC voltage value, and performing a warm run of the magnetron, A magnetron driving circuit, comprising: control means for operating the switching element during an on-time output from the arithmetic unit after the oscillation of the magnetron. 4. The arithmetic unit according to claim 1, wherein the control unit limits a maximum voltage applied to the switching element after the magnetron oscillates to a voltage value set in accordance with a maximum fluctuation value of the AC voltage. 4. The magnetron drive circuit according to claim 3, wherein the switching element is operated during the output ON time.