JPH02184273A - High-frequency heating device - Google Patents

Abstract

PURPOSE:To restrict a spike voltage and improve the reliability of the title device by connecting a capacitor in parallel to the transistor of an inverter circuit. CONSTITUTION:A high-frequency heating device is driven by changing the high AC voltage of a boosting transformer 20 into a high DC voltage through a voltage doubling rectifier 10 and impressing it on a magnetron 11. The device is provided with a resonance circuit 7 consisting of a resonant inductance 6 used both for the primary winding of said transformer 20, a resonant capacitor 5, and a transistor 8 switching in a driving circuit 13 by a resonance frequency. In this case, another capacitor 12 is connected in parallel to the transistor 8 to form said resonance circuit 7 by the capacitor 12.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a high frequency heating device having an inverter circuit.

Conventional technology The conventional technology will be explained below with reference to the drawings.

Figure 3 shows conventional high-frequency heating! FIG. 2 is a circuit diagram showing the circuit configuration of I@. In FIG. 3, a commercial power source 1 is converted into direct current by a rectifier 2, and is applied to a resonant circuit 7 through a filter composed of an inductor 3 and a capacitor 4. The capacitor 4 also serves to smooth the DC current having ripples. The resonant circuit 7 is made up of a parallel circuit of a resonant capacitor 5 and a resonant inductor 6, and the resonant inductor 6 also serves as the primary winding of the step-up transformer 20. The inductor 14 connected to the resonant circuit 7 is a parasitic inductance caused by wiring of components. The transistor 8 maintains the resonant operation of the resonant circuit 7 by switching at a resonant frequency determined by the values of the resonant inductor 6 and the resonant capacitor 5 of the resonant circuit 7. The drive circuit 13 of the transistor 8 supplies a signal to its base so that the transistor 8 switches at the resonant frequency of the resonant circuit 7. A diode 18 is connected in parallel between the collector of the transistor 8 and the emitter. The step-up transformer 20, in which the resonant inductor 6 of the resonant circuit 7 also serves as a primary winding, steps up the voltage of the resonant inductor 6, and generates an AC high voltage in its secondary winding Io9.

This AC high voltage is converted into a DC high voltage by the voltage doubler rectifier circuit 10, and this DC high voltage is applied to the magnetron 11 to drive the magnetron 11 and generate microwaves. This microwave is guided into the open chamber of the high-frequency heating H1if by a waveguide, and is supplied to the food in the oven to heat and cook the food.

Problems to be Solved by the Invention However, such high frequency heating devices have the following problems.

FIG. 4 shows the waveforms of the voltage VCE applied between the collector and emitter of transistor 8, the collector current 1c flowing through the collector, and the diode current flowing through diode 18 in the circuit shown in FIG. As shown in FIG. 4(b), the waveform of the collector voltage Vei is a resonant waveform, which is close to a sine wave. However, the moment the transistor 8 turns off, a spike voltage 5ffi is generated. This spike voltage S is very large, and therefore there is a problem in that the collector 8C of the transistor 8 breaks down withstand voltage.

The cause of this spike voltage S will be explained using FIG. 5. Figure 5 shows the capacitor 4 for Hiraisei, the capacitor 5 for resonance, the inductor 6 for resonance in the circuit diagram of Figure 3,
2 is a simplified circuit diagram with a parasitic inductor 14 and a transistor 8 extracted. FIG. At the moment when the transistor 8 is turned off, energy is stored in the resonant inductor 6, and during the period when the transistor 8 is turned off, a resonant current shown as a loop current 15 attempts to flow. However, due to component wiring, etc., a parasitic inductor 14 exists between the transistor 8 and the resonant circuit 7, and when the transistor 8 is on, current flows through the parasitic inductor 14 to the collector of the transistor 8. is flowing, so
At the moment when the transistor 8 is turned off, a spike voltage S shown in FIG. 4 is generated at the collector 8C&: of the transistor 8. This parasitic inductance 14 is caused by component wiring, etc., so it is very difficult to eliminate it completely.
There has been a problem in that it is very difficult to prevent spike voltages from occurring.

The present invention solves the above-mentioned g1rJi, and aims to provide a high-frequency heating device that does not cause voltage breakdown of transistors in an inverter circuit due to spike voltage.

