JP3207948B2 - Induction heating cooker - Google Patents

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 for improving a control method of a switching element for an inverter circuit.

[0002]

2. Description of the Related Art A control for an inverter circuit in which a switching element applies a high-frequency current to a heating coil to generate a high-frequency magnetic flux to induce an eddy current in a load magnetically coupled to the heating coil and heat the load by the Joule heat. In an induction heating cooker having a circuit, as a method of generating an energizing pulse of a switching element, a signal similar to an inverter input voltage is used as a reference, and a resonance voltage of the inverter is compared with an ON signal (an energizing pulse signal) of the switching element. ) Is proposed in Japanese Patent Publication No. Sho 61-12636, etc.
A circuit having a resonance output detection circuit and a comparison amplifier circuit that operates in accordance with these output signals is proposed in Japanese Patent Application Laid-Open No. 57-878.

[0003]

However, in the above-described method, a signal similar to the inverter input voltage (for example, a signal obtained by dividing the inverter input voltage, etc.) or a signal similar to the input current (for example, input by a current transformer). Since the ON signal is generated by comparing the inverter's resonant voltage with the signal obtained by taking in the current, the inverter input voltage and the resonant voltage both approach zero volts near the zero crossing of the AC power supply, and the input voltage of the comparator increases. As inappropriate, the noise resistance deteriorates, abnormal oscillation occurs, abnormal drive pulse is applied to the switching element,
Overload and overvoltage may cause device destruction.

In order to prevent the inverter input voltage from becoming zero volts, a very large smoothing circuit must be inserted, which causes a problem that the size of parts and the cost increase.

In addition, the apparatus has a change rate detection circuit for detecting a 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 a second zero. A device which can always oscillate regardless of the shape and shape of the object is proposed in Japanese Patent Application Laid-Open No. 60-74380. However, the change rate of the terminal voltage of the switching element is small near the zero cross of the AC power supply. Therefore, there is a possibility that the intended oscillation operation becomes unstable.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and comprises a rectifier circuit for converting an AC power supply to a DC power supply, and a converter for converting the DC power supply into a high-frequency current for heating. In a dielectric heating cooker comprising an inverter circuit for supplying a coil and a control circuit for controlling 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. Means for shaping the output of the free-running oscillator triggered by the input of the trigger pulse and turning it into an on / off pulse of the switching element. In this trigger detecting circuit, the resonance detecting means detects the resonance voltage VCE waveform of the inverter circuit. Voltage detection circuit and inverter
Flatten the input voltage, input current, or resonance voltage waveform of the circuit.
A comparison voltage detection circuit for outputting a comparison voltage waveform similar to a smoothed waveform is provided, and a waveform shaping circuit for holding the comparison voltage waveform so as not to be lower than a predetermined voltage is added to the comparison voltage detection circuit.

The comparison voltage detection circuit preferably outputs a comparison voltage waveform similar to a waveform obtained by smoothing a waveform such as an input voltage VL, an input current, or a resonance voltage VCE of the inverter circuit.

[0008]

With the configuration described above, the resonance voltage of the inverter circuit is detected by the resonance voltage detection circuit, and the input voltage, the input current, and the input current of the inverter circuit are detected by the comparison voltage detection circuit.
Alternatively, a comparative voltage waveform obtained by smoothing a resonance voltage or the like is detected, a waveform shaping circuit shapes the waveform of the comparative voltage input, holds the comparative voltage waveform so that it does not fall below a certain voltage, and generates a trigger pulse by comparing with a trigger detecting circuit. Therefore, in the vicinity of the zero cross, at least the comparison voltage waveform does not become zero volts, abnormal oscillation does not occur, and the output of the free-running oscillator is stabilized.

[0009]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block circuit diagram of an induction heating cooker according to one embodiment of the present invention, FIG. 2 is a specific circuit diagram of a trigger detection circuit of the circuit, and FIG. 3 is an operation waveform diagram of each part in the detection circuit. FIG. 4 is a circuit diagram of a comparative voltage detection circuit according to the second embodiment of the present invention, in which a resonance voltage is smoothed and used as a comparative voltage.
FIG. 6 is a comparative voltage waveform diagram according to the circuit diagram, and FIG.
And FIG. 7 is a comparative voltage waveform diagram according to the circuit diagram.

In FIG. 1, an AC power supply 1 is input to a rectifier circuit 2 to be converted into a direct current, and an output of the rectifier circuit 2 is input to a choke coil 3 and a smoothing capacitor 4 constituting a filter circuit and smoothed, thereby forming a heating coil constituting a resonance circuit. 5 and a resonance capacitor 6, and further connected to a damper diode 7 and a switching element 8 connected in anti-parallel to form a closed circuit.
Construct an inverter circuit. Here, the inverter input voltage, which is the power supply voltage after the smoothing, 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 detection 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 similar voltage of the inverter input voltage VL by an appropriate voltage dividing resistor. The waveform shaping circuit 130 is a circuit that holds 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 receives the inverter input voltage VL and the resonance voltage VCE, compares them with the comparator 140, and generates a trigger pulse with appropriate timing.

