JPH02306574A - High frequency heating device - Google Patents

Abstract

PURPOSE:To miniaturize and lighten a high frequency heating device by making the 'on' time of a semiconductor switching element longer than half the cycle and shorter than one cycle of the resonance current of a series resonance circuit, and turning off a current converting circuit while the current flow through a semiconductor switching element is substantially zero. CONSTITUTION:In a control portion 29, an oscillating circuit 25 is oscillated by a frequency lower than the resonance frequency of the series resonance circuit of both a condensor 18 and a transformer coil 22, and the signal of the circuit 25 is received and then a timer circuit 36 measures the 'on' period of a semiconductor switching element 7, and an output circuit 37 drives the element 7 in response to signals of the circuit 36. The 'on' period here is longer than half the cycle and shorter than one cycle of the resonance current of the series resonance circuit. Thereby such a current converting circuit is realized that can be turned off while the current flow through the element 7 is substantially zero. Then switching loss of the semiconductor switch is remarkably reduced and the device is miniaturized and its weight is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improvements in high frequency heating devices such as microwave ovens.
More specifically, the present invention relates to an improvement of a so-called inverter power supply device for a magnetron in which a semiconductor switching element is used in the power supply circuit of the magnetron.

BACKGROUND OF THE INVENTION Conventionally, various high-frequency heating devices using this type of power supply circuit have been proposed, but the most representative ones are:
This is a voltage resonant inverter circuit having a parallel resonant circuit as shown in FIG.

This circuit rectifies a commercial power supply 1 with a bridge diode 2, an inductor 3, and a capacitor 4, and a pulsating DC voltage source (
A parallel resonant circuit consisting of a resonant capacitor 5 and a step-up transformer 6 that also serves as a resonant inductor, a transistor 7, and a diode 8 constitute a voltage resonant inverter circuit, and the high-voltage output of the step-up transformer 6 is connected to a capacitor. 9. It is rectified by diodes 10 and 11 and supplied to the magnetron 12. In addition, 13.
14 and 15 are the primary, secondary and heater windings of the step-up transformer 6, respectively, and 16 is a control circuit for driving the transistor 7.

In this power supply circuit, the transistor 7 is connected to the control circuit 16
Therefore, the conduction time To is controlled by so-called pulse width control.
n is controlled (by controlling Ton', To
n controlled). That is, by the gate signal VGE as shown in FIG. 6(al), the voltage waveform V as shown in FIG. 6(C) is generated.
A switching operation is performed by a transistor 7 and a diode 8 in CE. TOFF is determined by the circuit constant of the parallel resonant circuit, and on the other hand, the magnitude of the power converted by the inverter circuit is determined by TON, so the magnitude of the power supplied to the magnetron 12 is determined by TON, and the magnitude of the power converted by the inverter circuit is determined by TON. The lower the operating frequency, the greater the power supplied to the magnetron 12. Control circuit 1
Reference numeral 6 controls this TON to adjust the magnitude of the output of the magnetron 12 and stabilize the output against fluctuations in power supply voltage.

Problems to be Solved by the Invention Although the switching state of the transistor 7 ideally has a waveform as shown in FIG. 6(b), it actually has a switching waveform as shown in FIG. 7. That is,
As shown in FIG. 5A, when the gate voltage VGE becomes 0.times. at time 1-1.degree., the collector current IC of the transistor 7 rapidly decreases.

Then, the collector-emitter voltage VCE draws a resonant waveform and rises as shown in FIG. 2(b). However, in switching circuits that handle large amounts of power (for example, IKW or higher), it is impossible for semiconductor switching elements such as transistors to achieve ideal switching operation, and the actual collector current becomes zero at t. =t
It's time for 2. In particular, when a switching element with a relatively large tail current (Ita) such as an insulated gate bipolar transistor is used as the semiconductor switching element, the switching loss caused by the tail current at turn-off becomes quite noticeable.

