JPS6325473B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an induction heating cooker that heats a pan or the like by induction heating, and particularly to a control circuit thereof.

[Prior art]

Conventional induction heating cookers use the collector-emitter voltage (V _CE ) after the transistor is turned off as the reference timing for turning on the switching transistor, or detect the period in which current flows through the damper diode to set the timing. Methods such as taking them were used. However, since the pots, which are the load of the electromagnetic cooker, come in a variety of materials, shapes, etc., it has been difficult to always obtain the appropriate timing using these methods.

Furthermore, in the past, a group of antiparallel diodes was connected to the output of a current transformer, as shown in, for example, Japanese Patent Application Laid-Open No. 52-74821. It was difficult to get the timing right.

[Summary of the invention]

This invention was made in view of the above problems.
By setting the on-timing of the switching transistor based on the coil current of the work coil and by providing a series circuit of a resistor and a diode in antiparallel as the load of the current transformer that detects the coil current, the on-timing of the switching transistor can be controlled based on the coil current of the work coil. To provide an induction heating cooker that can always obtain proper timing.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an inverter circuit for an induction heating cooker. In this figure, numeral 1 denotes a work coil, on which a top plate (not shown) is placed, and a pan is further placed thereon. 2 is a capacitor which is a resonance capacitor connected in parallel to this work coil 1;
3 is a switching transistor, the collector of which is connected to the work coil 1, and the damper diode 4 connected between the emitter and collector. A current transformer 5 detects a coil current, and is provided between the work coil 1 and the resonance capacitor 2. Further, 6 is a control circuit that performs output adjustment, on-timing setting, small object load detection, etc. Note that 7 is a power supply circuit consisting of a full-wave rectifier circuit 8 and a smoothing circuit 9.

In the above circuit, when the switching transistor 3 is turned on and off at a frequency of about 20 to 40 KHz, current and voltage waveforms as shown in FIG. 2 are generated. In this figure, I _L is the coil current flowing through the work coil 1, of which T ₁ is the ON (conduction) period of the switching transistor 3;
The period T ₂ is an off period, and during this T ₂ the coil current flows to the resonance capacitor 2 to perform charging and discharging. V _CE is the collector-emitter voltage of the switching transistor 3 that occurs during this resonance. _T3 is a period during which the damper diode current flows, and normally when the switching transistor 3 is turned on during this damper period, the switching transistor current flows again as at _T4 .
In this way, a nearly sinusoidal current flows through the work coil 1 continuously, and an eddy current is generated in the pan placed on the work coil 1 to heat it.

By the way, if the on-timing of the switching transistor 3 is performed before T ₃ (state of T ₂ ),
Since charge remains in the resonance capacitor 2, there is a risk that a large current of capacitor discharge will flow into the switching transistor 3 at the same time it is turned on, causing it to be destroyed. Also, if the on timing is later than _T3 , it will be difficult to receive input. Therefore, the on-timing must be performed within the period _T3 .

Furthermore, since there are various types of pots, which are the load of the electromagnetic cooker, such as materials and shapes, the inductance when placed on the work coil 1 changes depending on the load. Therefore, the resonance frequency formed by the work coil 1 and the resonance capacitor 2 also changes constantly.
Also, the on time T ₁ of the switching transistor 3
Since the output is controlled by changing the frequency, the frequency changes from this aspect as well. Therefore, the on-timing of the switching transistor must be controlled to follow the frequency. Methods for this control include the aforementioned method of detecting V _CE and method of detecting the damper period, but the present invention attempts to control the on-timing by detecting the coil current.

Detection of the coil current is performed by a current transformer 5 as shown in FIG.
0 is connected. And the voltage across this resistor 10
V _CT1 has the waveform shown in FIG. By the way, the coil current I _L has an asymmetric positive wave and a negative wave, and generally the positive wave is large and has a DC component. When this current is detected by the current transformer 5, the waveform changes to be positive or negative with respect to the average level. This relationship is shown in Figure 4, from the coil current 0 line to the current transformer current 0 line.
The line gets higher. It is desirable to perform the on-timing at a point slightly ahead of point A, which is the 0 line of the coil current, but if the on timing is set at the 0 line point of the current transformer 5, it will be at point B, which will be later than point A. become. To solve this problem, it is possible to shift the detection level of V _CT1 to the negative side, but if it is fixedly shifted, the point at which the coil current I _L changes between negative and positive changes depending on the load, so it is difficult to always obtain the appropriate timing. It's impossible.

Therefore, in the present invention, as shown in FIG. 5, series circuits of a resistor 11 and a diode 13 and a resistor 12 and a diode 14 are connected in antiparallel as the load of the current transformer 5. The positive side resistor 11 has a lower value than the negative side resistor 12.

When configured in this way, the output voltage V _CT2 is the 6th
It will look like the figure. In this figure 6, the solid line is the 4th line.
This is V _CT1 shown in the figure, and the dotted line is the voltage V _CT2 of the configuration shown in FIG. As is clear from this figure, by setting the load resistance so that the positive side < negative side, the waveform on the negative side advances from the 0 line point at V _CT1 , and the negative → positive rising point becomes point C. This point C is ahead of the point A on the 0 line of the coil current I _L , and if the switching transistor 3 is turned on at this point C, it can always be turned on at the appropriate timing within the period _T3 in Figure 2. This allows normal operation.

[Effects of the Invention] As explained above, according to the present invention, a series circuit of a resistor and a diode is provided in antiparallel as a load of a current transformer that detects the coil current of a work coil, and the positive and negative resistance values are changed. Therefore, there is an effect that the on-timing of the switching transistor can always be properly obtained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an inverter circuit of an induction heating cooker according to the present invention, FIG. 2 is a waveform diagram of a coil current and a collector-emitter voltage of a switching transistor in the inverter circuit, and FIG.
FIG. 4 is a circuit diagram showing a load circuit of a general current transformer, FIG. 4 is a waveform diagram of the generated voltage in the circuit of FIG. 3, and FIG. 5 is a circuit diagram showing a load circuit of a current transformer according to an embodiment of the present invention. 6 is a waveform diagram of the voltage generated in the circuit of FIG. 5. 1... Work coil, 2... Resonance capacitor, 3... Switching transistor, 4... Damper diode, 5... Current transformer, 6
... Control circuit, 11 ... Positive side load resistance, 12 ...
Negative side load resistance, 13, 14...diode. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1. In an induction heating cooker using an inverter circuit consisting of a work coil, a resonance capacitor, a switching transistor, a damper diode, etc., a current transformer is provided to detect the current of the work coil, and a current transformer is provided as a load of the current transformer. A series circuit of a resistor and a diode is provided in antiparallel and its resistance value is small when the load is on the positive side and becomes large when the load is on the negative side, and the turn-on timing of the switching transistor is obtained from this output signal. An induction heating cooker featuring: