JPH06176862A - Induction heating cooker - Google Patents

Abstract

(57) [Abstract] [Purpose] To improve the stability of the trigger detection method in the vicinity of the zero cross of the input AC power supply. [Constitution] In the induction heating cooker, the control circuit compares the resonance voltage waveform VCE of the inverter circuit with the input voltage VL of the inverter circuit, the input current, or a comparison voltage waveform similar to the smoothed resonance voltage waveform, and triggers In the trigger detection circuit 100, a trigger detection circuit 100 for generating a pulse and a means for shaping the output of the free-running oscillator 9 triggered by the input of this trigger pulse into an on / off pulse of the switching element 8 are provided. A resonance voltage detection circuit 110 that detects the resonance voltage VCE waveform of the inverter circuit and a comparison voltage detection circuit 120 that detects the comparison voltage are provided, and the comparison voltage detection circuit 120 holds the comparison voltage waveform so that it does not fall below a certain voltage. The circuit 130 is added.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating cooker with an improved control method for a switching element for an inverter circuit.

[0002]

2. Description of the Related Art Control of an inverter circuit for applying a high frequency current to a heating coil by a switching element to generate a high frequency magnetic flux to induce an eddy current in a load magnetically coupled to the heating coil and to heat the load by Joule heat thereof. In an induction heating cooker having a circuit, a method of generating the energizing pulse of the switching element is to use a signal similar to the inverter input voltage as a reference, compare the resonant voltage of the inverter with the ON signal (energizing pulse signal) of the switching element. ) Is proposed in Japanese Examined Patent Publication No. 61-12636, and the like.
A circuit having a resonance output detection circuit and a comparison amplification circuit which operates according to these output signals is proposed in Japanese Patent Laid-Open No. 57-878.

[0003]

However, in the above method, a signal similar to the inverter input voltage (for example, a signal obtained by dividing the inverter input voltage) or a signal similar to the input current (for example, input by a current transformer). Since the ON signal is generated by comparing the current-acquired signal, etc.) with the inverter resonance voltage, both the inverter input voltage and the resonance voltage approach zero volt near the zero cross of the AC power supply, and the input voltage of the comparator , The noise resistance deteriorates, abnormal oscillation occurs, and an abnormal drive pulse is applied to the switching element.
There is a risk that the device may be destroyed due to overload or overvoltage.

In order to prevent the inverter input voltage from becoming zero volt, a very large smoothing circuit must be inserted, which causes problems that the parts are upsized and the cost is increased.

In addition, it has a change rate detection circuit for detecting the change rate of the terminal voltage of the switching element and a trigger circuit for generating a trigger signal when the voltage change rate reaches 0 for the second time. A device that can always oscillate regardless of the shape and material of an object has been proposed in Japanese Patent Laid-Open No. 60-74380. However, the rate of change of the terminal voltage of the switching element is small near the zero cross of the AC power supply. Therefore, the intended oscillation operation may become unstable.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a rectifier circuit for converting an AC power supply into a DC power supply, and this DC power supply is converted into a high frequency current by a switching element and heated. In an induction heating cooker including an inverter circuit that supplies a coil and a control circuit that controls the inverter circuit, the control circuit compares a resonance voltage waveform of the inverter circuit with a comparison voltage waveform and generates a trigger pulse. And a means for shaping the output of the free-running oscillator triggered by the input of this trigger pulse into an on / off pulse of the switching element, and a resonance detecting the resonance voltage VCE waveform of the inverter circuit in the trigger detection circuit. A comparison voltage detection circuit that detects the comparison voltage and the comparison voltage detection circuit is provided. It is obtained by adding a waveform shaping circuit for holding so as not fall below the constant voltage.

Further, it is preferable that the comparison voltage detecting circuit outputs a comparison voltage waveform similar to a waveform obtained by smoothing the waveform of the input voltage VL, the input current, or the resonance voltage VCE of the inverter circuit.

[0008]

With the above configuration, the resonance voltage detection circuit detects the resonance voltage of the inverter circuit, and the comparison voltage detection circuit detects the input voltage, input current, and
Alternatively, a comparison voltage waveform that smooths the resonance voltage, etc. is detected, and the waveform shaping circuit performs waveform shaping on the comparison voltage input and holds the comparison voltage waveform so that it does not fall below a certain voltage, and the trigger detection circuit compares and generates a trigger pulse. Therefore, at least near the zero cross, the comparison voltage waveform does not become zero volt, abnormal oscillation does not occur, and the output of the free-running oscillator is stabilized.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1 is a block circuit diagram of an induction heating cooker according to an embodiment of the present invention, FIG. 2 is a specific circuit diagram of a trigger detection circuit of the same circuit, and FIG. 3 is an operation waveform diagram of each part in the detection circuit. FIG. 4 is a comparison voltage detection circuit diagram for smoothing a resonance voltage as a comparison voltage according to the second embodiment of the present invention, and FIG.
Is a comparative voltage waveform diagram based on the same circuit diagram, and FIG.
FIG. 7 is a circuit diagram in which the input current waveform obtained by the embodiment of FIG.

