JPS6074380A - Induction heating cooking device - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an induction heating cooker equipped with a frequency conversion device for use in general households.

Conventional configuration and its problems In a conventional induction heating cooker equipped with an inverter that excites a resonant circuit of a heating coil and a resonant capacitor with a semiconductor switching element, the voltage between the element terminals that occurs after the switching element is turned off is After detecting that the signal becomes zero again due to the parallel resonance phenomenon of the resonant circuit, a conducting signal of a predetermined time width is generated again to the switching element. Although this makes it possible to reduce switching loss when the switching element conducts, oscillation cannot be continued unless the element terminal voltage becomes zero due to resonance. To be more specific, the 11 loads of the inverter are pots that are used, and depending on the shape, material, and position 64, electrical load fluctuations vary widely.

However, a control method generates a conductive signal again by detecting that the voltage at the switching element terminal has decreased to a predetermined value or more, or by detecting that the current in the switching element (including the flywheel diode) flows in the opposite direction. In the conventional example, there was a risk of oscillation stopping unless the inverter was designed to connect the heating coil to the pot so that the voltage would always be zero under all load conditions.

Of course, it is physically impossible to confirm whether the design will work safely for all pots that will be used, so the combination of the heating coil and the pot should be designed with a safety factor. The method used was to reduce the load fluctuation by making the load worse.

However, in order to do so, the same output cannot be obtained unless more current than necessary is passed through the heating coil, which poses an obstacle to improving efficiency and other aspects.

OBJECTS OF THE INVENTION The present invention solves these conventional problems and provides an induction heating cooker that improves the stability of the oscillation operation of the inverter and the efficiency of the device.

Composition of the Invention The induction heating cooker of this invention uses semiconductor switching to excite a filter circuit consisting of a choke coil and a filter capacitor connected to a commercial power source, and a resonant circuit of a heating coil and a resonant capacitor connected to both ends of the filter capacitor. a frequency conversion circuit consisting of an element, a rate-of-change detection circuit that detects the rate of change of the terminal voltage of the semiconductor switching element, a trigger circuit that generates a signal when the rate of change of pressure reaches zero for the second time, and and a drive circuit connected to the output of the trigger circuit to supply an ON signal for a predetermined period of time 11'' to the semiconductor switching element.

DESCRIPTION OF THE EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings. In FIGS. 1 and 2, the rectifier output terminal of the rectifier 3 connected to the commercial power supply 1 via the switch 2 has a
A filter circuit consisting of a choke coil 4 and a filter capacitor 5 is connected.

Furthermore, the filter capacitor 5 includes a parallel resonant circuit of a heating coil 6 and a resonant capacitor 7, and a transistor 8 (NPN).
And the inverse of flywheel diode 9 ＼ 1 [The series circuit of the column circuit is connected and frequency conversion! .

20. 11111 of transistor 8;
J"l+l: Pressure v1 is input to the rate of change detection circuit 10, and every time the rate of change of ■1 becomes zero, an output is generated to the trigger circuit 11. The trigger circuit 11 is the second output of the rate of change detection circuit 1o. generates an output to the drive circuit 12 in synchronization with
Thereafter, a drive signal with a predetermined pulse width is supplied to the base terminal of the transistor 8.

FIG. 2 explains the block diagram of the embodiment shown in FIG. 1 in more detail.
The operation of each part will be explained with reference to the figures. The transformer 13 connected to the commercial power supply 1 has a secondary winding with a center tap, and each has a rectifier diode 14, 15, a smoothing capacitor 16.17, and a voltage regulator diode 1'18.1.
Two power supplies, positive and negative, are supplied to the transistor 9, and the midpoint thereof is connected to the emitter and ivy of the transistor 8. Change rate detection circuit 1
A differential circuit consisting of two resistors 102 and 103 connected in series with a capacitor 101 constituting 0 is connected between the terminals of the transistor 8, and is connected to the input terminal of the comparator 106 connected to the midpoint of the resistors 102 and 103. Input the differential waveform of 1.

Further, a constant negative voltage is supplied as a reference voltage to the other input terminal of the comparator 106 by two voltage dividing resistors 104 and 105 connected between the negative power supply.

