JP2003257613A - Inverter device for microwave oven - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inverter device for a microwave oven capable of properly controlling an anode current of a magnetron at lower cost than a conventional inverter device. <P>SOLUTION: A voltage rectified by a rectification circuit 2 in a direct current power supply circuit 31 is divided by voltage dividing resistances 17a and 17b. A control circuit 37 detects changes of a power supply voltage by referring to the divided voltage to change an output condition of an inverter circuit 32 based on its change condition in order to control an anode current of the magnetron 16. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic device configured to supply a high frequency current to a primary winding of a high frequency transformer for generating a drive voltage to be supplied to a magnetron on a secondary side by a half bridge type inverter circuit. The present invention relates to a range inverter device.

[0002]

The inverter system, which is used in a microwave oven and which drives a magnetron for generating a microwave, is widely adopted because of its easy continuous variable output control. In the inverter type drive circuit, the DC power source generated from the commercial AC power source is converted into a high frequency power source and supplied to the primary winding of the high frequency transformer. Then, the high frequency voltage generated in the secondary winding of the transformer is rectified by a voltage doubler rectifier circuit and the rectified voltage is applied between the anode and cathode of the magnetron to drive the magnetron.

In recent years, a half-bridge type inverter circuit is often used in order to meet the demand for higher output as the capacity of microwave ovens increases. The half-bridge type inverter circuit has the advantages that high output can be achieved without using a high-voltage switching element, and that no DC current component remains in the primary winding of the high-frequency transformer, so that magnetic saturation is unlikely to occur. ing.

The output of the magnetron is proportional to the average value of the anode current, but if the peak value of the anode current is too high, an abnormal operation called "moding" (heater current is too small, anode current is too large) , The phenomenon that the magnetron operates in an abnormal oscillation mode may occur due to defective load impedance. As one of the conventional techniques for avoiding the occurrence of this "moding", there is a high frequency power supply device for a microwave oven disclosed in Japanese Patent Laid-Open No. 11-329707.

This prior art is equivalent to the difference between the voltage obtained based on the current detected by the current transformer flowing through the primary winding of the high frequency transformer and the reference voltage determined by the output setting of the magnetron. Error voltage to obtain.
Then, the half-bridge type inverter circuit is switched at a frequency proportional to the error voltage to suppress the current peak.

However, in the above-mentioned conventional technique, the current flowing through the primary winding of the transformer is detected by the current transformer, so that there is a problem that the cost increases accordingly.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an inverter device for a microwave oven capable of appropriately controlling the anode current of a magnetron at a lower cost.

[0008]

In order to achieve the above object, an inverter device for a microwave oven according to claim 1 rectifies an AC power source and smoothes it to generate a DC power source, and a DC power source. A series circuit of an inverter circuit composed of two switching elements connected in series between the positive and negative terminals of the circuit, and two resonant capacitors connected in series between the positive and negative terminals of this DC power supply circuit And a high frequency transformer for generating a magnetron drive voltage on the secondary side, the primary winding being connected between the output terminal of the inverter circuit and the common connection point of the two resonance capacitors, and the AC power supply. And a control circuit for controlling the output state of the inverter circuit to change based on the changing state.

That is, the AC power supply is rectified and smoothed by the DC power supply circuit. Generally, the smoothing action in the DC power supply circuit used in the microwave oven is intended to prevent the reduction of the power factor and improve the power supply efficiency. Is suppressed (that is, the capacity of the smoothing capacitor is set to be relatively small). Therefore, the DC power supply voltage output from the DC power supply circuit is in a state close to a pulsating current in which the amplitude fluctuations of the AC power supply voltage are reflected relatively large, and the high frequency generated by switching such a power supply. The waveform amplitude of the current is also affected by the amplitude fluctuation.