Means for Solving the Problems In order to solve the above problem, the high frequency heating device of the present invention has the following features:
In an inverter circuit having a parallel resonant circuit of a resonant capacitor and a resonant inductor, and a transistor, a capacitor is provided in parallel with the transistor of the inverter circuit, and the capacitance value of the capacitor provided in parallel with the transistor is determined by the resonance This configuration is such that the capacitance value is smaller than the capacitance value of the resonance capacitor of the circuit.

Effect With the above configuration, in an inverter circuit that has a parallel resonant circuit of a resonant capacitor and a resonant inductor, and a transistor, a spike occurs at the collector of the transistor the moment the transistor is turned off due to the presence of a parasitic inductor due to component wiring, etc. The voltage is suppressed by a capacitor connected in parallel with the transistor, thereby preventing voltage breakdown of the transistor due to spike voltage.

Furthermore, by making the capacitance value of the capacitor provided in parallel with the transistor of the inverter circuit smaller than the capacitance value of the resonant capacitor of the resonant circuit, the capacitor can be used as a clearing capacitor for smoothing DC ripples. The regenerated high frequency current is reduced, and the high frequency loss of the smoothing capacitor can be made smaller than the high frequency loss of the resonance capacitor of the resonant circuit.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram of a high frequency heating device which is an embodiment of the present invention. In FIG. 1, a commercial power source 1 is rectified by a rectifier 2 and converted into direct current, and this direct current is passed through a low-pass filter consisting of an inductor 3 and a capacitor 4 to a resonant circuit 7 consisting of a resonant capacitor 5 and a resonant inductor 6.
is applied to

The capacitor 4 also serves to smooth the pulsating DC current rectified by the rectifier 2. Resonant inductor 6 is step-up transformer 2
It also serves as the primary winding of 0. The transistor 8 maintains the resonance of the resonant circuit 7 by switching at a resonant frequency fa determined by the resonant circuit 7. The step-up transformer 20 consists of a resonant inductor 6 as a primary winding and a secondary winding 9. The step-up transformer 20 boosts the voltage of the primary winding, that is, the resonant inductor 6, and applies an AC high voltage to the secondary winding 9. Occur. This AC high voltage is converted into a DC high voltage by an 8-voltage rectifier circuit 10, and this DC high voltage is applied to the magnetron 11 to drive the magnetron 11.

A capacitor 12 is connected in parallel to the transistor 8, and this capacitor 12 forms a resonant circuit 7 together with a resonant capacitor 5 and a resonant inductor 6, and the capacitance value C2 of the capacitor 12 is equal to the capacitance value C1 of the resonant capacitor 5.
It is considered to be a smaller value.

The resonant frequency fo of the resonant circuit having such a configuration is expressed by the following equation, assuming that the self-inductance of the resonant inductor 6 is 1. That is, the drive circuit 13 of the transistor 8 supplies a base signal for switching the transistor 8 at the above-mentioned resonant frequency. Parasitic inductor 1
4 is a parasitic inductor caused by component wiring,
It is extremely difficult to completely eliminate this parasitic inductor 14.

Next, the operation of the above configuration will be explained.

FIG. 2 is a simplified circuit diagram showing the flow of resonant current when transistor 8 is turned off. Resonant current flows in two ways. One flow is the loop current 15 flowing through the closed loop formed by the resonant capacitor 5 and the resonant inductor 6, and the other flow is the loop current 15 flowing through the closed loop formed by the resonant inductor 6 and the resonant inductor 6.
This is a regenerative current 16 that passes through the parasitic inductor 14 and the capacitor 72 and is regenerated to the smoothing capacitor 4. Energy is stored in the resonant inductor 6 until the transistor 8 is turned off, and when the transistor 8 is turned off, the energy stored in the resonant inductor 6 is transmitted to the resonant capacitor 5, resulting in a resonant operation. This is represented by loop current 15. In addition, the resonance inductor 6
Part of the energy stored in the parasitic inductor 1
4 and is transmitted to the smoothing capacitor 4 via the capacitor 12. This is represented by regenerative current 16.

As mentioned above, the resonant frequency fo of the resonant circuit 7 is expressed by equation (1), and the resonant inductor 6, the resonant capacitor 5, and the capacitor 12 are connected so that the resonant frequency fo becomes a frequency of 20 K tl z or more. The value of is determined. When the resonant frequency f becomes lower than 20 KHz, it enters the audible frequency range and the vibration sound of the step-up transformer 20 can be heard by human ears, so the resonant frequency fa of the resonant circuit 7 becomes 2.
0 is selected as a frequency of 82 or higher.

