JPS5932878B2 - induction heating cooker - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an induction heating cooker, and more particularly to a device that detects a small object load and stops oscillation of an inverter circuit.

Configuration of conventional example and its problems Conventional induction heating cookers are equipped with a small object load detection device to detect the small object load, because there is a risk of burns when heating small object loads such as spoons and kitchen knives.

This conventional small load detection device detects the input current of an inverter circuit, and when this input current is smaller than a predetermined value, it determines that the load is a small object and stops the oscillation of the inverter circuit. However, in induction heating cooking using an output control method that controls the on/off repetition frequency of the power switching semiconductors that make up the inverter circuit or the conduction pulse width of the power switching semiconductors, it is difficult to control the output when heating a normal load such as a normal pot. If it is made smaller, the input current of the inverter circuit becomes smaller, and the small load detection device detects an input current of the same magnitude as that of a small load, which has the disadvantage that the oscillation of the inverter circuit stops. In addition, in order to solve the above-mentioned drawbacks, conventionally, the current flowing through the inverter circuit at startup is detected, the setting level is switched to two levels depending on the magnitude of this current, and this switched setting level and the inverter circuit are It has been proposed to compare the magnitude of the input current with the magnitude of the input current to determine whether the load is a small object or not.

However, even in this case, as mentioned above, when the output is reduced, the current flowing through the inverter circuit becomes smaller, and the above setting level is switched to a lower setting level, so there is a possibility that the judgment as to whether or not it is a small load may be made incorrectly. There was a drawback that there was. OBJECTS OF THE INVENTION The present invention eliminates the above-described drawbacks of the conventional inverter circuit, and controls the oscillation of an inverter circuit by detecting with high accuracy whether or not the load is a small object even when the output is reduced.

Structure of the Invention In order to achieve the above object, the induction heating cooker of the present invention particularly includes an oscillation control circuit that supplies a base drive signal to a power switching semiconductor constituting an inverter circuit, and an oscillation control circuit that supplies a base drive signal to a power switching semiconductor that constitutes an inverter circuit, and an oscillation control circuit that supplies an input current to the inverter circuit or An input detection circuit that detects the magnitude of input power and outputs an output signal proportional to this magnitude, and an input detection circuit that detects the magnitude of the voltage of the heating coil that makes up the inverter circuit and outputs a signal proportional to this magnitude. a power detection circuit that outputs a signal; an output signal of the input detection circuit and an output signal of the voltage detection circuit are compared; and if the output signal of the input detection circuit is smaller than the output signal of the voltage detection circuit; The oscillation control circuit includes a small object load detection circuit that supplies a signal to the oscillation control circuit to inhibit the output of the base drive signal from the oscillation control circuit.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 shows the electric circuit of the induction heating cooker of this embodiment,
Figure 2 shows the signal waveform of each part in the same electric circuit when the output is set to maximum, Figure 3 shows the signal waveform of each part in the same electric circuit when the output is reduced, and Figure 4 shows the input current and heating in the same electric circuit. The relationship between the voltages of the coils is shown.

In FIG. 1, 1 is a low frequency AC power supply, 2 is a rectifier circuit that converts AC power from the low frequency AC power supply 1 into DC power,
3 is an inverter circuit that converts DC power from the rectifier circuit 2 into high-frequency power, and 5 is a control circuit that controls oscillation of the inverter circuit 3.

The inverter circuit 3 includes a capacitor 30 connected between the output terminals of the rectifier circuit 2, a series circuit of a power switching semiconductor 31 and a heating coil 32 connected between the same output terminals, and a series circuit connected in parallel to the heating coil 32. It is composed of a resonance capacitor 33 and a damper diode 34 connected in parallel in the opposite direction to the power switching semiconductor 31. The series circuit is formed by connecting a power switching semiconductor 31 to the + side output terminal of the rectifier circuit 2, and connecting a heating coil 32 to one side output terminal of the rectifier circuit 2. The control circuit 5 includes an input detection circuit 51 which inputs a signal from a current transformer 50 for detecting the magnitude of the input current of the inverter circuit 3 and outputs an output signal proportional to the input current, and a heating coil 32. Edge: 22nd year of junior high school = W
The output signal of the input detection circuit 51 and the output signal of the voltage detection circuit 52 are input and compared, and if the output signal of the input detection circuit 51 is smaller than the output signal of the voltage detection circuit 52, the output signal of the input detection circuit 51 and the voltage detection circuit 52 are compared. The small object detection circuit 53 prohibits the supply of the base drive signal from the oscillation control circuit 54 to the power switching semiconductor 31, and the terminal voltage V of the heating coil 32
The oscillation control circuit 54 detects the zero voltage of L and supplies a base drive signal to the power switching semiconductor 31 to control the conduction of the power switching semiconductor 31, and the width of the base drive signal from the oscillation control circuit 54. and an output control circuit 55 that controls the output.

