JP2893942B2 - Induction heating device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating device, and more particularly to an induction heating device capable of heating a non-magnetic metal such as aluminum as an object to be heated.

2. Description of the Related Art Conventionally, various induction heating devices have been developed for heating a metal pan.

FIG. 3 shows a circuit configuration of a conventional induction heating device. Third
In the figure, a commercial power supply 1 is rectified by a diode bridge 2 and smoothed by a smoothing capacitor 3 to constitute a unidirectional power supply 4.
The unidirectional power supply 4 includes a resonance capacitor 5, a switching element 6,
And the like. The output of the unidirectional power supply 4 is converted into the output of the high-frequency power supply by the inverter circuit 7 and supplied to the heating coil 8. Magnetic flux is supplied to the object to be heated 9 such as a metal pot by the heating coil 8, and Joule heat is generated by eddy current flowing in the metal.

Problems to be Solved by the Invention However, such a configuration has a problem in that the type of metal to be heated is limited. For example, there is an aluminum pan. Aluminum has a very small magnetic permeability compared to a magnetic material such as iron, so that almost no eddy current is generated. For this reason, there was a problem that the aluminum pan could hardly be heated. In order to solve this problem, it is conceivable to operate the inverter circuit at a higher frequency, but in the conventional configuration, the loss of the switching element increases. Further, since the load of the aluminum pan has a small input impedance, a new problem arises in that load matching cannot be performed with the same power supply as the iron pan.

SUMMARY OF THE INVENTION An object of the present invention is to provide an induction heating apparatus capable of heating not only iron but also a metal object having a low magnetic permeability such as aluminum.

Means for Solving the Problems To achieve the above object, the present invention provides a unidirectional power supply, and a DC voltage converter capable of converting the unidirectional power supply to a low-voltage DC power supply and adjusting the voltage of the DC power supply. An inverter circuit including a semiconductor switching element for converting the output of the low-voltage DC power supply into a high-frequency output by the DC voltage converter and a drive control circuit for the semiconductor switching element; A heating coil for producing a high-frequency magnetic flux for heating, wherein the switching frequency of the inverter circuit is set to a switching frequency when the metallic object to be heated is made of a material having a low magnetic permeability; Is a material having a high magnetic permeability, the output voltage of the DC voltage converter is adjusted and set high.

Function The induction heating device of the present invention converts the power supply voltage to a high-frequency voltage by the inverter circuit after reducing the power supply voltage to a low voltage by the voltage conversion device. Therefore, the voltage applied to the switching element of the inverter circuit can be low. Therefore, it is possible to use an element such as a MOS / FET, which has a low withstand voltage but a small high-frequency loss. For this reason, high-frequency switching becomes possible and heating of a magnetic material such as aluminum becomes possible.

Further, the difference in input impedance with respect to a magnetic material load such as iron can be matched by changing the output of the voltage conversion device, and an induction heating device that can select any load can be realized.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

 FIG. 1 shows a circuit diagram of the induction heating apparatus of the present invention.

In FIG. 1, the output of a commercial power supply 11 is rectified by a diode bridge 12 and smoothed by a smoothing capacitor 13 to form a unidirectional power supply 14. The output voltage of the unidirectional power supply 14 is converted by the DC voltage converter 15 into a voltage of about 10 volts. The output of the low-voltage DC power supply thus produced is converted into a high-frequency output by an inverter circuit 20 including a resonance capacitor 17, a switching element 18 of a MOS / FET, and a drive control circuit 19 for driving the switching element.

FIG. 2 shows waveforms of the voltage Vs applied to the switching element 18, the current Is flowing through the switching element 18, and the loss of the switching element 18. In FIG. 2, when the switching element 18 is conducting, a loss occurs due to the ON resistance of the element, and a switching loss occurs during the ON / OFF operation. The circuit of this embodiment is a voltage resonance type switching circuit, and a resonance circuit is provided so that the voltage of the switching element becomes almost zero at the time of ON / OFF, thereby reducing the loss.

Conventionally, the frequency generally used for induction heating is several tens of kilohertz. Metals with low magnetic permeability, such as aluminum, have a large skin depth. That is, since the high-frequency magnetic flux is easily transmitted, almost no eddy current is generated by the high-frequency magnetic flux as compared with a ferromagnetic material such as iron. To compensate for this, it is necessary to operate the inverter circuit at a higher frequency.

