JPS5878384A - Induction 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 The present invention relates to an induction heating device for heating food in a cooking pot, for example, by fully utilizing a high frequency magnetic field to heat the cooking pot by induction.In general, this type of induction heating device In this case, the heating coil and the capacitor are connected directly to each other.
In addition to forming a resonant circuit (or in parallel), this resonant circuit is connected to an inverter circuit consisting of a large current transformer, and this inverter circuit is driven by the entire drive circuit to cause oscillation, thereby generating a high frequency signal in the heating coil. A high-frequency magnetic field is generated by passing an electric current, and this magnetic field is applied to the load to be heated, such as a cooking pot made of magnetic material, to generate eddy current noise.The eddy current loss causes the entire cooking pot to self-heat. Cooking↓The sea urchin is ready.

However, in conventional induction heating devices like this one, they are not initialized even when the power is turned off, so depending on the amount of charge in the capacitor when the current starts flowing, the transistor may be turned on, causing a momentary Due to the large voltage generated in It is an object of the present invention to provide an entire induction heating device which can be initialized so that a large voltage is not instantaneously generated when starting energization, can prevent destruction of transistors, and can improve reliability.

Hereinafter, an embodiment of the present invention will be described with reference to FIG. mlk.

In FIG. 1, an AC power source (not shown) is connected between input terminals 1 and 2, and a rectifier circuit 3 is also connected.

A smoothing capacitor 4 is connected between DC output terminals P (positive) and N (negative) of this rectifier circuit 3. This capacitor 4 is set to a small value in order to prevent the power factor from worsening, and its voltage value is close to pulsating current, and the voltage is several volts at the trough. An mth lance 6 for detecting the current of the AC power source is inserted into the connection line 5 of the input terminal 2. t7j.

One end of a heating coil 7 is connected to the output end P of the rectifier circuit 3, the other end of this heating coil 7 is connected to the collector of an NPN transistor 8, and the emitter of this transistor 8 is connected to the output end of the rectifier circuit 3. Connected to N. A capacitor 9 and a damper diode 1o with the polarity shown are connected between the collector and emitter of the transistor 8, which together with the heating coil 7 form a resonant circuit. 10, the high frequency oscillation circuit 11 is completely formed.

On the other hand, the power supply circuit 121J converts the sound of an AC power source (not shown) into a DC power source and outputs the same. A feedback circuit 13 is connected between the output terminals a and b of the power supply circuit 12, and the input terminal of the feedback circuit 13 is connected to the connection point between the power 1 heating coil 7 and the collector of the transistor 8. The feedback circuit 13 is
In response to the signal generated by the on-off operation of the transistor 8, a full signal tuned to the resonant frequency of the heating coil 7 and the capacitor 9 is outputted. The output terminal of the feedback circuit 13 is connected to the input terminal of an oscillation circuit 14, and both power supply terminals of the oscillation circuit 14 are connected to the output terminal a of the power supply circuit 12.

connected to b. The oscillation circuit 14 generates a sawtooth wave sound synchronized with the resonance frequency of the heating coil 7 and the capacitor 9.

Further, the output terminal a of the power supply circuit 12 is connected to the base of an NPN transistor 16 via a resistor 1.5, and also to the collector of an NPN transistor 17. The emitters of the transistors J6 and 77 are connected to the output terminal of the power supply circuit 12. The pace of the transistor 17 is controlled by the capacitor 4 through the resistor 18.
The initialization circuit 20 is connected to the heating coil 7 and the reading point, and is connected to the output terminal of the power supply circuit 12 via the resistor 19f. has been done. This initialization circuit 20
1 is used to detect a drop in the voltage value applied to the heating coil 7 and discharge the charging voltage of the capacitor 22, which will be described later. For example, when a voltage drop is not detected, the transistor 17 is turned on. When the transistor 16 is off and a voltage drop is detected, the transistor 17 is turned off and the transistor 18 is turned on. A series circuit of a resistor 21 and a capacitor 22 is connected between the output ends fi and b of the power supply circuit 12.

The connection point between the resistor 21 and the capacitor 22 is connected to the output terminal of the initialization circuit 20, that is, the collector of the transistor 16, and is also connected to the output terminal via a resistor 23. Further, the output terminal of an output control circuit 33, which will be described later, is connected to the connection point between the resistor 21 and the capacitor 22 via a resistor 24, and the non-inverting input terminal of a comparator 25 is connected via a resistor 265. The output terminal of the oscillation circuit 14 is connected to the inverting input terminal of the comparator 250. Here, a pulse width control circuit 27 is constituted by resistors 21, 23, 24° 26, a capacitor 22, and a comparator 25.

The output end of the pulse width control circuit 27, that is, the output end of the comparator 25, is connected to the input end of the drive circuit 28. This drive circuit 28 controls the transistor 8 on and off by repeatedly outputting the at signal, the 71 signal, and the "0" signal in accordance with the supplied signals. That is, the emitter of the PNP transistor 29 is connected to the output terminal a, and the collector is connected to the collector of an NPN transistor 32 via a resistor 30 and a diode 31 having the polarity shown. It is configured to be connected. The base of the transistor 8 is connected to the output end of the drive circuit 28, that is, the connection point between the resistor 3o and the diode 3.