Therefore, it is possible to control so as to change the output state of the inverter circuit with reference to the state where the AC power source changes. And unlike the conventional configuration, the configuration that refers to the changing state of the AC power supply can be easily configured without using expensive devices such as current transformers, so that the anode current of the magnetron can be controlled appropriately at a lower cost. can do. Further, since the changed state of the AC power supply is referred to, even when the power supply voltage is distorted, it is possible to control the change in quick response to the change.

In this case, as described in claim 2, the control circuit may be configured to change the output state of the inverter circuit by changing the switching frequency of the switching element. That is, in the half-bridge type inverter circuit, the output state can be easily changed according to the switching frequency of the switching element, and the control system can be easily configured.

Further, as described in claim 3, the control circuit is provided with a voltage dividing resistor for dividing the voltage rectified in the DC power supply circuit, and the voltage divided by the voltage dividing resistor is referred to. It is preferable to detect the change of the AC power source. With such a configuration, it is possible to detect a change in the AC power supply voltage extremely easily and inexpensively.

In the above case, as described in claim 4, the control circuit may be configured to increase the output of the inverter circuit near the zero cross point of the AC power supply voltage. That is, near the zero cross point of the AC power supply voltage,
Since the amplitude of the high frequency current supplied to the primary winding of the high frequency transformer also decreases, the anode current supplied to the magnetron also decreases. If such a period exists, the average value of the anode current will decrease and the output of the magnetron will also decrease. Therefore, if the output of the inverter circuit is controlled to increase during the period, it is possible to compensate for the decrease in the amount of high frequency current due to the decrease in the AC power supply voltage and maintain the output level of the magnetron to be more average. .

Further, as described in claim 5, it is preferable that the control circuit is configured to reduce the output of the inverter circuit during a period in which the AC power supply voltage is equal to or higher than a predetermined level. That is, as described above, if the peak value of the anode current of the magnetron rises too much, "moding" may occur, so it is necessary to suppress the peak value at an appropriate level. Also for this control, if the output of the inverter circuit is reduced in a period in which the AC power supply voltage is equal to or higher than a predetermined level, the amount of high-frequency current supplied to the primary side of the high-frequency transformer is reduced, and the peak The rise can be suppressed appropriately. Also in this case, the output level of the magnetron can be made more average.

Further, in the above case, as described in claim 6, a switching power supply circuit for generating a control power supply for supplying to the control circuit from an AC power supply is provided,
The oscillation reference timing of the operating frequency of the switching power supply circuit may be periodically synchronized with the oscillation reference timing of the switching frequency of the inverter circuit.

Generally, all of these frequencies are set to be sufficiently higher than the audible frequency, and are not recognized by human hearing alone. Also, even if each is set sufficiently higher than the audible frequency, if the frequencies are very close to each other, the frequency that is the difference between them may belong to the audible frequency range. It is set to be sufficiently large.

However, even in such a setting, if each oscillation signal includes a harmonic component,
In some cases, one fundamental wave component and the other harmonic component are close to each other, and in such a case, the difference frequency is recognized as an unpleasant oscillation sound. Therefore, according to the sixth aspect, the periodicity of the operating frequency of the switching power supply circuit can be reduced. The "oscillation reference timing" referred to here is a timing with reference to the time (phase) when the amplitude of the oscillation signal waveform shows a predetermined level in each cycle. For example, the amplitude indicates the maximum value or the minimum value. Such as the time. Then, "to periodically synchronize one oscillation reference timing with the other oscillation reference timing" means, for example, that when one oscillation signal waveform has a minimum value, the other oscillation signal waveform also has a minimum value at the same time. Means to adjust as shown.

With this adjustment, the oscillated signal on the synchronized side includes the oscillation frequency component on the synchronized side (fundamental wave or its harmonic), and as a result, the original synchronized signal on the synchronized side. The periodicity of the oscillation frequency will be substantially reduced. It should be noted that in the switching power supply circuit, even if the operating frequency fluctuates to some extent, it has little influence on the power generation operation. Therefore, the level of the higher harmonic wave component is accordingly reduced, and as a result, the level of the interference sound generated in the relationship between the one fundamental wave component and the other higher harmonic wave component can be reduced.