By the way, as a semiconductor switching element that currently handles about 1 kilowatt of power, a self-extinguishing element that is relatively easy to control is a bipolar transistor, IGBT, MOS / F.
There is ET. Among them, it is known that MOS • FETs having low switching loss and suitable for use at high frequencies are used. However, MOS-FETs do not recombine holes and electrons when conducting, unlike bipolar transistors, so charged particles remain in the crystal and block the flow of free electrons, increasing the on-resistance. . For this reason, the loss at the time of conduction, which is called on-loss, becomes large. In order to reduce the on-loss, it is necessary to make the channel layer of the element thinner to reduce the internal loss. However, in this case, since the distance between the drain and the source is short, the breakdown voltage of the element is low.

In the present embodiment, the voltage applied to the switching element 18 can be small because the voltage is set to more than ten volts by using the DC voltage converter 15. In other words, MOS suitable for high-frequency operation
・ FET can be used. MOS / F suitable for high-frequency switching is used as a switching element of the induction heating device of the present embodiment.
Uses ET. Thus, by setting the switching frequency to several hundred KHz, it is possible to heat the aluminum pan.

The power converted to high frequency by the inverter circuit 20 is supplied to the high-frequency heating coil 21 and is supplied to the metallic object to be heated 22 as high-frequency magnetic flux. The object to be heated is heated by the eddy current flowing through the metal.

Here, when the aluminum pan and the iron pan are connected to the heating coil 21 having the same configuration, the input impedance of the heating coil 21 is smaller in the aluminum pan. Therefore,
It is difficult to heat two types of pots with the same heating coil. However, in this embodiment, since the power supply voltage can be adjusted by the voltage conversion device 15, the same power supply can be performed even when the input impedance changes significantly. That is, in the case of an iron-based hot pot, the power supply voltage may be increased to adjust the power.

In order to measure the input impedance of the load, a predetermined minute input voltage is applied to the heating coil, and the magnitude of the input impedance is determined based on the current flowing at that time. That is, in the case of an aluminum load, a larger current flows than in the case of iron. In this way, the load is determined, and an appropriate power supply voltage and frequency are selected for the load.

In a conventional induction heating device that does not use a voltage conversion device, the driving frequency of the switching element is changed in order to match the fluctuation of the load. That is, in the case of a light load, the frequency is increased. However, for non-magnetic materials such as aluminum, simply changing the operating frequency
There was a problem that sufficient matching could not be obtained and output power was limited. As a solution to this problem, various measures have been considered, such as switching between a heating coil and a capacitor for resonance according to the load. was there. The induction heating device of the present invention realizes an induction heating device capable of heating a wide range of loads in order to adjust a power supply voltage by a DC voltage converter and obtain matching with a load.

Effects of the Invention As described above, the following effects can be obtained with the induction heating device of the present invention.

(1) MOS / FET with low withstand voltage, that is, low on-resistance, because the power supply voltage is lowered by the DC voltage converter.
Can be used. Therefore, switching at several hundred KHz is possible, so that an induction heating device capable of heating even a metal having low magnetic permeability such as aluminum can be realized.

(2) Since a DC voltage converter can adjust the power supply voltage, an induction heating device that can supply the same power to loads such as iron and aluminum that have greatly different input impedances with the same circuit configuration is used. realizable.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an induction heating device in one embodiment of the present invention, FIG. 2 is a voltage / current waveform diagram of a switching element of the device, and FIG. 3 is a circuit diagram of a conventional induction heating device. 14 ... unidirectional power supply, 15 ... DC voltage converter, 18 ... semiconductor switching element, 19 ... drive control circuit, 20 ... inverter circuit, 21 ... heating coil.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yuji Nakabayashi 1006 Kazuma Kadoma, Kadoma City, Osaka Inside Matsushita Electric Industrial Co., Ltd. (56) References −83047 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) H05B 6/12

Claims (1)

(57) [Claims] 1. A unidirectional power supply, a DC voltage converter for converting the unidirectional power supply to a low voltage DC power supply and capable of adjusting the voltage of the DC power supply, and an output of the low voltage DC power supply by the DC voltage converter. An inverter circuit comprising a semiconductor switching element and a drive control circuit for converting a semiconductor switching element into a high-frequency output, and a heating coil for generating a high-frequency magnetic flux for inductively heating a metallic object to be supplied with the output of the inverter circuit, The switching frequency of the inverter circuit is set to a switching frequency when the metallic object to be heated is made of a material having a low magnetic permeability. An induction heating device, wherein the output voltage of the converter is adjusted and set high.