In the figure, an AC power supply 1 is input to a rectifier circuit 2 to convert it into a direct current, and the output of the rectifier circuit 2 is input to a choke coil 3 and a smoothing capacitor 4 which form a filter circuit and smoothed, and a heating coil which forms a resonance circuit 5 and the resonance capacitor 6 and further connected in antiparallel to the damper diode 7 and the switching element 8 to form a closed circuit,
Configure an inverter circuit. Here, the inverter input voltage, which is the smoothed power supply voltage, is VL, and the resonance voltage on the switching element 8 side of the resonance circuit is VCE.

The trigger detection circuit 100 includes a resonance voltage detection circuit 110, a comparison voltage detection circuit 120, and a waveform shaping circuit 13.
0 and a comparator 140. The resonance voltage detecting circuit 110 detects a similar voltage of the resonance voltage VCE by an appropriate voltage dividing resistor, and is a circuit that clamps the detected output voltage to a certain voltage or less. The comparison voltage detection circuit 120 detects a voltage similar to the inverter input voltage VL by a suitable voltage dividing resistor. The waveform shaping circuit 130 is a circuit for holding the output of the comparison voltage detection circuit 120 at a certain voltage or more when the output becomes a certain voltage or less. Therefore, the trigger detection circuit 100 inputs the inverter input voltage VL and the resonance voltage VCE, compares them with the comparator 140, and generates a trigger pulse at an appropriate timing.

The free-running oscillator 9 that receives the output of the trigger detection circuit 100 is an oscillator that oscillates in a certain fixed period when a trigger pulse is not input from the outside and locks at the period of the trigger pulse when the trigger pulse is input. ,
In this embodiment, a substantially triangular wave is output. In the input setting circuit 10, one comparison value of the comparator 11 is set according to the heating power level set by the user. The comparator 11 compares the outputs of the free-running oscillator 9 and the input setting circuit 10 and generates a drive pulse having a predetermined duty ratio. The driver 12 drives the output of the comparator 11 to drive power of the switching element 8.

Next, the trigger detection circuit 1 will be described with reference to FIGS.
00 will be described.

The resonance voltage detection circuit 110 includes resistors 111 and 1
The voltage is divided by 12 and the overvoltage component is bypassed to the circuit power supply by the diode 113 to protect the input terminal of the comparator 140. This output waveform is V1.

The comparison voltage detection circuit 120 includes resistors 121 and 1
The voltage is divided by 22 and the noise component is removed by the capacitor 123. The waveform shaping circuit 130 divides the circuit power supply by the resistors 131 and 132, synthesizes it with the output of the comparison voltage detection circuit 120 by the diode 133, and shapes the waveform so that it holds a certain voltage even when the input voltage becomes zero volts. This output waveform is V2.

The comparator 140 compares the voltages V1 and V2 and outputs a trigger pulse V3 to the free-running oscillator 9.
If the free-running oscillator 9 is triggered by the trailing edge of the trigger pulse V3 and outputs a substantially triangular wave V4, the waveform of the circuit described above is as shown in FIG.

In FIG. 3, the section T1 is around the zero cross of the power supply, and the output V2 of the waveform shaping circuit 130 is valid. In the sections other than the section T1, the crossing point of the resonance voltage V1 and the comparison voltage V2 is not near zero volt in the input voltage range of the comparator 140, so that a normal trigger pulse V3 is output and the free-running oscillator 9 is caused by the trigger pulse. Lock it. During the period T1, both the resonance voltage V1 and the comparison voltage V2 approach zero volt, and the input of the comparator 140 is in a very unstable state due to the characteristics of the comparator itself, noise, and the like.

Therefore, when the waveform shaping circuit 130 is not provided, the comparison voltage becomes as indicated by the dotted line V2 ', and the comparator 140 and the free-running oscillator 9 generate an abnormal pulse or an abnormal waveform as shown in the waveform example, and the switching occurs. There is a high risk of causing a failure of the element 8.

In this embodiment, the waveform shaping circuit 130
Since the comparison voltage V2 is held up to a certain voltage as shown by the solid line, the comparator 140 itself operates stably even when the amplitude of the resonance voltage V1 approaches zero volts. During that time, the free-running oscillator 9 is not triggered, and the free-running oscillator 9 oscillates at its own frequency. Therefore, the failure of the switching element 8 due to the generation of the abnormal pulse does not occur.