Further, as described in claim 7, a switching power supply circuit for generating a control power supply for supplying to the control circuit from an AC power supply is provided, and the operating frequency of the switching power supply circuit is determined by the output state of the inverter circuit. It may be configured to match the switching frequency corresponding to the case where is the maximum.

That is, since the switching frequency of the inverter circuit changes according to the cooking state of the microwave oven, if the operating frequency of the switching power supply circuit matches the switching frequency corresponding to the maximum output state,
It is possible to reduce the level of the interference sound generated in the relationship between the one fundamental wave component and the other harmonic component as described above.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be described below with reference to FIGS. In FIG. 1 showing an electrical configuration of an inverter device used in a microwave oven, an AC input terminal of a diode bridge rectifier circuit 2 is connected to both ends of a commercial AC power source 1, and a DC output terminal thereof has a Choke coil 3 and smoothing capacitor 4
A filter consisting of is connected. These form the DC power supply circuit 31.

An IGBT is connected to both ends of the smoothing capacitor 4.
A series circuit of (switching elements) 5a and 5b and a series circuit of resonance capacitors 7a and 7b are connected.
Further, feedback diodes 6a and 6b are respectively connected in antiparallel between the collectors and the emitters of the IGBTs 5a and 5b. The two IGBTs 5a and 5b form an inverter circuit 32.

A common connection point of the IGBTs 5a and 5b (output terminal of the inverter circuit 32) and the resonance capacitors 7a and 7b.
A primary winding 8 of the high-frequency transformer 10 is connected between the common connection point of b and the primary winding 8 and the resonance capacitors 7a and 7b form a series resonance circuit 33.

The secondary winding 9 of the high frequency transformer 10 is connected to a voltage doubler rectifier circuit 34 including diodes 12 and 13 and capacitors 13 and 14, so that a high DC voltage is supplied to the magnetron 16. There is. Also,
A heater winding 11 for supplying a current to the filament of the magnetron 16 is arranged on the secondary side of the high frequency transformer 10.

Further, between the DC output terminals of the rectifier circuit 2,
A series circuit of the voltage dividing resistors 17a and 17b is connected, and their common connection point (indicating the divided voltage V1) is connected to the inverting input terminal of the operational amplifier 20 via the resistor 34. A reference voltage V2 is applied to the non-inverting input terminal of the operational amplifier 20. The output terminal of the operational amplifier 20 is
A voltage controlled oscillator (VC
O) 22 and the inverting input terminal through the diode 35. Further, the anode of the diode 21 is connected to the inverting input terminal via the resistor 36.

When the level of the divided voltage V1 is lower than the level of the reference voltage V2, the operational amplifier 20 sets the output terminal to the high level to bring the diode 35 into conduction. Also,
When the level of the divided voltage V1 becomes higher than the level of the reference voltage V2, the output terminal becomes low level and the diode 2
Make 1 conductive.

The voltage controlled oscillator (VCO) 22 is constructed so that the oscillation frequency of the output signal decreases as the input voltage increases, and a reference voltage Vp is applied to its voltage input terminal via a resistor 25. Has been. Voltage controlled oscillator 2
The output signal of 2 is given to the drive control unit 23. The output terminal of the drive control unit 23 is connected to the gates of the IGBTs 5a and 5b via the drivers 24a and 24b.

The switching power supply circuit 26 also generates a control power supply from the commercial AC power supply 1 to generate a voltage controlled oscillator 2.
2 and the drive control unit 23, and also generates a gate drive power source and supplies it to the drivers 24a and 24b. In the above configuration, the operational amplifier 20, the voltage controlled oscillator 22, and the drive controller 23 form a control circuit 37.

Next, the operation of this embodiment will be described with reference to FIG. FIG. 2 shows the AC power supply voltage (a), the input voltage V3 (b) of the voltage controlled oscillator 22, and the magnetron 1.
6 shows a waveform of the anode current (c) of No. 6.

