JPS6394589A - Control circuit of induction heating cooker - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to an induction heating cooker, and particularly to a control circuit thereof.

(B) Conventional technology Conventionally, this type of cooking appliance uses a transistor as a switching element and compares the collector voltage of this transistor with zero voltage in order to determine the turn-on timing of a semiconductor switching element to cause the inverter circuit to self-oscillate. There were some that obtained the timing by comparing the collector voltage and the DC power supply voltage. The former is disclosed as a conventional example in Japanese Patent Publication No. 58-36473, and the latter is disclosed as an embodiment in Japanese Patent Publication No. 5B-36473.

(c) Problems to be Solved by the Invention In the above-mentioned conventional technology, the envelope of the collector voltage of the switching transistor is synchronized with and similar to the waveform obtained by full-wave rectification of the AC power supply. In the former case, when the oscillation frequency is increased, the I-rector voltage does not drop to zero voltage, and even when compared to zero voltage, the on timing of the switching transistor cannot be obtained, and a period of non-oscillation occurs. Therefore, there was a problem that stable oscillation could not be performed.

On the other hand, in the latter case, in order to improve the former and expand the frequency variable range, the on-timing is obtained by comparing the collector voltage and the DC power supply voltage. When the full-wave rectified DC power supply voltage suddenly changes due to external noise such as a lightning surge, the voltage to which the collector voltage is compared changes, causing the oscillation waveform to be distorted and the switching transistor to This may increase loss or create harsh conditions for switching transistors.

Therefore, the present invention provides an induction heating cooker that maintains stable oscillation without being affected by external noise and has an expanded frequency variable range.

(d) Means for Solving the Problems The present invention solves the above-mentioned problems, and will be explained below based on examples.

This device inputs the collector voltage of a switching transistor 7) in an inverter circuit (3) having a heating coil 4) that generates a high-frequency alternating magnetic field and inductively heats a load, and allows the DC component of this collector voltage to pass through. It has a DC blocking capacitor (15) that blocks DC current, and the signal via this capacitor <15) is input to the input terminal on the ○ side, and is unaffected by fluctuations in the commercial AC power supply (1) and contains zero voltage. A comparator (17) that inputs a constant voltage signal near zero voltage to the ■ side input terminal.
), and an on-timing detection circuit (11) that generates a timing pulse is provided. Then, upon receiving the timing pulse from the on-timing detection circuit (11), a sufficiently amplified pulse is input to the base of the transistor (7) in synchronization with the rise of the timing pulse and to control the drive of the transistor (7). A pulse generation circuit (12) is provided. Then, a control circuit (9) is formed by an on-timing detection circuit (11) and a pulse generation circuit (12).
Configure.

(E) Effect The collector voltage of the switching transistor (7) is made into a voltage signal containing only AC components via the DC blocking capacitor (15), so that it crosses zero voltage reliably. Since the input signal on the ■ side of the device (17) is not affected by fluctuations in the AC power supply (1), fluctuations due to external noise are eliminated and timing pulse transmission is made stable.

(f) Example An example of the present invention will be described below with reference to FIGS. 1 to 7.

(1) is a commercial AC power supply, (2) is a rectifier circuit that full-wave rectifies the AC power supply (1), and (3> is an induction heating circuit when the output from the rectifier circuit (2) (output of terminal A) is applied. Coil capacitor 4〉・Resonant capacitor 5)・Damper diode (
6)・Includes a switching transistor (7), and the switching transistor (7) is controlled by a control circuit (9) to be described later.
) is an inverter circuit whose on/off is controlled,
Using the alternating magnetic flux generated by high-frequency current flowing through the heating coil (4) in the inverter circuit (3), a magnetic material made of magnetic material placed on the top plate (not shown) is used. The device heats a manufactured load (not shown) by induction.

(8) is connected to the commercial AC power supply (1) and performs step-down, rectification, and smoothing to generate a constant voltage near zero voltage, including zero voltage, which is less affected by fluctuations in the AC power supply (1). It is a DC power supply, and its output (output from terminal B) is input to the control circuit (9). (10) is the inverter circuit (3).
This is a starting signal generation circuit that generates a starting signal synchronized with the rise of the voltage of the AC power supply (1) in order to start oscillation by the AC power source (1).

The control circuit (9) inputs the constant voltage (including zero volts) from the DC power supply (8) and the collector-emitter voltage of the switching transistor (7) in the inverter circuit (3), and outputs the switching transistor from both of them. (7
) is obtained, and a pulse is sent out based on that timing, and this pulse is input to the base of the switching transistor <7).

FIG. 1 shows an example of the control circuit (9), which is composed of an on-timing detection circuit (11) and a pulse generation circuit (12), and the on-timing detection circuit (11) is connected to the collector voltage of the switching transistor (7). Terminal CD to get
A series circuit of a resistor (13) and a resistor (14) is connected between them, and a capacitor (15) and a resistor (14) are connected in parallel to the resistor (14).
Connect the series circuit with 6). And the capacitor (1
5) and the connection point (P) between resistor (16)? Comparator (17)
In addition to connecting to the input terminals of ○, the diode (18)
is connected to the ground through the diode (19) in the forward direction, and connected to the output terminal B (voltage is set to VDD) of the DC power supply (8) through the diode (19) in the forward direction. On the other hand, the input terminals of the comparator (17) are grounded through a resistor (20) and connected to VOO through a resistor (21). The output terminal (E) of this comparator (17) is read directly to the pulse generating circuit 12). The pulse generation circuit (12) synchronizes with the output of the comparator (17), that is, the rise of the timing pulse, and amplifies the transistor (7) sufficiently to control the drive of the switching transistor (7). The on-pulse is generated from the output terminal F toward the base of the transistor (7>).

