JPS6310880B2 - - Google Patents

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 the resonant circuit of the heating coil and the resonant capacitor using 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 having a predetermined time width is generated again to the switching element. This makes it possible to reduce switching loss when the switching element becomes conductive, but oscillation cannot be continued unless the element terminal voltage becomes zero due to resonance. To be more specific, the load on the inverter is the pot being used, but electrically, the load fluctuates in an extremely wide variety depending on its shape, material, location, etc. However, the conduction signal is generated again by detecting that the voltage at the switching element terminal has dropped below a predetermined value, or by detecting that the current in the switching element (including the flywheel diode) flows in the opposite direction. In conventional control systems, 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 has been an obstacle to improving efficiency and other aspects.

OBJECTS OF THE INVENTION The present invention provides an induction heating cooker that solves these conventional problems and 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 the present invention consists of a filter circuit consisting of a choke coil and a filter capacitor connected to a commercial power supply, and a semiconductor switching element that excites a resonant circuit of a heating coil and a resonant capacitor connected to both ends of the filter capacitor. a frequency conversion device consisting of a frequency converter, a rate-of-change detection circuit that detects the rate of change of the terminal voltage of a semiconductor switching element, a trigger circuit that generates a signal when the rate of voltage change reaches the second zero, and an output of the trigger circuit. and a drive circuit connected to the semiconductor switching element for supplying an ON signal of a predetermined duration to the semiconductor switching element.

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

Furthermore, the heating coil 6 is attached to the filter capacitor 5.
A parallel resonant circuit of the resonant capacitor 7 and a series circuit of an anti-parallel circuit of the NPN transistor 8 and the flywheel diode 9 are connected to form a frequency converter 20. The terminal voltage V ₁ of the transistor 8 is sent to the rate of change detection circuit 10.
0 is the trigger circuit 11 every time the rate of change of V ₁ becomes zero.
Generates output to. The trigger circuit 11 triggers the drive circuit 12 in synchronization with the second output of the rate of change detection circuit 10.
After that, a drive signal with a predetermined pulse width is supplied to the base terminal of the transistor 8.

FIG. 2 provides a more detailed explanation of the block diagram of the embodiment shown in FIG. 1, and the operation of each part will be explained below along with FIG. 3 which shows the waveforms of each part. The transformer 13 connected to the commercial power supply 1 has a secondary winding with a center tap, and changes two positive and negative power supplies using rectifier diodes 14, 15, smoothing capacitors 16, 17, and voltage regulating diodes 18, 19, respectively. The signal is supplied to the rate detection circuit 10, the trigger circuit 11, and the drive circuit 12, and the midpoint thereof is connected to the emitter of the transistor 8. A differential circuit consisting of two resistors 102 and 103 connected in series with a capacitor 101 constituting the rate of change detection circuit 10 is connected between the terminals of the transistor 8, and a comparator 106 connected to the midpoint of the resistors 102 and 103. Input the differential waveform of V ₁ to the + input terminal.

In addition, the − input terminal of the comparator 106 is connected to two voltage dividing resistors 104 and 1 connected between the load power supplies.
05 supplies a constant negative voltage as a reference voltage.

Here, V ₁ 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 FIG. The result is shown in Figure 3b. In the figure, the dashed line indicates the reference voltage of the comparator 106. FIG. 3c represents the output signal of comparator 106 and resistor 10
7 and is supplied as an output signal of the rate of change detection circuit 10 to the clock terminal of a D-flip-flop 111 (hereinafter referred to as D-F/F) constituting the trigger circuit 11. As can be seen from the _input signal, the comparator ₁₀₆
The output signal of is inverted, and in the circuit of the embodiment of FIG. 2, the output of comparator 106 changes from low to high at the point where dV ₁ /dt becomes zero for the second time after transistor 8 is turned off. D-F/F111 is NEC Corporation
A commercially available IC such as UPD4013BC made by Manufacturer can be used, and its output terminal and D terminal are connected.
D-F/F111 is D at the rising edge of the clock input.
Since the terminal signal is read and the inverted output is generated at the terminal, the output (Figure 3 d) is inverted at the point where dV ₁ /dt becomes zero for the second time (at the rising edge of Figure 3 c) as described above. . The reason why a thin negative pulse is generated after the rising waveform in Figure 3c is as described below.
This is because when dV ₁ /dt=0 is detected and transistor 8 is made conductive, V ₁ drops to zero, so the input voltage of comparator 106 becomes a signal with dV ₁ /dt<0, and its pulse width is mainly The turn-on time of the transistor 8 is approximately 1 to 3 μS. In this way, when transistor 8 is turned on, D-
Since two rising points occur in the clock input of F/F111 in a short period of time, the output signal is
The pulse becomes a narrow pulse as shown in FIG. d, and is connected to the trigger terminal of the monostable multivibrator 121 that constitutes the drive circuit 12. This monostable multivibrator 121 may be a general type made of transistors, but here we use a monostable multivibrator made by NEC Corporation.
UPD4528BC is used.

The monostable multivibrator 121 generates an output with a predetermined pulse width T ₁ determined by an external resistor 128 and a capacitor 129 as shown in FIG. 3e in response to the trigger signal (FIG. 3d), and Supplying base current to transistor 125. The signal inverted by the transistor 125 is again inverted and amplified by the output transistor 127 and then sent to the transistor 8.
A base current is caused to flow through the resistor 126. FIG. 3f shows the collector current of transistor 8. Then, after an hour _T1 , the output transistor 127 is turned on, applying a reverse bias to the transistor 8, turning it off, and generating the oscillatory _V1 again. Therefore, the operating frequency of the frequency converter 20 is determined by the resonant frequency (fixed) of the heating coil 6 and the resonant capacitor 7 and the output pulse width (T ₁ ) of the drive circuit 12.
The longer T ₁ is, the larger the high frequency current flows through the heating coil 6, so by controlling T ₁ ,
It is possible to vary the induction heating output.

As can be seen from the above explanation of operation, transistor 8 turns on when the terminal voltage is at its lowest, so it operates with minimum switching loss, making it compact and compact.
It can be inexpensive, and since it generates little heat, it is possible to reduce noise by making the cooling fan smaller, and to make the product smaller and thinner by making the heat dissipation fin smaller.

The above example uses the transistor 8 as a semiconductor switching element, but instead of a transistor, an element that can be turned on or off using a control signal, such as a GTO (gate turn-off thyristor) or a FET (field effect transistor), can be used. There are no restrictions. 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 heated object, the semiconductor switching element always operates in such a way that the loss is minimized, making the equipment more efficient, 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. Since the second minimum point of the voltage is detected to drive the semiconductor switching element, the inductance of the heating coil has a saturation characteristic depending on the material of the pot, etc., and even if the oscillating voltage is a non-sinusoidal wave (distorted wave), it will accurately reach the minimum point. It is fired at a point to minimize switching losses.

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. 4... Chi York coil, 5... Filter capacitor, 6... Heating coil, 7... Resonance capacitor, 8... Semiconductor switching element (transistor), 10... Rate of change detection circuit, 11... Trigger circuit, 12... Drive circuit, 20...frequency conversion device.

Claims (1)

[Claims] 1 A frequency conversion device consisting of a filter circuit consisting of a chiyoke 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 the terminal voltage; a trigger circuit that generates a signal when the rate of change of the voltage reaches a second zero;
An induction heating cooker comprising a drive circuit connected to the output of the trigger circuit to supply an ON signal of a predetermined duration to a semiconductor switching element.