While the voltage level of the commercial AC power supply 1 is low,
That is, in the vicinity of the zero-cross point, the level of the divided voltage V1 becomes lower than the level of the reference voltage V2.
Makes the diode 35 conductive, and the input voltage V3 becomes equal to the reference voltage V2. Then, during the period when the voltage level of the commercial AC power supply 1 is high, the level of the divided voltage V1 is the reference voltage V1.
The voltage becomes higher than the level of 2 and the operational amplifier 20 makes the diode 21 conductive, and acts as an inverting amplifier to lower the input voltage V3 below the reference voltage V2.

As a result, the oscillation frequency of the voltage controlled oscillator 22 is lowered and the switching frequency in the inverter circuit 32 is lowered during the period when the voltage level of the commercial AC power supply 1 is low. Then, since the amount of high frequency current supplied to the primary side of the high frequency transformer 10 increases, the magnetron 16
The anode current of increases. Further, since the oscillation frequency of the voltage controlled oscillator 22 rises during the period when the voltage level of the commercial AC power supply 1 is high, the switching frequency in the inverter circuit 32 also rises and the anode current decreases.

Here, the DC power supply voltage output from the DC power supply circuit 31 is in a state close to a pulsating current in which the amplitude fluctuation of the AC power supply voltage is reflected relatively large because the capacity of the smoothing capacitor 4 is set small. Has become. Therefore, the waveform amplitude of the high frequency current generated by switching such a power supply is also affected by the amplitude fluctuation.
That is, near the zero-cross point of the AC power supply voltage, the amount of high frequency current supplied to the primary winding 8 of the high frequency transformer 10 also decreases, so that the anode current supplied to the magnetron 16 decreases.

Then, as shown by the broken line in FIG. 2C, when the control of the present embodiment is not performed, the anode current which was almost zero during the period when the voltage level of the commercial AC power supply 1 was low is the present embodiment. By performing the control of the example, it is increased as shown by the solid line. Therefore, the magnetron 16
The period during which it can be driven is longer.

In order to control as described above, the level of the reference voltage V2 is set to be higher than the normal control level. Then, when the diode 21 becomes conductive and the input voltage V3 drops, the control level is set to the normal control level.

As described above, according to this embodiment, the voltage rectified by the rectifying circuit 2 in the DC power supply circuit 31 is divided by the voltage dividing resistors 17a and 17b, and the control circuit 37 is operated.
Detects a change in the power supply voltage by referring to the divided voltage, and based on the change state, the inverter circuit 3
The output state of No. 2 is changed.

Therefore, unlike the prior art, it is possible to appropriately control the anode current of the magnetron 16 at a lower cost by using the voltage dividing resistors 17a and 17b without using an expensive device such as a current transformer. it can. Further, by referring to the changed state of the AC power supply 1, even when the power supply voltage is distorted, it is possible to quickly respond to the change and control.

Further, according to the present embodiment, the control circuit 37
Changes the output state of the inverter circuit 32 by changing the switching frequency of the IGBTs 5a and 5b. That is, since the half-bridge type inverter circuit 15 can easily change the output state according to the switching frequency of the IGBTs 5a and 5b, the control system can be easily configured.

Further, according to this embodiment, the control circuit 37
Causes the output of the inverter circuit 32 to rise near the zero crossing point of the AC power supply voltage, so that the inverter circuit is reduced during the period in which the amplitude of the high frequency current supplied to the primary winding 8 of the high frequency transformer 10 decreases as the AC power supply voltage changes. Three
By increasing the output of No. 2, it is possible to compensate for the decrease in the amount of high frequency current due to the decrease in the AC power supply voltage, and to maintain the output level of the magnetron 16 at a more average level. Therefore, it is possible to improve the driving efficiency.

FIG. 3 shows a second embodiment of the present invention. The same parts as those of the first embodiment are designated by the same reference numerals and the description thereof will be omitted. Only different parts will be described below. The configuration of the second embodiment is basically the same as that of the first embodiment, but the control concept is different.