In particular, when the power supply device is further miniaturized by increasing the switching frequency, the loss of the transistor 7 becomes a problem due to this switching loss, which inevitably limits the practical maximum switching frequency. According to experiments conducted by the inventors, the power converted by the inverter circuit is IK-, the insulated gate bipolar transistor is attached to a heatsink that can be regarded as an infinite heatsink, and the insulated gate bipolar transistor is installed under sufficient heat dissipation conditions. It was found that as the operating frequency fO of the inverter was increased, thermal runaway occurred at approximately 70 kHz to 80 kHz, leading to destruction, as shown in FIG. This is the time during which the so-called tail current Ita flows (Δ - 3
-t2) is 1 to 2 μs even at room temperature, and since this time Δt has a positive temperature coefficient, if an excessive loss occurs, the resulting temperature rise will lead to runaway.

Therefore, although this operating frequency limit changes depending on the size of the package and chip of the transistor 7,
Approximately 70kHz to 80kHz if economical design conditions are considered
It is believed that there is an operating frequency limit for z, and it has been difficult to realize an inverter power supply device that operates at a higher frequency than this limit.

For this reason, we have created an inverter circuit that operates at a high frequency of 100kHz or higher, making power supplies smaller, lighter, and more compact.
However, it has been difficult to provide a power supply device that is significantly more compact and lower cost than conventional power supplies.

The present invention solves these conventional problems and provides a high-frequency heating device having a low-cost power supply device.

Means for Solving the Problems In order to solve the problems of the prior art, the present invention has the configuration described below.

That is, a power source obtained from a commercial power source, a battery, etc., a semiconductor switching element with a self-commuting function, a series resonant circuit consisting of a resonant capacitor and a resonant inductor, and a resonant voltage of the series resonant circuit boosted and supplied to the magnetron. a step-up transformer that controls the semiconductor switch; and a controller that controls the semiconductor switch; and an output circuit that drives the semiconductor switching element with a signal from the timer circuit, and the conduction time is longer than a half cycle of the resonant current of the series resonant circuit, and the conduction time is longer than one cycle of the resonant current of the series resonant circuit. It is designed to be shorter.

Further, a reverse bias detection circuit for detecting the reverse bias state of the semiconductor switching element is provided, and the output pulse of the output circuit is cut off by the output of this reverse bias detection circuit.

Furthermore, a diode for rectifying the output of the step-up transformer is provided, and the rectified output of this diode is supplied to the magnetron, and a diode current detection means for detecting the current flowing through the diode, and an output of this diode current detection means are provided. and an error amplification control circuit that compares the current with a set value and controls the oscillator frequency of the oscillator using the error signal, and controls the oscillator so that the diode current reaches a predetermined value. .

Effects With the above configuration, the present invention has the following effects.

That is, a power conversion circuit is configured with a series resonant circuit and a semiconductor switching element, and a timer circuit provided in the control section generates a drive pulse having a time width between a half cycle and one cycle of the resonance cycle. By using a configuration that supplies power to the semiconductor switch element, it is possible to realize a power conversion circuit that can turn off the current flowing through the semiconductor switch element in a state where the current is virtually zero. This significantly reduces the switching loss of the semiconductor switch element, making it possible to It is possible to operate the semiconductor switch element at a frequency one order of magnitude higher than that achieved by conventional techniques, and to reliably control on/off the semiconductor switch element having a self-commuting function.

In addition, the configuration detects the reverse bias state of the semiconductor switching element and cuts off the output pulse of the output circuit, so even if the constant of the series resonant circuit changes significantly due to the temperature characteristics of the magnetron, capacitor, inductor, etc., it can be reliably operated. In addition, a driving pulse having a time width between a half period and one period of the resonance period can be supplied to the semiconductor switching element having a self-commuting function, so that the current flowing through the semiconductor switching element is substantially zero in any case. It is possible to realize a power converter circuit that can turn off the power converter in a state where the power converter is turned off, and the above-mentioned effect can be realized.

Furthermore, the output of the power conversion circuit is stabilized by detecting the current flowing through the output rectifying diode of the step-up transformer, and controlling the oscillation frequency of the oscillator by feedback control using the error signal between this detection signal and the set value. The high-frequency output of the magnetron can be stabilized against fluctuations in power supply voltage.

EXAMPLES Hereinafter, examples of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram of a high-frequency heating device showing an embodiment of the invention, and the same reference numerals as in FIG. 5 indicate corresponding components.

The output of the commercial power supply 1 is rectified by the diode bridge 2, resulting in a full-wave rectified voltage waveform output.

