JPS625589A - High frequency heater - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to improvements in high-frequency heating devices for performing so-called dielectric heating such as microwave ovens, and more specifically,
The present invention relates to an improvement of a high-frequency heating device configured to use an inverter as the power supply device, generate high-frequency power by the inverter, boost the voltage with an external pressure transformer, and drive a magnetron.

BACKGROUND OF THE INVENTION Various configurations of high-frequency heating devices of this type have been proposed in order to reduce the size, weight, or cost of the power transformer.

FIG. 12 is a circuit diagram of a conventional high-frequency heating device. In the figure, power from a commercial power source 1 is rectified by a diode bridge 2 to form a unidirectional power source. 3 is an inductor, and 4 is a capacitor, which serves as a filter for the high frequency switching operation of the inverter.

The inverter includes a resonant capacitor 5, a step-up transformer 6, a transistor 7, a diode 8, and a drive circuit 9. The transistor 7 performs a switching operation at a predetermined period and duty (ie, on-off time ratio) by a base current supplied from the drive circuit 9. As a result, a current Icd centered around the collector current Ic and the diode current Idt as shown in FIG. 13(a) flows through the primary winding 10 of the boost transformer 76, as shown in FIG. 13(b).
) flows through the high-frequency current IL. Therefore, high frequency high voltage power and high frequency low voltage power are generated in the secondary winding 11 and the tertiary winding 12, respectively. This high frequency, low voltage power is supplied between the cathode terminals of the magnetron 17 via capacitors 13, 14 and choke coils 15, 16e, while the high frequency, high voltage power is supplied between the anode and cathode as shown. As shown in U.S. Pat. No. 4,318,165, capacitor 5 and magnetron 17 are shown in FIG.
When the t currents shown in c) and (d) flow, the magnetron 17 oscillates and dielectric heating becomes possible.

With this configuration, transistor 7 is operated at 20 KHz-1
When operated at a frequency of about 0.00KHz, the weight and size of the step-up transformer can be reduced from a few minutes to a dozen times less than when boosting the voltage at the commercial power frequency, making the power supply part smaller and lower in cost. It has the feature that it is possible.

In particular, the high-frequency heating device shown in U.S. Pat. No. 4,318.165 uses a so-called flybank converter circuit structure in which the polarities of the primary winding 10 and secondary winding 11 of a step-up transformer 6 are as shown in the figure. As a result, the magnetron 17 can be driven without using a high-voltage diode normally used for high-voltage rectification, and a high-frequency heating device as shown in FIG. 12 is realized.

Therefore, there is no need for a high-voltage, high-frequency diode that is very expensive and large in size, making it possible to further reduce the size, weight, and cost of the high-frequency heating device.

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

The anode current IA of the magnetron 17 is shown in FIG.
), the current waveform had to have a large peak value. This is because the converter is of the so-called flyback type, in which the energy stored in the primary winding 10 while the transistor 7 is conducting is released during the non-conducting period.

Further, since current flows through the magnetron only while the transistor 7 is non-conducting, the peak value of the anode current IA has to become even larger in order to obtain a predetermined average current.

For this reason, the emission capacity of the magnetron's cathode had to be increased, making the magnetron expensive. In addition, if the rising peak value of the anode current IA is large, the so-called moding phenomenon tends to occur due to the emission capacity margin of the cathode, which significantly shortens the life of the magnetron and increases the amount of radio wave leakage from the high-frequency heating device. These disadvantages limit the reduction in price of high-frequency heating devices and reduce their reliability.

Means for Solving the Problems The present invention has been made to solve the drawbacks of the conventional high-frequency heating apparatus, and is a high-frequency heating apparatus constructed by the means described below.

That is, an inverter including a semiconductor switch element such as a transistor, a resonant capacitor connected in series or in parallel to the semiconductor switch element, a drive circuit for driving the semiconductor switch element, and an inverter connected in series to the semiconductor Nutsch element. It is equipped with an IJ-cage type step-up transformer and a full-wave rectified DC power source obtained by rectifying the commercial power supply that supplies power to the inverter, and a magnetron and a high-voltage capacitor are connected in parallel to the secondary winding of the external pressure tone. and power supply voltage detection means for detecting the instantaneous voltage of the full-wave rectified DC power supply, and a signal from the power supply voltage detection means to reduce the conduction time of the semiconductor switching element at least near the maximum value of the instantaneous voltage of the commercial power supply. The present invention is configured to include instantaneous voltage correction means for controlling the drive circuit.

