JPS62152374A - High frequency power source device - Google Patents

Abstract

PURPOSE:To obtain a stable high frequency power source, by controlling the ON-time of a switching element organizing an inverter circuit, shorter on the high wave of a power waveform and longer on the low wave. CONSTITUTION:Resistances 23-25 are set so that control pulse length may be made largest in a state that no input current flows, and the input of voltage obtained from the state is inputted to the non-inversion input terminal of a comparator 26. When an inverter is worked and input current flows, then it is detected by a current transformer 20, and the output voltage of a rectifier block 22 generates. The high potential side of the output resistance 23 is connected to the power source side, and so the potential of the dividing point of the resistances 23, 24 is lowered. As a result, the potential of the dividing point of the resistances 24, 25 drops, and the time width of ON-signal generated by the comparator 26 gets smaller.

Description

[Detailed Description of the Invention] Technical Field> The present invention converts a commercial power source into a rectified power source, further generates a high frequency power source from the rectified power source, and uses the high frequency power source to power the magnetron of a microwave oven, the heating coil of an electromagnetic cooker, etc. The present invention relates to a high frequency power supply device for driving a.

<Prior Art> Taking as an example a high frequency power supply device that drives a magnetron of a microwave oven, an outline will be explained based on the schematic electric circuit diagram shown in FIG.

A commercial power source 1 is rectified by a rectifier bridge 2 and smoothed by a choke coil 3 and a smoothing capacitor 4 to form a DC power source. Step-up transformer 5 for magnetron drive is connected to this DC power supply.
and connect a resonant capacitor 6 in series or parallel (in series in the figure) to this transformer 5 to form a resonant circuit,
An inverter circuit is constructed by connecting a diode 7 and a switching element (transistor) 8 to this resonant circuit.

The secondary side circuit of the step-up transformer 5 for driving the magnetron that constitutes the inverter circuit includes a magnetron 9 and a high voltage diode 10 for driving the magnetron 9.
, a high voltage capacitor 11 is connected, and a control circuit 12
optimally controlled by That is, the control circuit 12 detects the input voltage, the current of the step-up transformer 5 for driving the magnetron, the magnetron current, etc., and outputs control signals such as output limiting to the drive circuit 13 based on the results. The switching element 8 of the circuit is turned on and off for optimum control.

Through the operation of this inverter circuit, power is supplied to the secondary side of the step-up transformer 5, causing the magnetron 9 to oscillate, and high-frequency heating output is obtained.
It is controlled by the ON signal time (the OFF signal time is controlled to be substantially constant), and the longer the ON signal time, the greater the high frequency heating output.

FIGS. 7(a) and 7(b) are waveform diagrams showing the VCE (collector emitter voltage) and IC (collector current) characteristics of the switching element (transistor) 8 during operation of the inverter circuit.

Conventional inverter circuit control involves detecting the input current, heating coil current, etc., rectifying and smoothing it, and using the output to create a control signal to control the inverter circuit.

When we look at the inverter status during one cycle of the commercial power supply using this kind of control, the signal created by rectification and smoothing changes more slowly than the one cycle of the power supply, and it approaches the zero cross of one voltage of the power supply. The length of the ON signal near the voltage peak is also almost the same. Therefore, in the inverter circuit, the capacitance of the smoothing capacitor 4 is reduced to improve the power factor, so an operating voltage VCE (voltage between the collector and emitter of the transistor 8) is generated that is approximately proportional to the power supply voltage, and an envelope proportional to the power supply voltage is generated. It becomes a line. The same applies to the magnetron current, and the peak current is high and moding is likely to occur, making the inverter circuit unstable. Further, the operating voltage fluctuates greatly with respect to the power supply voltage fluctuation, and the control response is slow. The operating voltage VCE generated during inverter operation is a high voltage, and it has poor response to sudden fluctuations in the power supply voltage, so the operating voltage becomes even higher and the switching element 8 may be destroyed. The element 8 must also have a high withstand voltage.

In addition, since the magnetron 9 has low impedance during oscillation, in order to obtain stable output, it is necessary to supply the magnetron drive circuit with an inverter output that is sufficiently stable against fluctuations in the input power supply. The invention has been made in view of the above, and aims to enable the use of low voltage switching elements and obtain a stable high frequency output power source by stabilizing the circuit overnight.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. 1st
The figure is a schematic electric circuit diagram showing an example of inverter circuit control according to the present invention. The state of the input current of the inverter circuit is detected by a current transformer (current transformer) 20, noise is removed by a capacitor 21, and rectified. Full-wave rectification is performed in block 22,
It is output to the resistor 23. The resistor 23 is connected in series with the resistor 24.25 to divide a constant voltage (+V) by t, and the voltage division point of the resistor 24.25 is connected to the non-inverting input terminal of the comparator 26.