That is, in the first embodiment, the reference voltage V2 is set higher than the normal control level, and is set to the normal control level when the diode 21 conducts and the input voltage V3 decreases. By doing so, the control was performed so as to compensate for the decrease in the anode current amount near the zero cross point of the AC power supply voltage. On the other hand, in the second embodiment, the reference voltage V2 is set to a normal control level, and the diode is operated during a period in which the level of the AC power supply voltage is a predetermined value or more (for example, 30 to 50 V or more in the case of AC100V). When 21 becomes conductive, the input voltage V3 is made lower than the normal control level.

That is, when the level of the AC power supply voltage rises, the output of the inverter circuit 32 rises accordingly and the anode current of the magnetron 16 rises, so that the driving efficiency of the magnetron 16 is lowered or the above-mentioned "moding" is performed. May occur. Therefore, during a period in which the level of the AC power supply voltage is equal to or higher than a predetermined value, the output of the inverter circuit 32 is reduced and the peak of the anode current is reduced as shown in FIG.

As described above, according to the second embodiment, the control circuit 37 lowers the output of the inverter circuit 32 during the period when the AC power supply voltage is above the predetermined level, so that the primary side of the high frequency transformer 10 during that period. It is possible to reduce the amount of high-frequency current supplied to the device and suppress the rise of the peak appropriately, while suppressing the occurrence of "moding",
The driving efficiency of the magnetron 16 can be improved.

The present invention is not limited to the embodiments described above and shown in the drawings, but the following modifications and expansions are possible. The oscillation reference timing of the operating frequency of the switching power supply circuit 26 may be periodically synchronized with the oscillation reference timing of the switching frequency of the inverter circuit 32. In general, all of these frequencies are set to be sufficiently higher than the audible frequency, and are not recognized by human hearing alone. Also, even if each is set sufficiently higher than the audible frequency, if the frequencies are very close to each other, the frequency that is the difference between them may belong to the audible frequency range. It is set to be sufficiently large. However, even in such a case, if each oscillation signal contains a harmonic component, one fundamental wave component and the other harmonic component may be close to each other, In such a case, the difference frequency is still recognized as an unpleasant oscillation sound. Therefore, if configured as described above, the periodicity of the operating frequency of the switching power supply circuit 26 can be reduced.

The "oscillation reference timing" referred to here is
The timing is based on when the amplitude of the oscillation signal waveform shows a predetermined level in each cycle (phase).
For example, when the amplitude shows the maximum value or the minimum value. That is,
"Periodically synchronize the oscillation reference timing of the operating frequency of the switching power supply circuit 26 with the oscillation reference timing of the switching frequency of the inverter circuit 32" means, for example, that the oscillation signal waveform that defines the switching frequency of the inverter circuit 32 has a minimum value. Means that the oscillation signal waveform that defines the operating frequency of the switching power supply circuit 26 is also adjusted to show the minimum value at the same time. With such adjustment, the oscillation signal on the synchronized side includes the oscillation frequency component on the synchronization side (fundamental wave or its harmonic), and as a result, the original oscillation frequency on the synchronized side is present. The periodicity that is occurring will be substantially reduced. (In the switching power supply circuit 26, even if the operating frequency slightly fluctuates, the influence on the power generation operation is small.) As a result, the level of the harmonic component also decreases, and as a result, one of the fundamental wave components The level of the interference sound generated in relation to the other harmonic component can be reduced. Further, the synchronous-asynchronous relationship in this case may be set to be reversed.

Further, the operating frequency of the switching power supply circuit 26 may be made to coincide with the switching frequency corresponding to the maximum output state of the inverter circuit 32. That is, since the switching frequency of the inverter circuit 32 changes according to the cooking state of the microwave oven, if the operating frequency of the switching power supply circuit 26 is made to coincide with the switching frequency corresponding to the maximum output state, as described above. It is possible to reduce the level of the interference sound generated in the relationship between the one fundamental wave component and the other harmonic component. The control circuit may be configured as one microcomputer. In the first embodiment,
The control circuit may be configured to increase the input voltage of the voltage controlled oscillator circuit 22 near the zero cross point of the AC power supply voltage. The control of the first embodiment and the control of the second embodiment may be performed simultaneously. Similar control may be applied to the quasi-E class inverter circuit.

