JPH03163785A - Induction heating device - Google Patents

Abstract

PURPOSE:To prevent the breakage of a switching element due to the increase of turn-on loss or the rise of the connection temperature by increasing the inductance of an induction heating coil with a switching means when a small output is set. CONSTITUTION:When a small output with a small value or below is set by a variable resistor 14, a comparator 17 compares the signal from the variable resistor 14 with a reference value determined by resistors 18 and 19, then a relay coil 16 is excited. A relay contact 15 does not short-circuit both ends of an auxiliary induction heating coil 92, and both a main induction coil 91 and the auxiliary induction coil 92 function as induction heating coils. The inductance of the induction heating coil is the sum of inductance of the main induction heating coil 91 and the auxiliary induction heating coil 92, and it is larger than that when a large output is set. The occurrence of the rush current is prevented when a switching element 11 is turned on, and the breakage of the switching element 11 due to the increase of turn-on loss or the rise of the connection temperature can be prevented.

Description

[Detailed description of the invention] (b) Industrial application field The present invention relates to an induction heating device.

(b) Conventional technology A conventional general induction heating device is
This is shown in Publication No. 8, etc. By the way, an actual induction heating cooker, which is a type of induction heating device, is configured so that the heating output can be adjusted depending on the type of cooking. In normal cooking, heating is performed at a high output, and in the case of keeping warm, etc., the operation is performed at a low output.

FIGS. 4 and 5 illustrate such a configuration.

1 is a commercial AC power supply, 2 is a power switch, 3 is a fuse,
Reference numeral 4 represents a power supply circuit comprising a diode bridge 5 for full-wave rectification of the commercial AC power supply 1, a choke coil 6, and a smoothing capacitor 7, and reference numeral 8 represents an inverter circuit comprising an induction heating coil 9, a resonant capacitor 10, and a switching element 1l. The switching element 11 is usually a Darlington transistor with a built-in diode, and the inverter circuit 8 is supplied with power by the power supply circuit 4.

12 is a current transformer that detects the current flowing through the diode bridge 5, and 13 is a control circuit consisting of peripheral circuits such as a so-called microcomputer and an input/output interface. The control circuit 13 inputs the detection result of the current transformer 12, the voltage at the connection point A between the choke coil 6 and the induction heating coil 9, and the connection point B between the induction heating coil 9 and the switching element 1l, and turns the switching element 11 on and off. to control the oscillation of the inverter circuit. 14 is the control circuit 1
This variable resistor serves as a setting means for setting the heating output power by setting the ON period of the switching element l1.

When heating is performed, the inverter circuit 8 oscillates and a high-frequency magnetic field is supplied from the induction heating coil 9 to a cooking utensil such as a pot, thereby performing induction heating.

Figure 5 shows the voltage waveforms at connection points A and B and switching element 1.
Figure 1 (a) shows the case where a relatively high output is set by the variable resistor 14, and Figure (b) shows the case where a relatively small output is set by the variable resistor 14. . When a large output is set, the on-time of the switching element 1l is lengthened and the oscillation frequency is lowered to increase the output, and when a small output is set, the on-time of the switching element 11 is shortened. Lower the output by increasing the oscillation frequency.

Here, the control circuit 13 sets the timing at which the switching element 11 is turned on when the potential at the connection point B, that is, the collector voltage of the switching element 11, becomes lower than the potential at the connection point A (indicated by T in FIG. 5). It is detected, and a control signal is issued to turn on the switching element 11 with a slight delay from time T.

In such a lift mode, if a large output is set, as shown in FIG. 5(a), the switching element 11 is turned on after the potential at the connection point B becomes 0, so no problem occurs.

However, if a small output is set,
), the potential of the connection point B decreases slowly after the on-timing T, so the potential of the connection point B becomes 0.
The switching element 11 is turned on before this happens.

At this time, when the switching element 11 is turned on, a rush current as shown in the figure flows, causing a turn-on loss, which causes problems such as an increase in junction temperature and damage to the switching element 11.

(c) Problems to be Solved by the Invention The present invention prevents the generation of rush current when the switching element 11 is turned on when a small output is set, and prevents the switching element 11 from increasing turn-on loss or increasing junction temperature. An object of the present invention is to provide an induction heating device that can prevent damage to the device.

