JPS5941278B2 - High frequency induction heating device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A high frequency current is supplied to a work coil by driving a switching element with pulses having a high repetition frequency to turn it on and off.

High frequency induction heating devices are already known. When this device is turned on, the internal oscillator operates, and the resulting high-frequency drive pulse is supplied to the switching element, thereby supplying high-frequency current to the work coil. However, voltage is applied from the rectifier circuit to the work coil and the switching element almost at the same time as the power is turned on, whereas the oscillator cannot operate almost at the same time as the power is turned on.
This is because a large-capacity smoothing capacitor is inserted on the power supply side of the oscillator, and it takes time to charge it.
This is because it takes time until a normal DC voltage is applied to the oscillator. Generally, the power source for the work coil and the low voltage power source for the oscillator are provided separately, and a smoothing capacitor, that is, a large capacity capacitor, is not inserted in the power source for the work coil, and the external AC power source is connected from the induction heating device. A small-capacity capacitor is inserted to form a filter for high-frequency signals leaking to the PCB, so this capacitor charges quickly and voltage is applied to the work coil and switching element almost at the same time as the power is turned on. will be done. If a driving pulse is applied to the base or gate of the switching element to turn it on and off after a voltage is applied to the switching element, such as immediately after the power is turned on, the switching element may be damaged.

The present invention avoids these drawbacks, and therefore detects the rise of the oscillator after the power is turned on, and uses this detection output to temporarily reduce the voltage applied to the work coil and switching element. At this point, a driving pulse is supplied to turn the switching element on and off.

To explain the drawings below, 1a and Ib are AC power supply terminals, 2 is a power switch, and 3 is a rectifier circuit.

4 is a filter for high frequency signals, 4a is its choke coil, and 4b is a capacitor.

The current passed through this filter 4 is supplied to a work coil 6 through a capacitor 5. Further, a damper diode 7 and a switching element 8 such as a GCS are connected in parallel to the series circuit of the capacitor 5 and the work coil 6. 9 is an oscillator, for example 20kHz
A drive pulse with a repetition frequency of
The secondary coil 1 is supplied to the secondary coil 11a.
As usual, the pulse obtained at 1b is supplied to the gate of GCS8.

The output (current) from the rectifier circuit 3 is further passed through a reverse blocking diode 12 to a relatively large capacity smoothing capacitor 13.
is supplied to obtain a DC voltage across the capacitor 13,
This is supplied to the starting circuit 15 and used as a power source for its operation.

Therefore, if the rectifier circuit 3 is used as a first power source, the diode 12 and capacitor 13 constitute a second power source 14. When this starting circuit 15 starts operating, a DC voltage c is obtained from it, and the above-mentioned oscillator 9 is thereby started to operate. A secondary coil 11c is further wound around the drive transformer 11, and the output (
The current) is supplied to a rectifier circuit 18 consisting of a diode 16 and a capacitor 17, so that a DC voltage Vd is obtained at its output side.

Therefore, this rectifier circuit 18 constitutes a third power supply in this example. Since the voltage d of this rectifier circuit 18 cannot be obtained unless the oscillator 9 is fully operated, this circuit forms a detection circuit for the operation of the oscillator. This voltage Vd is applied to the zero-volt pulse generation circuit 19 and the control circuit 20 as their operating power supply voltages, and these circuits 19
and 20 are designed to operate at this voltage Vd.

A series circuit of resistors 24a and 24b is connected to the AC power supply terminals 1a and 1b, and the connection point thereof is grounded through a resistor 24c and connected to the input terminal of the zero-volt pulse generation circuit 19, and a power supply is connected to the output side. The pulse Pe is obtained at the time when the AC signals applied to the terminals 1a and 1b cross the zero level line. This circuit 1
9 is well known. This pulse Pe is the control circuit 2 of the next stage.
0, from which the pulse Pf and signal Sg are obtained. On the other hand, a resistor 21 is connected to the output side of the rectifier circuit 3.
A series circuit with a switching element 22 such as an SCR is connected to the secondary coil 1 of the drive transformer 11.
On the output side of 1b is a transistor 2 that constitutes a gate circuit.
3 collector emitters are connected, and the pulse Pf mentioned above is supplied to the gate of the SCR 22, and the signal Sg is supplied to the base of the transistor 23.