[0046]

According to the inverter device for a microwave oven of the present invention, the control circuit controls so as to change the output state of the inverter circuit based on the state in which the AC power source changes. It can be easily constructed without the use of expensive devices such as current transformers, and the magnetron anode current can be properly controlled at a lower cost. Further, since the changed state of the AC power supply is referred to, even when the power supply voltage is distorted, it is possible to control the change in quick response to the change.

[Brief description of drawings]

FIG. 1 is a diagram showing an electrical configuration of an inverter device used in a microwave oven according to a first embodiment of the present invention.

2A is a diagram showing an AC power supply voltage, FIG. 2B is a diagram showing an input voltage V3 of a voltage controlled oscillator, and FIG. 2C is a diagram showing a waveform of an anode current of a magnetron.

FIG. 3 is a view corresponding to FIG. 2 showing a second embodiment of the present invention.

[Explanation of symbols]

1 is a commercial AC power supply, 5a and 5b are IGBTs (switching elements), 7a and 7b are resonance capacitors, 8 is a primary winding, 10 is a high frequency transformer, 16 is a magnetron, 1
7a and 17b are voltage dividing resistors, 26 is a switching power supply circuit, 31 is a DC power supply circuit, 32 is an inverter circuit, 37
Indicates a control circuit.

Continued front page    F term (reference) 3K086 AA01 AA09 BA08 CC02 CD11                       DA02 DB15 FA02 FA03 FA04                 5H730 AA02 AA14 AA15 AS04 BB26                       BB57 BB76 CC01 DD02 DD12                       EE06 EE07 EE72 EE79 FD11                       FG07 FG26 XX03 XX15

Claims (7)

[Claims] 1. A DC power supply circuit for rectifying and smoothing an AC power supply to generate a DC power supply, and a DC power supply circuit connected in series between positive and negative terminals of the DC power supply circuit.
An inverter circuit composed of switching elements and a DC power supply circuit connected in series between the positive and negative terminals 2
A primary winding is connected between the series circuit of the resonance capacitors and an output terminal of the inverter circuit and a common connection point of the two resonance capacitors, and a drive voltage of the magnetron is generated on the secondary side. An inverter device for a microwave oven, comprising: a high-frequency transformer; and a control circuit that controls the output state of the inverter circuit to change based on a state in which the AC power source changes. 2. The inverter device for a microwave oven according to claim 1, wherein the control circuit changes the output state of the inverter circuit by changing the switching frequency of the switching element. 3. A voltage dividing resistor for dividing a voltage rectified in the DC power supply circuit, wherein the control circuit detects a change in the AC power supply by referring to the voltage divided by the voltage dividing resistor. 3. The inverter device for a microwave oven according to claim 1, wherein the inverter device is a microwave oven. 4. The inverter device for a microwave oven according to claim 1, wherein the control circuit raises the output of the inverter circuit near the zero cross point of the AC power supply voltage. 5. The inverter device for a microwave oven according to claim 1, wherein the control circuit reduces the output of the inverter circuit during a period when the AC power supply voltage is at a predetermined level or higher. 6. A switching power supply circuit for generating a control power supply for supplying to a control circuit from an AC power supply, wherein an oscillation reference timing of an operating frequency of the switching power supply circuit is set to an oscillation reference timing of a switching frequency of an inverter circuit. The inverter device for a microwave oven according to claim 1, wherein the inverter device is configured to be periodically synchronized. 7. A switching power supply circuit for generating a control power supply for supplying to the control circuit from an AC power supply, and switching the operating frequency of the switching power supply circuit when the output state of the inverter circuit is maximized. 6. Matching the frequency to the frequency.
An inverter device for a microwave oven according to any one of 1.