JPH0260086A - induction heating device - Google Patents

Abstract

PURPOSE:To prevent mis-energizing of a switching element and destruction thereof by keeping the switching element on an off only for a time from turn-on of AC power until an output voltage of DC power circuit reaches a normal operation voltage of a control circuit. CONSTITUTION:When an AC power is turned on, the drain source voltage VDS of a switching element 10c rises and then, an output voltage VCC of a DC power circuit 12 rises behind VDS. A gate source voltage VGS of the element 10 begins rising in a period of time for which VCC does not reach a voltage at which a control circuit 11 works normally. However, when VGS reaches such a voltage that a base current can be supplied to a transistor Tr 13f of a protection circuit 13 through a resistor 13d, the Tr 13f conducts so that VGS never rises up higher than that voltage. If a control voltage is set lower than the threshold voltage of the element 10c, the VGS of the element 10c does not exceed the threshold voltage, so that it is possible to prevent misenergizing of the element 10c and its destruction.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an induction heating device having a high frequency inverter including switching elements.

Conventional technology.

A conventional induction heating device has a configuration as shown in FIG.

In FIG. 3, 1 is an AC power source and 2 is a switch, which is shown as a power plug in the figure. 3 is a rectifier circuit that rectifies the AC power supply 1, 4 is a high frequency inverter connected to the output of the rectifier circuit 3, and includes a heating coil 4alt1 capacitor 4b,
It has a switching element 4C. Reference numeral 5 denotes a control circuit for controlling conduction/non-conduction of the switching element 4C, which includes an oscillation circuit 5a, a resistor 6b, a transistor 5C, etc. as shown in FIG. 3(b). A DC power supply circuit 6 serves as a power source for the control circuit 5, and converts the AC power supply 1 supplied via the power plug 2 into a DC voltage suitable for the control circuit 6.

The operation when the AC power is turned on will be described below with reference to FIG.

When connecting the power plug 2 to the AC power source 1, that is, when turning on the AC power source, the rectifier circuit 3. The drain of the switching element 4c, the lease voltage VOS via the resonant capacitor 4b
increases as shown in FIG. 4(a).

At this time, since the DC power supply circuit 6 includes a large-capacity smoothing capacitor, the output voltage VCC of the DC power supply circuit 6 rises with a delay from vDs as shown in FIG. 4(b). The lower limit of the normal operating voltage of the oscillation circuit 6a is V in FIG. 4(b).
Assuming that B, the transistor 5C in the control circuit 5 is generally in an open collector state during the period TS when Vcc<va. Furthermore, since a parasitic capacitance Coa generally exists between the drain and gate of the switching element 3C, the gate and source voltages yGs rise according to VDS through this parasitic capacitance as shown in FIG. 4(C). After the period Ts, if the oscillation circuit 5a operates normally, the transistor 5C becomes conductive and VaS becomes zero.

Problems to be Solved by the Invention In the conventional configuration described above, since the gate and source voltages VGS of the switching element 4C rise when the AC power is turned on, the switching element 3C becomes erroneously conductive when the threshold voltage of the switching element 4C is exceeded. However, there was a problem that there was a risk of destruction.

The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a highly reliable induction heating device that can prevent the risk of destruction of switching elements when AC power is turned on.

Means for Solving the Problem In order to achieve this object, the induction heating device of the present invention provides protection for keeping the switching element in the OFF state only during the period until the output voltage of the DC power supply circuit reaches the normal operating voltage of the control circuit. It is equipped with a circuit.

Effect: With this configuration, the switching element is maintained in the OFF state from the time the AC power is turned on until the control circuit operates stably, so that erroneous conduction of the switching element can be prevented and the risk of destruction thereof can be avoided.

EXAMPLE Hereinafter, an example of the present invention will be described with reference to FIGS. 1 and 2. In Figure 1, 7 is an AC''
The power source 8 is a switch, which is shown as a power plug in the figure, but it may also be constituted by a switch, for example. 9 is a rectifier circuit, 1o is a high frequency inverter, and heating coil 10a
, a resonant capacitor 10b, and a switching element 10C, and in this embodiment, M is used as the switching element 10C.
OSFET is used. 11 is a switching element 10
A control circuit 12 for controlling conduction/non-conduction of 0 is a DC power supply circuit that serves as a power source for the control circuit 11. 13 indicates that the output voltage of DC power supply circuits 1 and 2 is normal operation of the control circuit.
This is a protection circuit that limits the gate and source voltages of the switching element 10C only until the voltage reaches i. Zener diode 13a, resistors 13b, 13c, 13d, transistor 13θ

It has 13f'.