Function The present invention has the following functions based on the above structure. have That is, the high-frequency heating device of the present invention connects a high-voltage capacitor and a magnetron directly, that is, without using a rectifier, to the secondary winding of an IJ-cage step-up transformer that is connected in series to a semiconductor switch such as a transistor. In addition, since the configuration is such that an instantaneous voltage correction means is provided to control the conduction time of the semiconductor switch so that it decreases near the maximum value of the power supply voltage, the peak value of the anode current supplied to the magnetron can be reduced while eliminating the need for a high-voltage diode. The magnetron can be driven with a current waveform that suppresses the

In particular, when power is obtained from a full-wave rectified DC power source obtained by rectifying a commercial power source, the anode peak current near the maximum voltage can be suppressed, so when driving the magnetron without using a high-voltage diode. It is possible to prevent problems such as moding due to excessive anode peak current near the maximum voltage, which tends to occur in the case of a magnetron, and to drive the magnetron stably and safely.

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

FIG. 1 is a block diagram of a high-frequency heating device showing an embodiment of the present invention, and the same reference numerals as in FIG. 9 are corresponding components, and the explanation thereof will be omitted.

In FIG. 1, power from a commercial power supply 1 is sent to a full-wave rectified DC power supply 18 and supplied to an inverter 19. The inverter 19 consists of a semiconductor Nutsch 7, etc., and an external pressure transformer 6.
The magnetron 17 is energized to supply high voltage power to the magnetron 17. Transistor 7, transformer primary winding M10. The currents flowing through the resonant capacitor 5 and the magnetron 17 are shown in FIG. 2 (a>, (b), (C), and (
As shown in d), the anode voltage V of the magnetron 17
The AK becomes as shown in the same figure (a). This is external pressure transformer 6
This is because the polarity of the primary winding and secondary winding is as shown in the figure, the external pressure transformer 6 is a leakage type transformer, and the high voltage capacitor 13 is connected in parallel to the magnetron 17. It is something.

FIG. 3 is a circuit diagram of a high-frequency heating device illustrating the first main part of the embodiment of the present invention shown in FIG. 1, and the same reference numerals as in FIG. do.

In FIG. 3, the current IL of the step-up transformer 6.

The current ICH of the high-voltage capacitor 13 and the anode current IA flow as shown by the arrows in the figure because the polarities of the primary and secondary windings of the step-up transformer 6 are as shown in the figure.
Current flows through the Toki magnetron when the transistor is on.

FIG. 4 is a primary side equivalent circuit diagram of the circuit in FIG.
is the self-inductance of the primary winding 10, and K is the coupling coefficient between the primary winding and the secondary winding. The magnetron 17 can be replaced with a series circuit of a resistor RM, a diode DM, and a zener diode ZDM, and a capacitor CH is connected in parallel to this series circuit. As shown in FIG. 5, the characteristics of the magnetron are extremely nonlinear, and the dynamic impedance is extremely small. Therefore, by increasing the leakage of the transformer and making the leakage inductance (1-K)Ll larger than that of a normal conventional transformer, the impedance Zi seen from the secondary side of the transformer is increased and the transformer has constant current source properties. can be set.

Therefore, the resonance operation of the inverter 19 can be stabilized, and problems such as destruction of the transistor can be prevented, and the magnetron can be driven favorably.

In addition, by providing a leakage inductance and a capacitor CH, the anode current IA has a trapezoidal waveform without sharp peaks as shown in Figure 2(d), which suppresses cathode deterioration and moding, making it safe and reliable. This makes it possible to realize a high-frequency heating device with high performance.

Furthermore, when the magnetron is reverse biased, the capacitor CH prevents an abnormally high voltage as shown in Figure 6, which is generated in the secondary winding 11 due to stray capacitance, and reduces the reverse voltage as shown in Figure 2(e). This has the effect of suppressing the temperature to a relatively low value. Therefore, the 1-Ranno or magnetron may have a relatively low breakdown voltage, and can be manufactured at low cost.

Furthermore, by appropriately selecting the capacitance of the capacitor CH, the high voltage when the multiple magnetrons are reverse biased can be suppressed to a low value.

As shown in FIG. 7, there is a phase difference of 9σ between the magnetron current A and the current ICH of the capacitor CH.