The capacitor (15) used is a DC blocking capacitor that blocks the passing of the DC component of the input signal and allows only the AC component to pass.

Next, the operation based on this example will be explained. However, the explanation will be made assuming that a power switch (not shown) is turned on with an appropriate load placed on the top plate. When the power switch is turned on, a start signal is generated by the start signal generation circuit (10), and the oscillation of the inverter circuit (3) can be started. In order to continue this oscillation, the control circuit (9>) functions. That is, the oscillation of the inverter circuit (3) changes the voltage between the collector and emitter of the switching transistor (7). This voltage is turned on. The timing detection circuit (11) detects the voltage, and the resistors (13) and (14) step down the voltage (see Figure 3 (d)), and the capacitor (15) cuts the DC component, leaving only the AC component (see Figure 3(d)). Figure 5 (
a)). However, this AC component is connected to a diode (18
), the negative part is clipped and the comparator (17)
At , it is compared with a reference potential determined by resistors (20) and (21) to obtain on-timing. Figure 5 shows a waveform diagram of the input/output voltage relationship of the comparator (17).
17), zero volt is adopted as the reference potential. However, this reference potential is not limited to zero volts, and the circuit described above selects the reference potential to be around zero volts. Then, the pulse generating circuit (12) generates a pulse synchronized with the rising edge of the output of the comparator (17), turning on the switching transistor (7) in the inverter circuit (3). This on-state retention period is determined by changing the time constant in a pulse width determining section in the pulse generating circuit (12) by operating an output control section (not shown) to determine the on-pulse width. That is, by lengthening this on-pulse width (equivalent to lowering the oscillation frequency), the peak value of the current flowing through the heating coil (4) increases, and the heating output increases accordingly. Conversely, when the on-pulse width is shortened, the heating output is reduced. In addition, the collector
When the emitter voltage is made up of only alternating current components and the on-pulse timing is obtained, the timing is faster than the conventional one, and the switching transistor (7)
In order to prevent an on-pulse from inputting and increasing loss when the collector voltage is excessive, a resistor (
It is desirable to insert a phase adjustment capacitor in parallel with 16).

When the load placed on the top plate is removed, the load detection circuit (not shown) is activated and the pulse generation circuit (1
The self-oscillation of the inverter circuit (3) is stopped by not sending out the pulses for automatic oscillation (2). At this time, the collector of the switching transistor (7)
The emitter voltage is the charging potential of the resonant capacitor (5) (
(about 140 V) (see FIG. 3(c)), and the DC component of this voltage is cut off by the capacitor (15) and becomes zero potential (see FIG. 7(a)). Therefore, it is not possible to obtain a timing pulse that reaches the on-timing detection circuit (11), no on-pulse is generated, and the oscillation of the inverter circuit (3) is stopped.

The present invention is as described above, and the collector-emitter voltage of the switching transistor (7) is controlled by the capacitor (1).
By cutting the DC component through step 5) and leaving it with only the alternating current and nine components, it is possible to reliably generate the zero volt intersection and obtain the timing for sending the on-pulse of the switching transistor (7>). By ensuring the timing of on-pulse generation, the output controllable range can be expanded and stable oscillation can be maintained at all times. Since the comparison target voltage is obtained from a constant voltage source that has little influence on external noise,
The oscillation is not disturbed by external noise and the loss due to the switching transistor (7) is not increased.
harsh conditions for the switching transistor (7)
Don't give the situation away.

(g) Effects of the Invention The present invention reliably generates a zero-volt crossing point by cutting off the DC component of the voltage between the collector and emitter of the switching transistor through a capacitor and leaving only the AC component. On the other hand, what can be compared with this AC component?
Since tJIE is introduced from a DC power supply that generates a constant voltage that is hardly affected by zero volts or sudden changes in the AC power supply, there is almost no influence from external noise and stable on-pulse generation timing can be obtained. This makes it possible to expand the output control range and realize stable oscillation.

[Brief explanation of the drawing]

Each figure shows an embodiment of the present invention. Figure 1 is an electric circuit diagram of a control circuit, Figure 2 is a schematic circuit diagram of an induction heating cooker,
Figure 3 is a diagram of main signal waveforms, Figure 4 is an enlarged view of section X in Figure 3, Figure 5 is an input/output waveform diagram of the comparator in section X of Figure 3, and Figure 6 is a diagram of the input and output waveforms of the comparator in section X of Figure 3. FIG. 7, an enlarged view of the Y section, is an input/output waveform diagram of the comparator in the Y section of FIG. (1)... AC power supply, (3)... Invar 4 circuit,
(4)...Heating coil, (7>...Switching transistor, (8)...DC power supply, (9)...
Control circuit, <11>...On-timing detection circuit,
(12)...Pulse generating circuit, (15)...Capacitor, (17)...Comparator. Figure 2 Figure 6 Figure 7

Claims (1)

[Claims] 1. An induction heating cooker that is equipped with an inverter circuit that includes a switching transistor for generating high-frequency alternating current and has a heating coil that generates a high-frequency alternating magnetic field to inductively heat a load, and inputs the collector voltage of the transistor. The on-timing detection circuit generates a timing pulse by a comparator whose one input is a signal via a capacitor that blocks the passage of a DC component, and whose other input is a constant voltage around zero voltage including zero voltage. A control circuit for an induction heating cooker, comprising: a pulse generation circuit that receives a timing pulse from the detection circuit and sends out a pulse for controlling driving of the transistor.