The free-running oscillator 9 which receives the output of the trigger detection circuit 100 oscillates at a certain period when a trigger pulse is not input from the outside, and locks at the period of the trigger pulse when a trigger pulse is input. ,
In this embodiment, a substantially triangular wave is output. The input setting circuit 10 sets one comparison value of the comparator 11 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 the switching element 8.

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

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

The comparison voltage detection circuit 120 includes resistors 121, 1
The voltage is divided at 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, combines the divided voltage with the output of the comparison voltage detection circuit 120 by the diode 133, and shapes the waveform so that the input voltage becomes zero volt and is maintained up to a certain voltage. This output waveform is defined as V2.

The comparator 140 compares the voltages V1 and V2 and outputs a trigger pulse V3 to the free-running oscillator 9.
Assuming that the free-running oscillator 9 is triggered by the falling 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, a section T1 is around the zero cross of the power supply, and is a section in which the output V2 of the waveform shaping circuit 130 is valid. In a section other than the section T1, the cross 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. Lock. During the interval T1, the resonance voltage V1 and the comparison voltage V2 both approach zero volts, and the input of the comparator 140 is in a very unstable state due to characteristics of the comparator itself, noise, and the like.

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

In this embodiment, the waveform shaping circuit 130
As shown by the solid line, the comparison voltage V2 is held up to a certain voltage, so that the comparator 140 itself operates stably even when the amplitude of the resonance voltage V1 approaches zero volts. During that time, no trigger is applied to the free-running oscillator 9, and the free-running oscillator 9 oscillates at a unique frequency. Therefore, the switching element 8 does not fail due to the generation of the abnormal pulse.

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

FIG. 4 shows an example of a circuit that smoothes the resonance voltage VCE and uses it as a comparison voltage. The resonance voltage VCE is divided by resistors 152 and 153 via a backflow prevention diode 151, and smoothed by a capacitor 154. Waveform VCE smoothed
The waveform is made similar to the waveform, and this output is
, The waveform of this circuit is as shown in FIG. The dotted line in the figure indicates the output voltage when the waveform shaping circuit 130 is not provided.

FIG. 6 shows an example of a circuit using an input current waveform as a comparison voltage. The input current is detected by a current transformer 161 and full-wave rectified by a rectifier 162.
Divided by 3,164, taking a noise component removal have as a similar waveform to the input current by the capacitor 165, connecting the output to the waveform shaping circuit 130, the waveform of this circuit is as shown in FIG. The dotted line in the figure indicates the output voltage when the waveform shaping circuit 130 is not provided.

In any case where there is no load or an improper load, almost no input current flows.
Even in that case, due to the effect of the waveform shaping circuit 130,
Since the voltage does not fall 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 wave obtained by smoothing an input voltage, an input current or a resonance voltage of the inverter circuit are provided.
Compare a resonance voltage and a comparison voltage waveform with a comparison voltage detection circuit that outputs a comparison voltage waveform similar to the shape, and a waveform shaping circuit that shapes the waveform of the comparison voltage input and keeps the comparison voltage waveform so that it does not fall below a certain voltage. Since it is provided in the trigger detection circuit that generates the trigger pulse, it has a simple configuration without inserting a very large smoothing circuit, etc., near the zero crossing of the AC power supply, the resonance voltage of the inverter circuit, or the input voltage and input of the inverter circuit. Even if the current or the like approaches zero volts, at least the comparison voltage waveform does not become zero volts, and abnormal oscillation etc. does not occur in the signal for driving the switching element, so that the switching element due to overcurrent or overvoltage It has the effect of preventing destruction.

[Brief description of the drawings]

FIG. 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 circuit.

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

FIG. 4 is a comparison voltage detection circuit diagram according to a second embodiment of the present invention, in which a resonance voltage is smoothed and used as a comparison voltage.

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

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

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

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 AC power supply 2 Rectifier circuit 5 Heating coil 8 Switching element 9 Self-propelled oscillation circuit 100 Trigger detection circuit 110 Resonance voltage detection circuit 120 Comparison voltage detection circuit 130 Waveform shaping circuit

Claims (1)

(57) [Claims] A rectifier circuit for converting an AC power supply to a DC power supply; an inverter circuit for converting the DC power supply to a high-frequency current by a switching element and supplying the high-frequency current to a heating coil; In a dielectric heating cooker comprising 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, and a trigger detection circuit (100). Means for shaping the output of the free-running oscillator (9) triggered by the input of the trigger pulse to form an on / off pulse of the switching element (8);
A resonance voltage detection circuit (11) for detecting a resonance voltage (VCE) waveform of the inverter circuit in the trigger detection circuit (100).
0), the input voltage (VL) of the inverter circuit, the input current,
Alternatively, a comparison voltage detection circuit (120) that outputs a comparison voltage waveform similar to a waveform obtained by smoothing the resonance voltage waveform (VCE) is provided, and the comparison voltage detection circuit (120) has a comparison voltage waveform equal to or less than a certain voltage. An induction heating cooker characterized by adding a waveform shaping circuit (130) for holding the cooker so that it does not become stale.