Therefore, the current IL flowing through the transformer becomes a composite current of these when the magnetron is forward biased, while it becomes a current flowing through the capacitor CHON when the magnetron is reverse biased. FIG. 8 is a diagram for explaining this matter using a model. In the figure, the transformer current IL changes from ◯ to ILP at the time of positive and negative polarity voltages (only the absolute value is considered). This is because a transformer can be considered a constant current source.

Consider the case where capacitor CH has a certain capacitance value.

When the magnetron is forward biased, the operating point is from 0 to the solid line I in the figure.
Go to VAKO on CH, and when IA starts flowing, ICH+
Go on the IA line until IL=ILP and return to 0. Next, when the magnetron is reverse biased, the operating point is from 0 to the solid line I in the figure.
Go to IIL=ILP on CH and return to 0. Therefore,
The magnetron voltage VAK (transformer secondary voltage) is V
AK=VAK1. By the way, if the capacitor CH
If the capacitance value is smaller, the solid line CH becomes a dashed-dotted line lCH2. Therefore, the voltage VAK of the magnetron during reverse bias is VAK = VAK2 ) VA
KI,! : The larger the capacitance value of the capacitor CH, the smaller the voltage VAK'i of the magnetron can be. However, if the capacitance value of the capacitor CH is too large, a so-called beat phenomenon occurs between the inverter 19 and the resonance capacitor 5, and the operation of the inverter becomes unstable. Therefore, when the primary side equivalent capacitance value of the capacitor CH is CH1 and the capacitance value of the resonant capacitor 5 is 05, it is necessary that C5 and CHI are not close values. And (:'5 (because stable operation of the inverter cannot be expected with CHI, C3) it is necessary to be CHI.

As described above, the capacitor CH has extremely important functions and effects, and is very useful in realizing a high-frequency heating device that is low-cost, highly reliable, and can guarantee stable performance.

FIG. 1 will be explained again. In Figure 3! The pressure capacitor 13 is also used as a Magnelon filter capacitor in FIG. 1, and is the first and second high voltage capacitors 13 and 14. A capacitor 2o is provided between the cathode terminals of the magnetron, and choke coils 15 and 16 are bifilar-wound on the same core. Therefore both high voltage capacitors 13.1
The combined capacitance of 4 performs the function of the high voltage capacitor 13 in FIG. By doing this, high voltage can be supplied to the magnetron from both cathode terminals, so when driving the magnetron with high frequency voltage, the potential difference between the cathode terminals can be reduced and stable oscillation of the magnetron can be promoted. , has the effect of suppressing the generation of harmonics. Furthermore, since the high-frequency current can be shunted to the high-voltage capacitors 13, 14, heat generation in the high-voltage capacitors 13, 14 can be suppressed, and lower costs and higher reliability can be realized. Furthermore, by configuring the high voltage capacitors 13 and 14 as integrated feedthrough capacitors, they can also be used as the capacitor 20, making it possible to make the high voltage capacitor group more compact and lower in price.

Next, the second main part of the present invention will be explained with reference to FIG.

When the capacitance of the voltage VC4 of the capacitor 4 is small, the waveform becomes more like a full-wave rectification as shown in FIG. 9(a).

Therefore, when the output capacity of the magnetron 17 is large, the anode peak current IA becomes the voltage VO2 as shown in FIG.
That is, the permissible value IAP is exceeded near the maximum value of the commercial power supply voltage, resulting in problems such as occurrence of moding. Therefore, in the present invention, as shown in FIG. Instantaneous voltage correction means 22 is provided to control the voltage. The instantaneous voltage correction means 22 receives a waveform signal as shown in FIG.
), the conduction time ton (see Figure 10) of the transformer current 7 is controlled, and ton'
decrease. The relationship between this ton and the transformer output voltage VL is as shown in FIG. 11, so by such correction, 9
IA can be suppressed to be smaller than 91AP, as shown in FIG. 3(d), and the required output can be obtained without causing the above-mentioned disadvantages.

The specific configuration of the drive circuit 9, the power supply voltage detection means 21, and the instantaneous voltage correction means 22, which reduce the conduction time ton of the semiconductor switch near the maximum value of the power supply voltage, can be easily constructed using known techniques. Since these can be realized in many ways, the detailed configurations thereof will be omitted. for example,
Switching regulator I04 using general-purpose timer rc555 and monostable multivibrator etc.
This can be easily realized by using a configuration in which the operating time of the monostable multivibrator is corrected with respect to the power supply voltage using an operational amplifier or the like using a 94 and a monostable multivibrator.