Next, another embodiment of the comparison voltage detecting circuit 120 will be described.

FIG. 4 shows an example of a circuit for smoothing the resonance voltage VCE to obtain a comparison voltage. The resonance voltage VCE is divided by the resistors 152 and 153 via the backflow prevention diode 151, smoothed by the capacitor 154, and the output is obtained. Waveform shaping circuit 13
When connected to 0, the waveform of this circuit looks like FIG.
The dotted line in the figure is the output voltage when the waveform shaping circuit 130 is not provided.

FIG. 6 is an example of a circuit in which an input current waveform is used as a comparison voltage. The input current is detected by a current transformer 161, and full-wave rectified by a rectifier 162, and then a resistor 16 is provided.
When the voltage is divided by 3, 164, the noise component is removed by the capacitor 165, and this output is connected to the waveform shaping circuit 130, the waveform of this circuit becomes as shown in FIG. The dotted line in the figure is the output voltage when the waveform shaping circuit 130 is not provided.

In either case, if there is no load or if the load is improper, almost no input current may flow.
Even in that case, due to the effect of the waveform shaping circuit 130,
Since it does not drop below a certain voltage, no abnormal trigger signal is generated. Therefore, the failure of the switching element 8 due to the generation of the abnormal pulse does not occur.

[0024]

According to the present invention, a resonance voltage detecting circuit for detecting a resonance voltage of an inverter circuit, and a comparison voltage detecting circuit for detecting a comparison voltage waveform obtained by smoothing an input voltage, an input current, a resonance voltage or the like of the inverter circuit. Since the circuit and the waveform shaping circuit that shapes the waveform for the comparison voltage input and holds the comparison voltage waveform so that it does not fall below a certain voltage are provided in the trigger detection circuit that compares the resonance voltage and the comparison voltage waveform and generates the trigger pulse, With a simple configuration without inserting a very large smoothing circuit, etc., near the zero cross of the AC power supply, even if the resonant voltage of the inverter circuit, or the input voltage, input current, etc. of the inverter circuit approaches zero volts,
At least the comparison voltage waveform does not become zero volt, and abnormal oscillation or the like does not occur in the signal that drives the switching element. Therefore, there is an effect of preventing the switching element from being damaged by overcurrent or overvoltage.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of an induction heating cooker showing an embodiment of the present invention.

FIG. 2 is a specific circuit diagram of a trigger detection circuit of the same circuit.

FIG. 3 is an operation waveform diagram of each part in the detection circuit.

FIG. 4 is a comparison voltage detection circuit diagram for smoothing a resonance voltage as a comparison voltage according to the second embodiment of the present invention.

FIG. 5 is a comparison voltage waveform diagram according to the circuit diagram.

FIG. 6 is a circuit diagram in which an input current waveform according to a third embodiment of the present invention is used as a comparison voltage.

FIG. 7 is a comparison voltage waveform diagram according to the circuit diagram.

[Explanation of symbols]

 1 AC power supply 2 Rectifier circuit 5 Heating coil 8 Switching element 9 Free-running oscillation circuit 100 Trigger detection circuit 110 Resonance voltage detection circuit 120 Comparative voltage detection circuit 130 Waveform shaping circuit

Claims (4)

[Claims] 1. A rectifier circuit (2) for converting an AC power supply (1) into a DC power supply, and an inverter circuit for converting this DC power supply into a high frequency current by a switching element (8) and supplying it to a heating coil (5). In an induction heating cooker including a control circuit for controlling the inverter circuit, the control circuit compares a resonance voltage (VCE) waveform of the inverter circuit with a comparison voltage waveform to generate a trigger pulse (100).
And the switching element (8) by shaping the output of the free-running oscillator (9) triggered by the input of this trigger pulse.
The trigger detection circuit (100) includes a resonance voltage (VC) of the inverter circuit.
E) A resonance voltage detection circuit (110) for detecting a waveform and a comparison voltage detection circuit (120) for detecting a comparison voltage are provided, and the comparison voltage detection circuit (120) holds the comparison voltage waveform so as not to fall below a certain voltage. Wave shaping circuit (130)
An induction heating cooker characterized by being added with. 2. The induction heating cooker according to claim 1, wherein the comparison voltage detection circuit (120) outputs a comparison voltage waveform similar to the input voltage (VL) of the inverter circuit. 3. The induction heating cooker according to claim 1, wherein the comparison voltage detection circuit (120) outputs a comparison voltage waveform obtained by smoothing the resonance voltage waveform (VCE) of the inverter circuit. 4. The induction heating cooker according to claim 1, wherein the comparison voltage detection circuit (120) outputs a comparison voltage waveform similar to the input current.