Next, its operation will be explained with reference to the waveform diagram in FIG.

A signal (AC voltage) V shown in FIG. 2A is applied to terminals 1a and 1b.
When a is applied, turn on power switch 2 (second
At time T in the figure), this alternating current signal is full-wave rectified by the rectifier circuit 3, so that the voltage Vb shown in FIG. 2B is obtained at its output terminal. This voltage b is further charged to the capacitor 13 through the diode 12, whereby the starting circuit 15 starts operating, and the output terminal is connected to the capacitor 13 as shown in FIG.
A voltage Vc shown in is obtained. This voltage Vc is supplied to the oscillator 9 as its operating DC power supply voltage, so that the oscillator 9 starts oscillating at time T2.

As a result, a voltage can be obtained in the secondary coil 11c of the transformer 11, and therefore a voltage is applied to both ends of the capacitor 17.
The DC voltage Vd shown in Figure D is now obtained. This voltage d becomes a predetermined voltage Vd' when the oscillator 9 becomes fully operational, and this point is indicated by T3 in FIG. 2D. In this way, a voltage d is obtained across the capacitor 17, and when this reaches a predetermined value Vd',
That is, when the oscillator 9 starts to operate completely, this voltage Vd
The zero-volt pulse generating circuit 19 and the control circuit 20, which use the operating DC power supply voltage as the operating DC power supply voltage, each start operating, and the zero-volt pulse generating circuit 19 obtains the pulse Pe shown in FIG. 2E.

This pulse Pe is obtained at the time when the signal Va crosses the zero level. When the pulse Pe is supplied to the control circuit 20, it causes the pulse Pf shown in FIG. 2F and the signal S shown in FIG. 3G (by the third pulse Pe in the example of FIG. 2).
g is obtained at time T4. In this example, this signal Sg is configured to be 1 (high level (c)) before time T4, and O (0 level (c) after time T4,
Since this is supplied to the base of the transistor 23, the transistor 23 is in an on state before time T4, and therefore the output of the secondary coil 11b of the transformer 11 is short-circuited by this transistor 23, and the switching element 8 No driving pulse is supplied to T4.
Thereafter, since the transistor 23 is turned off, the drive pulse obtained in the secondary coil 11b of the transformer 11 is supplied to the gate of the switching element 8, which turns on and off. This state is shown in FIG. 2H. Ph is a drive pulse. On the other hand, since the above-mentioned pulse Pf is applied to the gate of the SCR 22 at time T4,
This turns on, and therefore, at this time, the voltage Vb at the output end of the rectifier circuit 3 is significantly stepped down.

Since this time T4 is the time when the instantaneous value of the AC signal Va becomes zero, even if the output side of the rectifier circuit 3 is short-circuited at this time, a large current will not flow through the rectifier circuit 3, which will cause damage. I won't even receive it. When starting to supply the five drive pulses Ph to the switching element 8 in this manner, the output voltage b from the rectifier circuit 3 is once significantly lowered by turning on the SCR 22. As is clear from the above explanation, according to the present invention, the operating state of the oscillator 9 is detected by the output voltage Vd of the rectifier circuit 18 as a detection circuit, and when the oscillator 9 is completely in the operating state, The pulse Pf from the control circuit 20 turns on the SCR 22 to once drop the output voltage from the rectifier circuit 3 to a large extent, and at the same time, the driving pulse Ph can be supplied to the switching element 8. This feature has the feature that it is possible to avoid the risk of damaging the 8. Conventionally, for example, as a DC power supply for the oscillator 9, the voltage applied to the AC power supply terminals 1a and 1b has been stepped down and rectified using a step-down transformer. However, since the above-described embodiment of the present invention does not use such a transformer, the above-mentioned drawbacks can be avoided.

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing an example of the device according to the present invention, and FIG. 2 is a waveform diagram for explaining its operation. 2 is a power switch, 6 is a work coil, 8 is a switching element, and 9 is a drive pulse oscillator.

Claims (1)

[Claims] 1. In a high-frequency induction heating device that supplies a high-frequency current to a work coil by turning on and off a switching element, detecting the rise of a drive pulse oscillator that drives the switching element after the power is turned on, The voltage applied to the work coil is at least temporarily lowered by the detection output, and the driving pulse is supplied to the switching element from the point at which the voltage is lowered. High frequency induction heating equipment.