This output is connected to the inductor 17 which acts as a constant current power supply.
, a series resonant circuit consisting of a capacitor 18, a step-up transformer 19, and an insulated gate bipolar transistor (I
The power is supplied to a power conversion circuit (inverter) comprised of a switching circuit including a GBT (GBT) 7 and a diode 8, and a snubber circuit including a resistor 20 and a capacitor 21, and is converted into high frequency power of 100 to 300 kHz. This high frequency power is transferred to the primary winding 22 of the step-up transformer 19.
Since a high frequency voltage is generated in the secondary winding 23 and the heater winding 24, high frequency high voltage and high frequency low voltage power are respectively induced, and the high frequency high voltage is rectified by the diode 25, 26 and the capacitor 27, 28. DC voltage power is supplied to the magnetron 12, while high frequency low voltage power is supplied as is to the cathode of the magnetron 12 to heat the cathode. The current and voltage waveforms flowing through the IGBT 7 and diode 8 are as shown in FIGS. 2(a) and 2(b), respectively. The IGBT 7 is driven by a control section 29, which will be described later, using a gate voltage waveform VGE shown in FIG.

When a switch 30 such as a relay is closed, the control unit 29
The operation starts by forming a control power source consisting of a resistor 31, a diode 32, a capacitor 33, and a zener diode 34. Voltage controlled oscillator 35 is started and its output energizes timer circuit 36. The timer circuit 36 can be easily configured with, for example, a monostable multivibrator with a reset terminal, and the voltage controlled oscillator 35 can be easily realized with an astable multivibrator with a voltage control terminal. The output of the timer circuit 36 is sent to an output circuit 37, subjected to impedance conversion, and then supplied to the IGBT 7 via a resistor. As is clear from FIG. 2, when the gate pulse VGE force is supplied to the IGBT 7, the collector current 1c begins to flow resonantly as shown in the figure. After time t1 passes, Ic becomes zero, and then the diode current 1d
flows, and this Id is also 1. It becomes zero after some time. IGBT
It is extremely important for the safe operation of IGET 7 that the gate pulse voltage VGE for driving IGET 7 has a pulse width ton which is longer than t2 and shorter than t2. if,
If ton is shorter than tl, the IGBT 7 will be turned off during the period when Ic is flowing, resulting in a large turn-off loss as in the conventional technology. On the other hand, if it is longer than t2, a so-called commutation failure phenomenon occurs, and a short circuit current flows through the IGBT 7, resulting in destruction. That is, when using a switching element having a self-commuting function such as an IGBT, the conduction time ton is shorter than the resonant period L2 of the resonant circuit, and the resonant half period 1. The condition that the length is longer than that is extremely important and indispensable in order to guarantee safe and reliable operation, as well as to operate with high efficiency and high frequency.

The reverse bias detection circuit 3 in FIG. 1 detects the point in time (D+time after the elapse of time) in FIG. 2 when Id starts flowing,
After a time delay of Δ, the timer circuit 36 is reset to cut off the output pulse GE. This reverse bias detection circuit 39 allows the gate pulse VG[
! The pulse width (i.e. conduction time) ton is always t
The conditions of being longer than l and shorter than t2 can be satisfied, and stable and low-loss switching operation can be realized. Of course, the reverse bias detection circuit 3
9 is not necessarily necessary. For example, if the variation range of the circuit constants of the series resonant circuit due to changes in the characteristics of the magnetron 12, capacitor 18, transformer 19, etc. described above is relatively small, the timer circuit 3
If the count time of is set to be the conduction time ton in Fig. 2, even if there is some variation in the circuit constant of the series resonant circuit, the period during which Id is flowing (i.e., 1.c
becomes zero, and the gate pulse VGE can be turned off during the period in which VCE is zero. Therefore, in this case, the timer circuit 3 is a simple monostable multivibrator.
This means that only 6 is required, resulting in a simple and inexpensive configuration.