In the conventional case without the capacitor 12, as described above, a spike voltage is generated at the collector 8C of the transistor 8 at the moment the transistor 8 is turned off due to the parasitic inductor 14, which may destroy the transistor 8.

However, if a capacitor 12 is connected in parallel to the transistor 8 as shown in FIG. A loop is made to return to the smoothing capacitor 4 through the capacitor 4. Therefore, the spike voltage generated at the collector 8C of the transistor 8 by the parasitic inductor 14 is suppressed by the capacitor 12, and the voltage breakdown of the transistor 8 is prevented. However, by providing the capacitor 12, the regenerative current 16 is generated during the period when the transistor 8 is off.
flows into the smoothing capacitor 4, and this regenerative current 16 is
Resonant inductor 6 and resonant capacitor 5 of resonant circuit 7
, the resonant frequency determined by the size of the capacitor 12,
That is, it is a high frequency current with a frequency of 20 KHz or more. In this manner, since the high frequency regenerative current 16 with a frequency of 20 KHz flows through the smoothing capacitor 4 during the period when the transistor 8 is off, the high frequency loss of the capacitor 4 increases.

The ratio of the magnitudes of the loop current 15 and the regenerative current 16 when the transistor 8 is turned off is expressed by the following equation using the capacitance value C1 of the resonance capacitor 5 and the capacitance value C2 of the capacitor 12.

Therefore, if the capacitance value C2 of the capacitor 12 is increased, the loop current increases, so the smoothing capacitor 4
High frequency loss increases. Therefore, if the capacitance value C2 of the capacitor 12 is made larger than the capacitance value C1 of the resonance capacitor 5, the high frequency loss of the smoothing capacitor 4 will become greater than the high frequency loss of the resonance capacitor 5, which is not preferable. . However, the capacitance of the resonance capacitor 5 &! iCt and the capacitance value C of capacitor 12
2 is C1>02 (3), so the high frequency loss of the smoothing capacitor 4 can be made smaller than the high frequency loss of the resonance capacitor 5.

Effects of the Invention As described above, according to the present invention, since a capacitor is connected in parallel with a transistor in an inverter circuit, it is possible to suppress the spike voltage that occurs at the moment the transistor turns off due to parasitic inductors caused by component wiring, etc. As a result, voltage breakdown of the transistor can be prevented and reliability can be improved.

Furthermore, it is possible to suppress the abnormal voltage generated in the transistor due to the tube discharge of the magnetron, and it is also possible to prevent dielectric breakdown of the transistor, further improving reliability.

Furthermore, the relationship between the capacitance 11Cz of the capacitor connected in parallel with the transistor and the capacitance value C1 of the resonance capacitor is set to 0.
By setting 2<CI, the high frequency loss of the smoothing capacitor can be made smaller than the high frequency loss of the smoothing capacitor.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a high-frequency heating device according to an embodiment of the present invention, Fig. 2 is a simplified circuit diagram by extracting the main parts of the circuit diagram of Fig. 1, and Fig. 3 is a circuit diagram of a conventional high-frequency heating device. figure,
Figures 4g(a) and (b) are waveform diagrams showing the waveforms of the voltage applied to the transistor of the conventional high-frequency heating Iff and the flowing current, and Figure 5 is a simplified diagram with the main parts of the circuit diagram in Figure 3 extracted. FIG. DESCRIPTION OF SYMBOLS 1... Commercial power supply, 2... Rectifier, 4... Smoothing capacitor, 5... Resonant capacitor, 6... Resonant inductor, 7... Resonant circuit, 8... Transistor, 11... Magnetron, 12... Capacitor, 1
4... Parasitic inductor, 20... Valve pressure transformer. Agent Yoshihiro Morimoto Figure 1/-l Commercial power supply 2-Smooth duct 71 Just supply Σ4 B-tora〉jiλri11・-Magnetron fz-f〉te°′)′wa′ 14・Intercover with 20 Katana-6 tiger〉nu

Claims (1)

[Claims] 1. A voltage resonant inverter circuit that has a parallel resonant circuit of a capacitor and an inductor and a transistor to convert a commercial power source or DC power source into a high-frequency power source, a step-up transformer that steps up the output of this inverter circuit, and this step-up transformer. It includes a rectifier circuit that rectifies the output of the transformer to drive the magnetron, and a capacitor connected in parallel with the transistor of the inverter circuit, and the capacitance value of the capacitor connected in parallel with the transistor of the inverter circuit is determined by the parallel resonance. A high-frequency heating device whose capacitance value is smaller than the capacitance value of the circuit capacitor.