JPH0636395B2 - Induction heating cooker - Google Patents

Description

The present invention relates to an output control circuit of an induction heating cooker. More specifically, the conduction time of a switching element for performing high frequency oscillation is set to a commercial AC power supply frequency. The present invention relates to an output control circuit of an induction heating cooker that changes in synchronization.

(B) Conventional technology As a conventional method for controlling the output of this type of cooker, a direct current (pulsating current) obtained by full-wave rectifying a commercial AC power source is 20 to 5
A pulse width modulation method (hereinafter referred to as a PWM method) is generally used to obtain a desired output by varying the conduction time of a switching element in an inverter circuit that converts a high frequency current of 0 KHz. Sho 59-
It is disclosed in Japanese Patent Publication No. 9992.

(C) Problems to be Solved by the Invention In the PWM method as disclosed in the above-mentioned prior art, when the output level desired by the user is set, the conduction time of the switching element in the inverter circuit is assumed to be constant. High frequency oscillation is performed according to the rule. However, at this time, the voltage V _CE applied to the switching element is the full-wave DC voltage V whose envelope is applied to the inverter circuit.
_It is similar to _C, and V _CE is small near the zero cross of the AC input power supply, and V _CE takes a peak value near the peak. On the other hand, if the conduction time of the switching element is changed in order to increase the set output, the value of V _CE increases and there is a danger of exceeding the withstand voltage of the switching element. Therefore, the upper limit of the output obtained by the inverter circuit is mainly limited by the withstand voltage or current capacity of the switching element, and their peak values occur near the peak of the AC input. The safe conduction time was set, and near voltage zero crossing of the AC input, the operation was performed while leaving a large margin for both voltage and current, and efficient high output could not be obtained.

Therefore, according to the present invention, the operating state near the zero crossing, that is, the conduction time is changed within a region where the switching element has a margin without changing the load on the switching element in the vicinity of the peak of the AC input, and the efficiency is improved. It is intended to provide a cooker capable of obtaining high output.

(D) Means for Solving the Problems The present invention provides a commercial AC power supply in an induction heating cooker including a heating coil that includes a switching element for generating a high frequency alternating current and generates a high frequency alternating magnetic field to induction heat a load. A full-wave rectifier circuit for full-wave rectifying the power supply, a high-frequency generation circuit having the switching element, a heating coil, and a bypass capacitor, a drive circuit for driving the switching element, and a frequency equal to the frequency of the rectifier circuit. A reference wave generating circuit that generates a reference wave that has a minimum value near both peak values of the power supply voltage and a maximum value near the zero crossing of the power supply voltage, and the reference wave generated from the reference wave generation circuit approaches the maximum value. As the conduction time of the switching element is increased, the conduction time of the switching element is increased as the reference wave approaches the minimum value. And an output control circuit for controlling the drive circuit so as to shorten the output voltage.

(E) Action The reference potential for determining the conduction time of the switching transistor (8) is maximized near the zero crossing of the power supply (1) by making it correspond to the periodical change of the commercial AC power supply (1) voltage. ) Is a cosine-shaped waveform having a frequency twice as high as the frequency of the power source (1) so that it becomes minimum near both peak values.

(F) Example One example of the present invention will be described below with reference to FIGS.

(1) is a commercial AC power supply, (2) is a full-wave rectifier circuit, (3) is a high-frequency generator circuit, bypass capacitor (4), induction heating coil (5), resonant capacitor (6), damper diode (7)・
Switching transistor as switching element
It is composed of (8) and. A low voltage circuit (9) receives a commercial AC power source (1) and generates a DC low voltage (hereinafter referred to as V _DD ).

(10) is a reference generation circuit that inputs the heating coil (5) side end of the bypass capacitor (4), and the input end is a resistor (11) and a resistance (1
It is grounded via a series circuit with 2). Both resistors (11)
The connection point (P) of (12) is connected to an impedance element such as a field effect transistor (hereinafter referred to as FET) (13) gate, the source is grounded, and the drain is a resistor (14) for comparison as an output control circuit. Connect to the side input of the device (20). On the other hand, it is grounded from V _DD via resistor (15) -variable resistor (16) -resistor (17), and the set point of variable resistor (16) is the comparator.
Connect to the side input terminal of (20). This reference wave generation circuit (10) has a frequency twice that of the power supply (1),
It generates a reference wave that takes a minimum value for both peak values and takes a maximum value near the zero cross of the power supply (1).

(21) is an oscillator which picks up the voltage across the switching transistor (8) and generates a pulse having a frequency corresponding to the voltage, for example, a sawtooth wave as shown in FIG. 3a, the output of which is the side of the comparator (20). It is input to the input terminal. The output of the comparator (20) is input to the drive circuit (22), amplified, and input to the base of the switching transistor (8) of the high frequency generation circuit (3). Reference numeral (23) is a starting circuit for generating a starting signal for causing the high frequency generation circuit (3) to oscillate automatically. (twenty four)
Is a load that is induction-heated by supplying a high-frequency current to the heating coil (5).