An output control circuit 33 is connected to the electric SL transformer 6, and a volume 34 operated by a knob (not shown) is connected to the output control circuit 33. The output control circuit 33 is a circuit that compares the input current detected in the electric SL transformer 6 with the output of the volume controller 34 and outputs an appropriate DC voltage. It is connected to the connection point of the capacitor 32.

Next, in this simple configuration, Figs. 2(a) to (d)
For example, suppose that a magnetic cooking pot containing cooked food is to be heated as an object to be heated. Then, when a power source (not shown) is turned on, direct current is supplied from the rectifier 3 to the heating coil 7. At this time, a DC current is also output from the power supply port W812, so that the oscillation circuit 1
A sawtooth wave is generated from 4. Also, at this time, the voltage of the capacitor 4 is ↓ Since the retransistor 17 is turned on and the transistor 16 is turned on, the capacitor 22 is charged. A comparison voltage t1 is set at the non-inverting input voltage of the comparator 25. Tori, comparator 25 is the second
As shown in Figure (b), the output of the oscillation circuit No. 4 is at voltage t1.
↓When the voltage is smaller than 1+, a ``1#'' signal is output, and when the voltage is greater than 1 (time t2), a ''0# signal is output. Then, when the comparator 25 outputs the "1# signal," the transistor 29 is off and the transistor 3 is turned off.
2 is turned on. In addition, when a 0" signal is output from the drive circuit 28, the transistor 8 is turned off. Furthermore, when a 0" signal is output from the comparator 25, the transistor 29 is on and the transistor 32 is turned off. When the signal becomes 0, the @t and $7 signals are outputted from the drive circuit 28, thereby turning on the transistor 8. As a result, a load current flows through the heating coil 2. As a result,
A charging current 11 flows through the capacitor 9 in a resonant circuit formed by the heating coil 7 and the capacitor 9, and when this current 11 finishes flowing, a discharging current I2 flows as a countercurrent.

By continuing the same operation as above repeatedly,
A signal with a predetermined period is supplied from the drive circuit 28 to the base of the transistor 8, so that the transistor 8 is turned on and off accordingly, and the high frequency oscillation circuit 11 oscillates at a predetermined frequency. In addition, a load current flows through the heating coil 1 by continuing to flow in the positive and negative directions alternately, and a high-frequency magnetic field is generated from the heating coil 7, which is applied to the magnetic cooking pot. . Therefore, eddy current loss occurs in the magnetic cooking pot, and the cooking pot self-heats due to the eddy current loss, and the food in the cooking pot is cooked.

By the way, when the power switch (not shown) is turned off,
The voltage across capacitor 4 drops rapidly due to the action of transistor 8. Then, when the value of capacitor 4 becomes less than several volts, transistor 17 is turned off, and transistor 1
8 turns on. In addition, the voltage of the capacitor 22 in the pulse width control circuit 27 is discharged and becomes zero potential.When the power switch is turned on again, the capacitor 22 is at zero potential, so the pulse width control circuit 27 will not become unstable.

In this way, when the power is cut off, the initialization circuit detects the voltage drop in the power supply voltage of the transistor, which rapidly discharges its charge completely, and in response to this detection signal, the charge of the capacitor in the pulse width control circuit is fully discharged. ↓, the charge in the pulse width control circuit is discharged, or initialized.

In the above embodiment, the initialization is performed to completely discharge the charge in the pulse width control circuit when the power is cut off. However, the present invention is not limited to this. It may be initialized by doing this.

As detailed above, according to the present invention, when the power is turned off, the charge inside the device is reliably initialized, so that it is possible to eliminate the momentary generation of a large voltage when the power is turned on. , XP can also prevent transistor breakdown and provide an induction heating device with improved reliability.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention; FIG. 1 is an electric circuit diagram showing the overall configuration, and FIGS. 2(a) to (d) are signal waveform diagrams of important parts to explain the operation. 0 3... Rectifier, 4... Capacitor, 7... Heating coil, 8... Transistor, 11... High frequency oscillation circuit, '12... Power supply circuit, 14... Oscillation circuit , 16
．． 17, j...transistor, 20...initialization circuit, 22...
- Capacitor, 25... Comparator, 27... Pulse width control circuit, 28... Drive circuit.

Claims (1)

[Claims] A DC power source, a heating coil connected to this power source to inductively heat an object to be heated, a transistor connected in series to this heating coil, and a transistor connected in parallel to this transistor to form a resonant circuit with the heating coil. It comprises a capacitor, a drive circuit that controls on/off of the transistor according to the output of the oscillation circuit, and an initialization circuit that detects a drop in the voltage applied to the heating coil, and an output signal of the initialization circuit. An induction heating device characterized in that a charging voltage in a control circuit that controls the drive circuit is completely initialized by the following.