(2) Means for Solving the Problems The induction heating device of the present invention includes an inverter circuit including an induction heating coil, a resonant capacitor, and a switching element, a power supply circuit that supplies power to the inverter circuit, and the switching element. a control circuit that turns on and off to control oscillation of the inverter circuit; a setting device that sets an on period of the switching element for the control circuit; and changing an inductance of the induction heating coil according to a setting value of the setting device. It is characterized by comprising a switching means.

(e) When the operation is set to a small output, the inductance of the induction heating coil is increased by the switching means, so that the peak of the collector voltage when the switching element 1l is off becomes high, and the drop after the on timing becomes steep. Therefore, the switching element 11 is turned on after its collector voltage becomes zero.

(F) Embodiment FIGS. 1 to 3 show an induction heating cooker which is an embodiment of the present invention. In the following description, parts that are the same as or correspond to those of the conventional example already described are given the same reference numerals, and the description thereof will be omitted, and only the different parts will be described.

In this embodiment, the induction heating coil is the main induction heating coil 9.
The main induction heating coil 9l has a smaller inductance than the main induction heating coil 9l, and a sub induction heating coil 92. 15 is a relay contact that short-circuits both ends of the sub-induction heating coil 92, and 16 is a relay contact that disconnects the relay contact 15. The relay coil to be replaced, 17, connects the number from the variable resistor 14 to the resistor 1.
8 and 19, and provides a drive signal to the relay coil l6.

When a high output is set by the variable resistor l4, the comparator 17 does not energize the relay coil 16, and therefore the relay contact 15 short-circuits both ends of the sub-induction heating coil 92, causing no heating. is performed only by the main induction heating coil 9l. The operation at this time is the same as when setting the high output in the conventional example.

When a small output below a certain level is set by the variable resistor 14, the comparator l7 compares the signal from the variable resistor 14 with a reference value determined by the resistors 18 and 19, and energizes the relay coil l6. . At this time, the relay contact 15 does not short-circuit both ends of the auxiliary induction heating coil 92, resulting in the circuit state shown in FIG. 1, and both the main induction heating coil 91 and the auxiliary induction heating coil 92 act as induction heating coils. In this state, the inductance of the induction heating coil is the sum of the main induction heating coil 91 and the sub induction heating coil 92, which is larger than when the high output is set. That is, the sub induction heating coil 92, the relay contact 15, the relay coil l6, and the comparator l7 correspond to switching means that changes the inductance of the induction heating coil according to the set value of the variable resistor 14.

FIG. 2 shows a waveform diagram when heating is performed with a small output setting at an on time corresponding to FIG. 5(b) according to this embodiment. In this case, as shown in Figure @3, the collector voltage when the switching element 11 is off has a higher peak than in the case of only the main induction heating coil 91 (indicated by the broken line in Figure 3), and the on-timing (T ) after which the decline becomes steeper. Therefore, the time from the on timing (T) until the collector voltage becomes 0 is short, and the switching element l1
It is turned on after its collector voltage becomes O. Therefore, even when a small output is set, a rush current does not occur when the switching element 11 is turned on, and it is possible to prevent an increase in turn-on loss and damage to the switching element l1 due to an increase in junction temperature. Furthermore, the minimum heating output, which could only be lowered to about 150 W in the past, can be set lower, and for example, it is possible to perform melting of chocolate for which about 50 W is optimal. Incidentally, since the sub-induction heating coil 92 is small and large, it hardly affects the heating output.

(g) Effects of the Invention According to the present invention, it is possible to prevent the generation of rush current when the switching element l1 is turned on when a small output is set, and to prevent the switching element from increasing turn-on loss or increasing junction temperature. 11 damage can be prevented. Further, heating operation can be safely performed at a lower output than conventionally.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of an induction heating cooker which is an embodiment of the present invention, Fig. 2 is a waveform diagram of the same main part, Fig. 3 is an enlarged view of the waveform of the main part, and Fig. 4 is a circuit showing a conventional example. 5(a) and 5(b) are waveform diagrams of the same essential parts. 13... Control circuit, l4... Variable resistor, 15...
- Relay contact, 16... Relay coil, 17... Comparator, 9l... Main induction heating coil, 92... Sub-induction heating coil.

Claims (1)

[Claims] (1) an inverter circuit including an induction heating coil, a resonant capacitor, and a switching element; a power supply circuit that supplies power to the inverter circuit; and a control circuit that turns on and off the switching element to control oscillation of the inverter circuit; An induction heating apparatus comprising: a setting means for setting an ON period of the switching element for the control circuit; and a switching means for changing the inductance of the induction heating coil according to a setting value of the setting means.