The operation of the induction heating apparatus of this embodiment constructed as described above will be described below with reference to FIG. 2.

First, when the power plug 8 is connected to the AC power supply 7, that is, when the AC power is turned on, the rectifier circuit 9. The drain and source voltages VDS of the switching element 10C rise through the resonance capacitor 3b as shown in FIG. 2(a). At this time, the output voltage VCC of the DC power supply circuit 12 is vD as in the conventional example.
It rises with a delay with respect to s as shown in FIG. 2(b).

During the period TS when VCC has not reached the voltage va at which the control circuit 11 operates normally, the switching element 1 is activated as in the conventional example.
The gate and source voltage VGS of 00 begins to rise. However, as shown in FIG. 2(C), when VGS reaches a voltage at which a pace current is supplied to transistor 13f of protection circuit 13 via resistor 13d, transistor 13f becomes conductive, and VGS does not rise any further. In other words, the pace and emitter voltage of the transistor 13f is Va! :+5f
When the resistance value of the positive resistor 13d is R+3d, the feedback current flowing through the parasitic capacitance COC of the switching element 10c when the AC power is turned on is Re, and the amplification factor of the transistor 13f is 11rE+sf, the limit voltage vcsc is as follows. Therefore, if hFz+sf is made large enough, VGS
C can be limited to a voltage slightly higher than vBE. If this limiting voltage vasc is set below the threshold voltage of the switching element 10C, O3 of the switching element 10c will not generate a threshold voltage when the AC power is turned on, thereby preventing erroneous conduction of the switching element 10c and preventing damage. Danger can be avoided.It goes without saying that the DC power supply circuit 12
The output voltage VCC of the control circuit 11 is the normal operating voltage Va
If vGs remains restricted even after the
The output (drive signal of the switching element 100) is not transmitted to the switching element 10C. Therefore, in the present invention, when the output voltage VCC of the DC power supply circuit 12 becomes equal to or higher than the normal operating voltage va of the control circuit, the Zener diode 13a of the protection circuit 13 becomes conductive and the resistors 13b, 13
0 causes transistor 136 to conduct. As a result, the transistor 13f is cut off, and the W dynamic signal of the switching element 13c supplied from the control circuit 11 is transferred to the protection circuit 13.
It is transmitted correctly without being influenced by

As described above, in this embodiment, by providing a protection circuit with an extremely simple configuration, when the AC power is turned on, the gate and source voltages of the switching elements are The switching element can be maintained in the off state by limiting the voltage to below its threshold voltage.

Effects of the Invention As is clear from the above embodiments, according to the present invention, the switching element is turned off only during the period from when the AC power is turned on until the output voltage of the DC power supply circuit reaches the normal operating voltage of the control circuit. Therefore, it is possible to prevent erroneous conduction of the switching element and prevent its destruction, and to realize a highly reliable induction heating device that can correctly transmit the output to the switching element during normal operation of the control circuit.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an induction heating device according to an embodiment of the present invention, Fig. 2 is a waveform diagram of the same operation, Fig. 3 is a circuit diagram of a conventional induction heating device, and Fig. 4 is a waveform diagram of the same operation. . 7... AC power supply, 9... Rectifier circuit, 10...
...High frequency inverter, 10C...Switching element, 11...Control circuit, 12...
...DC power supply circuit, 13...protection circuit. Name of agent: Patent attorney Shigetaka Awano and one other person

Claims (1)

[Claims] A rectifier circuit and a DC power supply circuit connected to an AC power source via a switch, a high frequency inverter including a switching element that converts the output of the rectifier circuit to a high frequency, and controlling conduction and non-conduction of the switching element, A control circuit receives power from the DC power supply circuit, and maintains the switching element in a non-conductive state only during a period until the output voltage of the DC power supply circuit reaches a normal operating voltage of the control circuit when the switch is closed. Induction heating device with protection circuit.