Effects of the Invention As described above, according to the present invention, the following effects can be obtained.

It is a step-up transformer leakage type that is energized by a semiconductor switch of an inverter, and has a configuration in which a high-voltage capacitor and a magnetron are connected in parallel to the secondary winding, and an instantaneous room pressure correction means is provided to increase the maximum value of the commercial power supply voltage waveform. The structure reduces the conduction time of the semiconductor switch in the vicinity, so (1) the peak value of the magnetron's anode current can be kept small even when the power supply voltage has a full-wave rectification waveform, and the expensive high-voltage diode can be removed to make the magnetron It is possible to use a magnetron with an inexpensive structure, and it is possible to provide an extremely low-cost high-frequency heating device.

(2) Also, since it is possible to prevent the generation of abnormally high voltage when the magnetron is reverse biased, there is no need to make the magnetron's reverse withstand voltage performance or the step-up transformer's withstand voltage performance excessively high, and even more affordable high-frequency A heating device can be provided.

(3) Furthermore, since the rise of the anode current peak value can be suppressed to a small value, it is possible to prevent the occurrence of moding of the magnetron, suppress the increase in the amount of radio wave leakage, and prevent the deterioration of the cathode, thereby improving safety. Furthermore, it is possible to provide a high-frequency heating device with extremely high reliability.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a high-frequency heating device showing an embodiment of the present invention, and FIGS. 2(a), (b), (c), (d), and (
, ) are operating voltage and current waveform diagrams of the same circuit, Figure 3 is a circuit diagram for explaining the main parts of the circuit, Figure 4 is an equivalent circuit diagram of the main part circuit, and Figure 5 is the operating voltage of the magnetron. Current characteristic diagram,
FIG. 6 is an abnormal voltage waveform diagram of the magnetron, FIG. 7 is a current vector diagram of the magnetron and high-voltage capacitor, FIG. 8 is an explanatory diagram of the action of the high-voltage capacitor, and FIG. 9 is an explanatory diagram of the action and effect of the present invention. (,) is a voltage waveform diagram of capacitor 4, (b) and (d) are peak anode current waveform diagrams, (
C) is a characteristic diagram showing changes in the conduction time of the semiconductor switch 7, Fig. 10 is a definition diagram of the conduction time of the semiconductor switch 7, Fig. 11 is a relation diagram between the conduction time and the step-up transformer output voltage, and Fig. 12 is a conventional Circuit diagram of the high frequency heating device, Figure 13 (a
), (b), (c), and (d) are operating voltage and current waveform diagrams of the same circuit. 5... Resonance capacitor, 6... Step-up transformer, 7... Semiconductor switch (transistor), 9... Old drive circuit, 10... Next winding wire,
11... Secondary winding, 13... High voltage capacitor (first high voltage capacitor), 14... Second high voltage capacitor, 17... Magnetron, 18
・・・・・・Full wave rectification DC power supply, 19・・・Inverter, 20・Capacitor, 21・Power supply voltage detection means, 22・・Instantaneous voltage correction means , T
······Trance. Name of agent: Patent attorney Toshio Nakao and one other person wI
Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 L Figure 9

Claims (2)

[Claims] (1) An inverter comprising a semiconductor switch element such as a transistor, a resonant capacitor connected in series or in parallel to the semiconductor switch element, a drive circuit for driving the semiconductor switch element, and a drive circuit connected in series to the semiconductor switch element. a leakage-type step-up transformer, and a full-wave rectified DC power source obtained by rectifying the commercial power supply that supplies power to the inverter, and a magnetron and a high-voltage capacitor are connected in parallel to the secondary winding of the step-up transformer. , power supply voltage detection means for detecting the instantaneous voltage of the commercial power supply; and the drive circuit that reduces the conduction time of the semiconductor switching element at least near the maximum value of the instantaneous voltage of the commercial power supply by a signal from the power supply voltage detection means. A high-frequency heating device equipped with instantaneous voltage correction to control (2) The high-frequency heating device according to claim 1, further comprising a transformer for supplying power to the drive circuit, and a power supply voltage detection means on the secondary side of the transformer.