JPH02119084A - 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 Field of the Invention The present invention relates to an induction cooking device having a plurality of heating coils.

Conventional technology A high-frequency current is passed through a heating coil in an inverter circuit, and the resulting high-frequency electric current is generated by placing a magnetic pot within the magnetic field and creating a vortex inside the pot? 1) First, the pot itself is heated to generate a loss of heat. At this time, slight vibrations are generated in the pot etc. due to magnetostriction, but since the frequency is higher than the audible range, no sound can be heard.

Problems to be Solved by the Invention However, conventional induction heating cookers having multiple heating coils have a problem in that resonance occurs when there is a difference in the frequency of the current flowing through each heating coil, resulting in a humming noise. Was.

In view of the above-mentioned problems, an object of the present invention is to prevent the generation of humming noise even when high-frequency current is applied to two or more heating coils at the same time in an induction heating cooker having a plurality of heating coils. do.

Means for Solving the Problems In order to achieve the above object, the present invention provides a period time detection means for detecting the cycle time in which each semiconductor switching element is turned on and off as one cycle, and compares the respective cycle times. A comparison means and a high-frequency current flowing through each heating coil are selected by selecting the minimum cycle time among those compared by the comparison means and varying the on-time width of each semiconductor switching element so that each cycle time becomes the minimum cycle time. This reduces the frequency difference.

Function: By detecting the on/off cycle time of each semiconductor switching element with the above configuration, and adjusting the conduction time width of the semiconductor switching element so that each semiconductor switching element has the same time, the high frequency current frequency of each heating coil is matched. It is possible to prevent the resonance phenomenon of pots, etc. from occurring and reduce the humming noise.

Example FIG. 1 is a block diagram showing an embodiment of the present invention.

1 is a commercial low-frequency power supply, 2 is a full-wave rectifier, and 3a and 3b are inverter circuits connected in parallel to the DC side of the full-wave rectifier 2. Inverter circuit 3a is power transistor 1
A heating coil 6a is connected in series with a diode 9a connected in antiparallel to 0a, and a pot is placed near the heating coil 6a to perform induction heating. Also heating coil θ
A resonant capacitor 7a is connected in parallel with the heating coil 6a to cause a high frequency current to flow through the &. 8a is a high frequency bypass capacitor, and 5a is a current transformer for detecting low frequency input current. Note that the inverter circuit 3b has the same configuration as the inverter circuit 2a, so a description thereof will be omitted.

The control circuit 11 includes an input detection circuit 12a that detects the input current using the current transformer 6a and detects the input to the inverter circuit 3a, and a setting circuit 14a using a potentiometer 13a that sets the input level to the inverter circuit 3.
The reference voltage is set. The amplification circuit 15a compares and amplifies the input detection circuit 12=1 and each output of the setting circuit 14a, and outputs the result to the timing circuit 16a. The timing circuit 16a detects each voltage waveform between the terminals of the high frequency bypass capacitor 8a and between the collector and emitter of the power transistor 10a, and determines the timing of base current supply to the power transistor 10a based on the output voltage of the RI amplifier circuit 15a. form a signal. This will be explained in more detail using FIG.

A is the voltage waveform C between the terminals of the high frequency bypass capacitor 8a
and the collector-emitter voltage waveform d of the power transistor IQa. B is a power transistor 10d
The combined waveform of the collector current (positive part) and the current flowing through the diode 9a (negative part) is shown. C is the triangular wave q formed in the timing circuit 16a and the increase "↑" circuit 15a.
D indicates the output voltage e of the timing circuit 16a, and D indicates the timing output waveform f of the timing circuit 16a. This inverter circuit 3a is of a self-excited type, and when the intersection of the downward slope of the waveforms C and d is detected, the triangular wave q changes from the downward slope to the downward slope.

The triangular wave q has a predetermined voltage V. When reaching this point, the slope changes again from a downhill slope to an uphill slope. The pulse output timing of the output waveform f is determined by the triangular wave q and the voltage e.

At this time, as the voltage e increases, the pulse width t1 of the output waveform f naturally becomes longer. Note that the downward slope of the triangular wave q is set so that the rising timing of the output waveform f falls within the conduction time of the diode 9a shown in FIG.

Now, if the output voltage of the setting circuit 14a is increased by one or two to increase the input setting, the output e of the comparator circuit 15a will rise, the pulse width t1V1 will increase as described above, the frequency will decrease, and the collector current of the power transistor 10a will increase. The input current increases until the output of the input detection circuit 12a becomes the same as the output of the setting circuit 14a. The drive circuit 17a transfers the signal of the output waveform f to the power transistor 10.
This is to supply a base current sufficient to turn on the transistor a.

The period measuring circuit 18a detects the output waveform f (+:, + t2
) is used to measure time. The time t1+t2 coincides with the period of the high frequency current flowing through the heating coil 6a.

J': J, when the two inverter circuits operate simultaneously as above, the outputs of the period measuring circuits 18a and 18b are transmitted to the selection circuit 19. The selection circuit 19 immediately compares the cycle times, and selects the setting circuit 14 for the inverter circuit with the longer cycle time (a and b will not be specified hereafter).
Regardless of the potentiometer 13, the output voltage of
This decreases until the periods 11+12 of the two inverter circuits are the same.

Incidentally, when the output voltage of the setting circuit 14 fluctuates at this time, stable control can be achieved by providing a constant time constant regardless of the input setting.

It is easily conceivable that the single cycle measuring circuit 18 and the selection circuit 19 could be replaced with a microcomputer for control.

Effects of the Invention As is clear from the embodiments, according to the present invention, the drive signal cycle of each power transistor driving a plurality of inverter circuits is detected, and the minimum cycle time is selected from among each cycle time. The on-time of the power transistors can be automatically varied so that the cycles of all power transistors match the minimum cycle time, and this eliminates humming noises caused by drive frequency differences for cooker users. 1゜ which can prevent unpleasant phenomena from occurring.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram of an induction heating cooker showing an embodiment of the present invention, and FIG. 2 is an operational waveform diagram of each part of the same. 3a, 3b...Inverter circuit, 6a, eb
... Heating coil, 7a, 7b ... Resonance capacitor, 9a, 9b ... Diode,
10a, 10b... power transistor, 11
...Control circuit, 18a. 18b...Period measurement circuit.

Claims (1)

[Claims] A semiconductor switching element, a diode connected in antiparallel to the semiconductor switching element, a plurality of inverter circuits each including a heating coil and a resonance capacitor connected in series with the semiconductor switching element, and each semiconductor switching element being turned on and off to perform heating. It consists of a control circuit that causes a coil to generate a high-frequency current, and the control circuit detects the cycle time of each semiconductor switching element, which is one cycle of on/off, and adjusts the cycle time of each semiconductor switching element so that it matches the minimum cycle time. An induction heating cooker characterized by varying the on-time width of a switching element.