On the other hand, the operating state of the inverter circuit (switching element 8
A timing circuit 28 extracts a timing signal necessary for continuing oscillation of the inverter circuit from the signal of the inverter operating state detection circuit 27 which detects the voltage VCE). In synchronization with the timing signal, the sawtooth wave generating circuit 29 generates a "sawtooth wave" as shown in FIG. 2(a), which is input to the inverting input terminal of the comparator 26. In the comparator 26, the signal from both input terminals causes the signal shown in FIG.
This 0N- generates an 0N-OFF signal as shown in
The OFF signal becomes the control signal for the inverter circuit, is amplified by the drive circuit 13 so as to operate the switching element (transistor) 8 of the inverter circuit, and is applied to the base of the transistor 8. The 0N-OFF signal generated by the comparator 26 The ON length of the inverter circuit contributes to the output and operating voltage of the inverter circuit, and the longer the ON signal time, the larger the output and operating voltage become. This O
The time width of the N signal is determined by the voltage input to the non-inverting input terminal (+ terminal).

In this embodiment, the resistors 23, 24 and 25 are set so that the maximum control pulse width is achieved when no input current flows, and the voltage input to the non-inverting input terminal is synchronized with the power supply as shown in Figure 3. Varies depending on

When the inverter operates and input current flows, the current transformer 2
0 is detected and the output voltage of the rectifier block 22 is generated,
Since the circuit time constant is small, there is almost no delay with respect to the power supply cycle. Since the high potential side of the output resistor 23 is connected to the power supply side, the potential at the voltage dividing point of the resistors 2, 3, and 24 decreases. As a result, the potential at the voltage dividing point of the resistors 24 and 25 also decreases, so that the width of the ON signal generated by the comparator 26 becomes narrower.

If this is followed over time, as shown in FIG. 4, the ON signal width is wide near the zero cross where the voltage is low, becomes narrower as the voltage increases, and becomes the minimum width near the peak voltage. After that, it widens as the voltage decreases, reaching its maximum width near the zero cross. The pulse width changes from maximum to minimum to maximum, but since the relationship between the input current and the pulse width is in a feedback state, the input current settles at a current value determined by the relationship with the pulse width.

FIG. 5 shows the VCE envelope waveform and magnetron current envelope waveform of the transistor 8 when the control method using such an input current waveform is compared with the conventional control method. Compared to the conventional control method (dotted line A) in which the pulse width does not change near the zero crossing of the input voltage and near the peak voltage, the peak voltage decreases as shown by the solid line, and the low voltage area increases, and the input current increases. The more the shape increases, the closer it becomes to a trapezoid.

The reduction in this peak voltage makes it possible to lower the withstand voltage of the conventionally used switching element 8. Also, when comparing the magnetron current, if it is controlled so that A = a + b as shown in the figure, the peak current will decrease even if the output is the same, and stable oscillation of the magnetron 9 will be achieved. It will be done.

Although the above embodiment is an application of the present invention to a microwave oven, similar effects can be obtained when the present invention is applied to an electromagnetic cooker having a configuration in which the transformer 5 is replaced with a heating coil. Input current and operating voltage comparison is the fifth
This is similar to the collector-emitter voltage envelope comparison shown in the figure.

In the embodiment, control is performed by detecting the input current, but control may also be performed by detecting the envelope of the coil current and the operating voltage VCE of the transistor 8.

In addition, similar control is possible not only for the input current but also for the input voltage waveform, and if the control circuit is equipped with a microcomputer, it is possible to have a constant for the input voltage and sequentially detect the input voltage. However, control can also be performed by varying the control pulse width.

Furthermore, the present invention is applicable to the above-mentioned microwave oven.

It goes without saying that it is not limited to electromagnetic cookers.The present invention includes a rectifier circuit that rectifies a commercial power source into a rectified power source, an inverter circuit that converts the rectified power source into a high frequency power source, and a commercial power source waveform or rectified power source. The inverter is equipped with a power waveform detection circuit that detects the power waveform and a control circuit that controls the ON time of the switching elements constituting the inverter circuit so that it is short when the power waveform is high and becomes long when the power waveform is low. It is possible to obtain a stable high-frequency power supply by lowering the peak voltage of the high-frequency power supply that is the output of the circuit and increasing the bottom voltage, and it is also possible to use low-voltage switching elements that make up the inverter circuit, which reduces costs. It is remarkable that it can measure down.

[Brief explanation of drawings]

Fig. 1 is a schematic electric circuit diagram showing an example of inverter circuit control of the present invention, Fig. 2 (a') and (b) are input and output waveform diagrams of the comparator, and Fig. 3 is the comparator ■ terminal input Figure 4 is a diagram showing the relationship between the VCE envelope of the transistor and the control signal. Figure 5 is a waveform diagram showing the vCE envelope waveform and magnetron current envelope waveform of the transistor. FIG. 6 is a schematic electrical circuit diagram of a microwave oven, and FIGS. 7(a) and 7(b) are waveform diagrams showing VCE and Ic characteristics of a transistor during operation of an inverter circuit. Code 1: Commercial power supply, 2: Rectifier bridge, 8 Niswitching elements, 12: Control circuit, 2o: Current transformer, 26:
Comparator. Agent Patent attorney Aihiko Fuku (2 others) Figure 1 Figure 2 Figure 1s3

Claims (1)

[Claims] 1. A rectifier circuit that converts a commercial power source into a rectified power source, an inverter circuit that converts the rectified power source into a high-frequency power source, and a power waveform detection circuit that detects a commercial power source waveform or a rectified power source waveform.
Turning on the switching elements constituting the above inverter circuit
A high-frequency power supply device comprising a control circuit that controls the time so that it is short when the power waveform is a high wave and long when the power waveform is a low wave.