A current transformer 40 is used to detect the current flowing through the diode 26. Since this diode current is about half of the anode current (proportional to radio wave output) flowing through the magnetron 12, a signal corresponding to the anode current can be obtained by detecting a small amount of current. Therefore, a small current transformer can be used. The output of this current transformer 40 is sent to an error amplifier 41.
controls the oscillation frequency of the voltage controlled oscillator 35 so that the output of the current transformer 40 becomes the set value. That is, when the output of the current lance 40 is smaller than the set value, the error amplifier 41 controls the voltage controlled oscillator 35 to increase the oscillation frequency, and conversely, when the output is larger than the set value, the error amplifier 41 controls the oscillation frequency to be lowered. Negative feedback control. The error amplifier 41 can be realized very easily using a well-known operational amplifier. By detecting the diode current and performing negative feedback control in this manner, it is possible to easily stabilize the radio wave output against voltage fluctuations of the commercial power supply.

With the above configuration, it is possible to realize an inverter circuit that can operate at a frequency approximately □-digits higher, which has been difficult in the past. For example, the conventional inverter i-path (Fig. 5)
When converting IKi power using
Even if GBT7 is used, which generates a tail current of Cost reduction can be achieved.

FIG. 3 shows a more detailed embodiment of the reverse bias detection circuit 39 shown in FIG.

FIG. 3 shows a more detailed embodiment of the reverse bias detection circuit 39 shown in FIG.

In FIG. 3, the same symbols as in FIG. 1 indicate corresponding components, and the collector terminal 43 of I' G B T' 7,
The voltage across the emitter terminal 44 (that is, the voltage across the diode 8) is supplied to the comparator 45 via the resistor 44.

Since the output of the comparator 45 is connected to the set terminal R of the timer circuit 36 via a delay circuit consisting of a resistor 46 and a capacitor 41, when the IGBT 7 is reverse biased, the timer circuit 36 is delayed by a predetermined delay time Δt.゛ is reset. Therefore, as shown in FIG. 2, during the period when Id is flowing, that is, I
Gate pulse VG is applied during the period when 7 is receiving reverse bias.
E is terminated, and the IGBT 7 can minimize its turn-off loss. In addition,
48 is a protective Zener diode for limiting the input voltage to the comparator 45.

FIG. 4 shows a more detailed embodiment of the control circuit 29,
Components with the same symbols as in FIG. 1 are corresponding components.

In the figure, the output of a current transformer 40 is converted into a voltage by a diode 49, a capacitor 50, and a resistor 51, and the error is amplified by an error amplifier consisting of an operational amblifier 52, a reference voltage 5'3.54, and resistors 57 to 58. The signal is then sent to the oscillation frequency control terminal 6 of the oscillator 59 (+1E555 timer IC). 6
0.61 is a resistor, and 62 is a capacitor. As is well known, the oscillation frequency is determined by these constants and the input voltage to the terminal 6. The output of this oscillator 59 is a monostable multivibrator 65 (
It is configured to be sent to the NE555 timer IC). Therefore, an output pulse having a frequency corresponding to the oscillation frequency of the oscillator 59 and a pulse width determined by the monostable multivibrator 65 is outputted from the monostable multivibrator 65 to the CMOS buffer 66 . CMO
Since the output of the S buffer 66 is configured to be supplied to the gate of the I'GBT 7, the IGBT 7 operates so that the current flowing through the diode 26 detected by the current transformer 40 becomes the value set by the reference voltage 53. Its switching frequency is controlled. 67 is a capacitor.

As described above, by using a control circuit with an extremely simple configuration, the current flowing through the diode 26 can be kept constant, and therefore the current flowing through the magnetron 12 is also controlled to be constant, so that its radio wave output can also be controlled to be almost constant. I can't stop thinking about it.

Effects of the Invention As described above, according to the present invention, commercial power supply and battery
a power supply unit obtained from the above, a semiconductor switching element having a self-commuting function, a series resonant circuit consisting of a resonant capacitor and a resonant inductor, and a step-up transformer that boosts the resonant voltage of the series resonant circuit and supplies it to the magnetron; a controller for controlling the semiconductor switch; and an oscillator that oscillates at a frequency lower than the resonance frequency of the series resonant circuit; The old enemy timer circuit and
and an output circuit that drives the semiconductor switch element with a signal from the timer circuit, and the conduction time is
longer than a half cycle of the resonant current of the series resonant circuit, and
Since it is configured so that the period is shorter than one cycle, it is possible to realize a current conversion circuit that can turn off the semiconductor switch element when the current flowing through the element is substantially zero. operate at an order of magnitude higher frequency than conventional technology, and
This makes it possible to reliably control on/off the semiconductor switching element having a self-commuting function.