Next, the operation in the above configuration will be explained. first,
The case where the output is set to the maximum output will be explained with reference to FIG. 2. In FIG. 2, A indicates the terminal voltage L of the heating coil 32, B indicates the collector current c of the power switching semiconductor 31 and the current 1d of the damper diode 34, and C indicates the base drive current of the power switching semiconductor 31. show.

The oscillation control circuit 54 detects the time TO at which the terminal voltage VL of the heating coil 32 crosses zero, and supplies the forward base current 1B1 to the power switching semiconductor 31 with a delay of about one pulse.

The width (Tpw time) of this forward base current IBl is set to the maximum by the output control circuit 55, and this width Tpw
After a lapse of time, a reverse base current 1B2 is supplied to turn off the power switching semiconductor 31. When the power switching semiconductor 31 is turned off, a flyback pulse voltage is generated in the heating coil 32 and the resonant capacitor 33. Then, when the terminal voltage of the heating coil 32 reaches the negative peak voltage VLP and then becomes higher than the DC power supply voltage i, the damper diode 34 becomes conductive. The flyback period is T2, and the conduction periods of the damper diode 34 and the power switching semiconductor 31 are Td and T, respectively.
c and T1=Td+Tc, the heating output from the heating coil 32 can be controlled by controlling the T1 period. That is, the heating output is controlled by adjusting the base current pulse width Tpw by the output control circuit 55. FIG. 3 shows a case where the base current pulse width Tpw is narrowed. At this time, the damper diode 34 is not conductive, and only the power switching semiconductor 31 is conductive.

The collector current c also becomes smaller, and the flyback voltage VLP
′ also becomes smaller. That is, the terminal voltage L of the heating coil 32 also becomes smaller. Of course, the input current of the inverter circuit 3 also decreases. In FIG. 4, the relationship between the input current and the terminal voltage of the heating coil 32 is shown as A for a normal load and B for a small load. For example, if the heating coil voltage is constant and compared, A relationship is necessarily established in which the input current for a normal load is larger than the input current for a small load. Therefore, it is possible to detect the small object load by comparing the heating coil voltage VL and the input current. In other words, in order to judge whether or not it is a small load based on the magnitude of the input current, it is possible to make a highly accurate judgment by changing the comparison setting value in proportion to the heating coil voltage. In the above embodiment, the DC voltage i is the heating coil voltage VL and the collector-emitter voltage CE of the power switching semiconductor 31.
is equal to the sum of That is, i = VL + VCE, and if the collector-emitter voltage CE is detected, it means that the heating coil voltage 1 has been detected, and even if this voltage CE is compared with the input current, it is determined whether it is a small load or not. It is possible to judge with high precision. As described above, the induction heating cooker of the present invention can reduce the load on small objects by comparing the signal proportional to the input current of the inverter circuit and the signal proportional to the terminal voltage of the heating coil constituting the inverter circuit. Since it is determined whether or not this is the case, the determination can be made with high accuracy even when the input current decreases, which occurs when the output is reduced by controlling the pulse width of the power switching semiconductor, or when the power supply voltage fluctuates.

Note that in the present invention, input current detection is not limited to a current transformer, and a resistor may be used, or a signal corresponding to input power may be detected.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of an induction heating cooker according to an embodiment of the present invention, Fig. 2 A, B, and C are waveform diagrams of various parts at the same high output, and Fig. 3 A'', B', and C'' are the same. Waveform diagram of various parts at low output, Part 4
The figure is a characteristic diagram showing the relationship between the human power current and the heating coil voltage. 1...Low frequency AC power supply, 2...Rectifier circuit, 31...Power switching semiconductor, 32
... Heating coil, 33 ... Resonance capacitor, 34 ... Damper diode.

Claims (1)

[Claims] 1 Consists of an inverter circuit that converts DC power into high-frequency power and a control circuit for this inverter circuit, the inverter circuit includes a power switching semiconductor, a heating coil, and a resonance capacitor, and the control circuit provides a base drive to the power switching semiconductor. an oscillation control circuit that provides a signal, an input detection circuit that detects the magnitude of the input current or input power to the inverter circuit and outputs an output signal proportional to this magnitude, and the magnitude of the voltage of the heating coil. and a voltage detection circuit that detects and outputs an output signal proportional to this magnitude, and compares the output signal of the input detection circuit with the output signal of the voltage detection circuit to determine whether the output signal of the input detection circuit is An induction heating cooker comprising an accessory load detection circuit that supplies a signal to the oscillation control circuit that prohibits the oscillation control circuit from outputting a base drive signal when the output signal is smaller than the output signal of the voltage detection circuit.