The configuration of the present invention is as described above, and its operation will be described below.

First, by turning on the power switch (not shown),
Power (1) is supplied. At this time, it is assumed that an appropriate load (24) is placed on the top plate of the cooker.

Assuming that the setting by the variable resistor (16) in the reference wave generation circuit (10) is maximized within the setting range, the side input of the comparator (20), that is, the reference potential is set to the maximum within the setting range. On the other hand, since the voltage V _C across the bypass capacitor (4) is applied across the series resistors (11) and (12), the full-wave rectified waveform is divided, and a waveform similar to the rectified waveform appears at point P. Derived. Therefore, the gate input voltage of the FET (13) increases or decreases according to the change of V _C. And the drain of FET (13)
The impedance between the sources changes with the gate input voltage, the impedance with increasing gate input voltage decreases, and the impedance with increasing gate input voltage increases. Since the drain of the FET (13) is connected to the set point of the variable resistor (16) via the resistor (14), the FET (1
When the impedance of 3) changes, the reference potential changes. That is, a change in V _C causes a change in the drain-source impedance of the FET (13) to change the reference potential. The change is accompanied by a series of operations of increase (decrease) of V _C > decrease (increase) of impedance of FET (13)> decrease (increase) of reference potential, and the reference potential is near the peak value of V _C. It becomes the minimum and the maximum near the zero crossing of V _C. (However, the minimum value of the reference potential is set to be equal to the conventional reference potential.)
As shown in Figures a and 3b, bypass capacitors
The conduction time of the switching transistor (8) changes with the change in the voltage V _C across (4) (minimum (T ₁ ) when V _C reaches a peak, maximum (T ₂ ) when zero crossing occurs) The envelope of the voltage across the switching transistor (8), which is not similar to the full-wave rectified waveform as shown in FIG. The area surrounded by the envelope and the zero line, that is, the heating output is larger than in the conventional case.

Next, if the setting by the variable resistor (16) in the reference wave generation circuit (10) is minimized within the setting range, the side input of the comparator (20) is set to the minimum, and the reference potential waveform accompanying it is set. The minimum value of becomes the minimum within the setting range. The operation with the minimum output setting is performed by the same operation as described above, and the peak value of the collector-emitter voltage of the switching transistor (8) becomes the minimum within the setting range (Fig. 2).
The height of the trapezoidal shaped envelope in (d) is minimized).

If the setting by the variable resistor (16) in the reference wave generation circuit (10) is varied within the setting range, the minimum value of the reference potential based on the setting is determined, and the peak value of the collector-emitter voltage is also determined.

As described above, the potential used as the reference for determining the conduction time of the switching transistor (8) is changed at a frequency twice as high as that corresponding to the periodic change of the power supply (1) voltage. It is possible to obtain a good conduction time of the switching element, improve the heating output waveform, and realize highly efficient heating.

(G) Effect of the Invention The present invention is as described above, and the reference potential for determining the conduction time of the switching element for performing high frequency oscillation is maximized in the vicinity of the zero cross in correspondence with the periodic change of the commercial AC power supply voltage. Therefore, it is possible to determine the conduction time with high efficiency and to change the heating output waveform with high efficiency by periodically changing the frequency at twice the power supply frequency so that it becomes minimum near both peak values. Can be one.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of the present invention, FIG. 2 is a signal waveform diagram of essential parts of the present invention, FIG. 3a is an enlarged waveform diagram near the time point of FIG. 2A, and FIG. FIG. 4 is an enlarged view in the vicinity of time B in the figure, and FIG. 4 is an electric circuit diagram showing a conventional example. (1) …… Commercial AC power supply, (3) …… High frequency generator, (4)…
… Bypass capacitor, (5) …… Heating coil, (8) …… Switching transistor, (10) …… Reference wave generation circuit, (1
3) ... FET, (16) ... variable resistance, (20) ... comparator, (2
1) …… Oscillator, (22) …… Drive circuit.

Claims (1)

[Claims] 1. An induction heating cooker including a heating coil that includes a switching element for generating high-frequency alternating current and generates a high-frequency alternating magnetic field to inductively heat a load, and a commercial alternating-current power source and full-wave rectification of the power source. A full-wave rectification circuit, a high-frequency generation circuit having the switching element, a heating coil, and a bypass capacitor, a drive circuit driving the switching element, and a power supply voltage having the same frequency as the rectification circuit. A reference wave generation circuit that generates a reference wave that becomes minimum near the peak value and becomes maximum near the zero cross of the power supply voltage, and the conduction time of the switching element as the reference wave generated from the reference wave generation circuit approaches the maximum value. The drive voltage is set so that the conduction time of the switching element is shortened as the reference wave approaches the minimum value. Induction heating cooker is characterized in that an output control circuit for controlling the blanking circuit.