Therefore, 100 kH while meeting economic design conditions.
By realizing an inverter circuit that operates at a high frequency of z or more, it is possible to make the power supply device smaller, lighter, and lower in cost, and it is possible to provide a power supply device that is significantly more compact and lower cost than conventional ones.

In addition, the configuration detects the reverse bias state of the semiconductor switch element and cuts off the output pulse of the output circuit, so even if the constant of the series resonant circuit changes greatly due to the temperature characteristics of the magnetron, capacitor, inductor, etc., it can be used reliably. A driving pulse having a time width between a half period and one period of the resonance period can be supplied to the semiconductor switching element having a self-commuting function, and in any case, the current flowing through the semiconductor switching element is substantially zero. It is possible to realize a power conversion circuit that can turn off the power supply in a single step, guarantee sufficiently high performance stability even against fluctuations in environmental conditions and manufacturing conditions, and make the power supply device smaller and more compact. Light weight, low cost, and compactness can be achieved.

Furthermore, by detecting the current flowing through the output rectifier diode of the step-up transformer, and controlling the oscillation frequency of the oscillator using the error signal between this detection signal and the set value, the output of the power conversion circuit is stabilized. It is possible to stabilize the magnetron's high-frequency output against power supply voltage fluctuations, etc., and it is also compact, lightweight, and
It is possible to provide a power supply device that has been significantly reduced in cost and size.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a high-frequency heating device showing an embodiment of the present invention, and Figures 2 (al, (b), and (C) are waveforms of collector current Ic of an insulated gate bipolar transistor of the same device, respectively. , a waveform diagram of collector voltage VCE, a waveform diagram of gate voltage VGH, and FIG. 3 is a circuit diagram of a more detailed embodiment of the reverse bias detection circuit 39 of the same device.
FIG. 4 is a circuit diagram 1 of a more detailed embodiment of the control section (control circuit) 29 of the device. FIG. 5 is a circuit diagram of a conventional high-frequency heating device, diagram (a). (b) and (C) are the gate voltage waveform diagram, collector current waveform diagram, and collector voltage waveform diagram of the insulated gate bipolar transistor of the same device, respectively. 8 is a detailed composite waveform diagram of the collector current and diode current of the insulated gate bipolar transistor, and a collector voltage waveform diagram. FIG. 8 is a characteristic diagram showing the relationship between the rise in the collector temperature of the transistor and the operating frequency in the same device. . 1.2...Power supply section (1...Commercial power supply,
2...Diode bridge), 7...
Semiconductor switch element, 18, 22...Series resonant circuit (18...Resonant capacitor, 22...
... step-up transformer), 22 ... step-up transformer,
25゜26...Diode, 29...
Control unit, 35... Oscillator, 36... Timer circuit, 37... Output circuit, 39...
. . . Reverse bias detection circuit, 40 . . . Diode current detection means, 41 . . . Error amplification control circuit.

Claims (3)

[Claims] (1) A power source obtained from a commercial power source, battery, etc.
A series resonant circuit consisting of a semiconductor switch element having a self-commuting function, a resonant capacitor, and a resonant inductor,
a step-up transformer that boosts the resonant voltage of the series resonant circuit and supplies it to the magnetron; and a control section that controls the semiconductor switch, the control section being an oscillator that oscillates at a frequency lower than the resonant frequency of the series resonant circuit. , a timer circuit that receives a signal from the oscillator and counts the conduction time of the semiconductor switch, and an output circuit that drives the semiconductor switch element with the signal from the timer circuit, and the conduction time is determined by the series resonant circuit. A high-frequency heating device configured to be longer than half a period and shorter than one period of a resonant current. (2) The high-frequency heating device according to claim 1, further comprising a reverse bias detection circuit for detecting a reverse bias state of the semiconductor switching element, and an output pulse of the output circuit is cut off by the output of the reverse bias detection circuit. (3) Provide a diode to rectify the step-up transformer output,
The rectified output of this diode is supplied to the magnetron, and the diode current detection means detects the current flowing through the diode, and the output of this diode current detection means is compared with a set value, and the error signal is used to oscillate the oscillator. and an error amplification control circuit that controls the frequency.
The high frequency heating device according to claim 1, wherein the oscillator is controlled so that the diode current reaches a predetermined value.