Here, vl is the voltage generated when the transistor 8 is off, and due to the parallel resonant circuit of the heating coil 6 and the resonant capacitor 7, it becomes an oscillating waveform as shown in Figure 3a.
The differential voltage waveform shown in FIG. 3 is as shown in FIG. In the figure, the dashed line indicates the reference voltage of the comparator 106. The output signal of the comparator 106 in FIG.
07 and constitutes the trigger circuit 11 as the output signal of the rate of change detection circuit 10.
The voltage is supplied to the black terminal of the knob 111 (hereinafter referred to as D-F/F), but as can be seen from the input signal of the comparator 106 shown in FIG. The output signal of the comparator 106 is rl, py-.
On the other hand, in the circuit of the embodiment of FIG. 2, the output of the comparator 106 changes from low to high the second time dV1/dt becomes zero after the transistor 8 is turned off. D-F/F
111, a commercially available IC such as UPD4013BC manufactured by Nippon Electric Corporation can be used, and its Q output terminal and D terminal are connected. D=F/F111 reads the D terminal signal at the rising edge of the clock input and generates the inverted output at the Q terminal, so as mentioned above, the second dV1/dt
At the point where Q becomes zero (at the rise of C in Figure 3), the output Q (the third
Figure d) is reversed. The reason why a narrow negative pulse is generated after the rising waveform in Figure 3C is due to dV1/d t as described later.
-o is detected and transistor 8 is turned on, vl
This is because the input voltage of the comparator 106 becomes a signal of dV1/dt(O), and the pulse width is mainly the turn-on time of the transistor 8, which is about 1 to 1.
It is about 3 μs. In this way, when transistor 8 is turned on, 2
Since the rising point of the temperature occurs in a short period of time, the Q output signal is a narrow pulse as shown in FIG. has been done. This monostable multivibrator 1
21 may be a general transistor made of transistors, but in particular, UPD 4528B C manufactured by NEC Corporation is used.

The monostable multivibrator 121 generates an output with a predetermined pulse width T1 determined by a resistor 128 and a capacitor 129 of the external signal as shown in FIG. 3e in response to the trigger signal (FIG. 3d), Supplying base current to transistor 125. The signal inverted by the transistor 125 is again inverted and amplified by the output transistor 127, and a base current flows to the transistor 8 via the resistor 126. Third
Figure f shows the collector current of transistor 8. And T
After one hour, the output transistor 127 is turned on and a reverse bias is applied to the transistor 8, turning it off, and the oscillatory 1 is generated again. The operating frequency of the frequency converter 2o is determined by the resonant frequency (fixed) of the heating coil 6 and the resonant capacitor 7 and the output pulse width (T) of the drive circuit 12.
1), and the longer T1 is, the larger the high frequency iZL flow flows through the heating coil 6, so by controlling T1, it is possible to vary the induction heating output.

As can be seen from the above explanation of operation, the transistor 8Cj turns on when the terminal voltage is at its lowest, so it operates with the least switching loss, so it can be small and inexpensive, and it generates less heat. It is possible to reduce noise by lowering the performance of the cooling fan, and to make the product smaller and thinner by making the radiation fins smaller.

As described above, the transistor 8 is used as a semiconductor switching element.
Although the embodiment has been described using a transistor, there is no restriction at all as long as it is an element that can be turned on and off by a control signal, such as a GTO (gateter/off thyristor) or a FET (field effect transistor) instead of a transistor. Further, even if the resonant capacitor 7 is connected in parallel to the transistor 8, the operation remains the same.

Effects of the Invention According to the induction heating cooker of the present invention, the following excellent effects are obtained.

1. Regardless of the shape or material of the object to be heated, the semiconductor switching element always operates in such a way that the loss is minimized, so the device can be made smaller, quieter, and less expensive.

2. Because oscillation conditions always exist despite variations in the shape and material of the heated object, stable equipment can be provided without oscillation stoppage or abnormal oscillation phenomena.

3. In order to detect the second minimum point of voltage and drive semiconductor switching loss, such as pot material.

The inductance of the heating coil has saturation characteristics, and even if the oscillating voltage is a non-sinusoidal wave (distorted wave), it will be ignited accurately at the minimum point, and switching loss will be minimized.

4. Simple configuration and high reliability.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an induction heating cooker showing an embodiment of the present invention, FIG. 2 is a circuit diagram thereof, and FIG. 3 is a waveform diagram showing an explanation of the operation of FIG. 2. 6...°°" heating coil, 7... resonant capacitor, 8... semiconductor switching element (transistor), 10... rate of change detection circuit, 11... to1tsuga [−711 path, 12...drive circuit, 20...
・Frequency conversion equipment.

Claims (1)

[Claims] A frequency conversion device consisting of a filter circuit consisting of a choke coil and a filter capacitor connected to a commercial power supply, a heating coil connected to both ends of the filter capacitor, a resonant capacitor, and a semiconductor switching element that excites the resonant circuit; A rate-of-change detection circuit that detects the rate of change of terminal voltage, and a voltage change rate of 2
An induction heating cooker comprising a trigger circuit that generates a signal when the temperature reaches zero, and a drive circuit that is connected to the output of the trigger circuit and supplies an on signal of a predetermined duration to a semiconductor switching element.