JPH0132637B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an inverter circuit for an induction heating device.

Conventional configurations and their problems Conventional series inverter circuits have only had diodes 3 and 4 connected in antiparallel to switching elements 1 and 2 constituting the inverter circuit as shown in Fig. 1, or as shown in Fig. 2. Furthermore, a series circuit of a resistor R and a capacitor _Cs was connected in parallel. In FIG. 1, since the voltages of the switching elements 1 and 2 rise sharply, the operating margin for the safe operation range of the switching elements is small and the switching loss is large. In Fig. 2, it is thought that the disadvantage that the resistance loss is large and reduces the conversion efficiency of the series inverter circuit can be eliminated by simply removing the resistor R shown in Fig. When operating an inverter circuit below the resonant frequency of a series circuit of resonant capacitors, if switching element 1 or 2 is turned on while capacitor _Cs is charged, a short-circuit current will flow through the switching element, causing the switching element to fall outside the safe operating area. Since the current reaches up to 1, it was necessary to provide a resistor R to limit the current.

Purpose of the Invention The present invention was made in view of the above drawbacks, and the present invention provides a switching element that constitutes an inverter circuit.
This aims to increase the operating margin in the safe operating range and reduce heat generation due to switching loss.

Structure of the Invention In order to achieve the above object, the inverter circuit for induction heating of the present invention includes a DC power supply, a load circuit consisting of a series resonant circuit of a load coil and a resonant capacitor,
It consists of a first switching element interposed in series between the DC power supply and the load circuit, and a second switching element connected in parallel to the load circuit, and a diode is connected in antiparallel to the switching element. In the series inverter circuit, at least one delay capacitor is connected between the connection point of the switching element and the load circuit and the DC power supply, and the circuit operates at a frequency higher than the resonant frequency of the series resonant circuit.

Description of Examples Hereinafter, the present invention will be described in detail according to Examples. In FIG. 3, a transistor 1 is interposed in series between a DC power source and a load circuit. 2 is a transistor connected in parallel to the load circuit. Diodes 3 and 4 are connected antiparallel to transistors 1 and 2, respectively. 5 is a load coil,
6 is a resonant capacitor. A load circuit consisting of a series resonant circuit including a load coil and a resonant capacitor 6 is connected between the connection point of the transistors 1 and 2 and the DC power supply. 8 is a full-wave rectifier consisting of a single-phase bridge to which AC power is applied to its input via power switch 7; 9 is a smoothing capacitor;
0 is a frequency higher than the resonant frequency of the series resonant circuit connected in parallel with the full-wave rectifier 8;
and a base drive circuit that drives 2. It is.

The operation will be explained in FIG. 3 using the waveform diagram in FIG. 4. Figure 4A is the base current of transistor 1, Figure B is the collector current of transistor 1, Figure C is the forward current of diode 3, Figure D is the collector-emitter voltage of transistor 1, Figure 4E is
is the base current of transistor 2, F is the collector current of transistor 2, and G is the diode 4.
, H is the collector-emitter voltage of transistor 2, I is the switching loss of transistor 1, J is the switching loss of transistor 2, and K is the voltage-current trajectory of transistor 1. The horizontal axis is the collector-emitter voltage and the vertical axis is the collector current. L in the same figure shows the voltage-current locus of the transistor 2, and the horizontal and vertical axes are the same as in K in the same figure. In Figure 4, the broken line is the delay capacitor C _d
This is the waveform in the case without , that is, in the conventional example shown in FIG.

In this embodiment, when the transistor 2 is cut off, a series resonant circuit consisting of the load coil 5, the resonant capacitor 6, and the delay capacitor _Cd is formed, and the collector of the transistor 2 is connected to the collector of the transistor 2 according to the natural angular frequency of the series resonant circuit. The emitter voltage rises. In the conventional example shown in FIG. 1, the series circuit is not formed, so the voltage between the collector and emitter of transistor 2 reaches its maximum value when transistor 2 is completely cut off. Therefore, Figure 4 D, H
As shown in FIG. 3, in the embodiment shown in FIG. 3, the rise of the voltage between the collector and emitter of transistor 2 is delayed by the charging time T of delay capacitor _Cd compared to the conventional example shown in FIG. Similarly, the rise of the collector-emitter voltage of transistor 1 is also delayed by the above-mentioned time T. In particular, above the resonance frequency, transistor 1,
2, the collector currents of transistors 1 and 2 (FIG. 4, B and F) are turned off before forming a complete sine wave, and rapidly decrease. This collector current has almost the same waveform as the current waveform flowing through the series resonant circuit. That is, when the transistor 1 is turned off from on, the current flowing through the series resonant circuit also begins to drop rapidly, and the current drops rapidly while energy is stored in the load coil 5. By the time the energy of the delay capacitor C _d is absorbed (the delay capacitor C _d is discharged) and the next time the transistor 2 is turned on, the terminal voltage of the delay capacitor becomes almost zero, and no short-circuit current flows through the transistor 2.

Utilizing the above-mentioned characteristics, the present invention makes it possible to eliminate the current-limiting resistor for suppressing short-circuit current, eliminate the reduction in conversion efficiency caused by the resistor, and reduce the steep voltage applied to the transistor by the delay capacitor. It is possible to suppress the voltage and increase the operating margin for the safe operating area of the transistor. 5 and 6 show other embodiments of the present invention, the operation of which can be easily inferred from the embodiment shown in FIG. In the figure, elements that operate in the same way as in FIG. 3 are given the same numbers.

Effects of the Invention As described above, by adding the delay capacitor of the present invention, as shown in FIG. 4
Switching loss can be reduced as shown in Figures I and J. In addition, since there is no loss due to the resistor R compared to the conventional example shown in FIG. 2, the conversion efficiency of the series inverter circuit can be improved.

[Brief explanation of drawings]

1 and 2 are circuit diagrams of a conventional series inverter circuit, FIG. 3 is a circuit diagram of an inverter circuit for an induction heating device showing an embodiment of the present invention, and FIG. 4A,
B, C, D, E, F, G, H, I, J, K, and L are waveform diagrams showing the operation of FIG. 3, and FIGS. 5 and 6 are circuits showing other embodiments of the present invention. It is a diagram. 1, 2...Transistor, 3, 4...Diode, C _d ...Delay capacitor, 5...Load coil,
6...Resonance capacitor.

Claims (1)

[Claims] 1. A DC power source, a load circuit consisting of a series resonant circuit of a load coil and a resonant capacitor, a first switching element interposed in series between the DC power source and the load circuit, and a first switching element connected in parallel to the load circuit. In a series inverter circuit comprising a second switching element and a diode connected in antiparallel to the switching element, at least one delay capacitor is connected between a connection point between the switching element and the load circuit and the DC power supply. , an inverter circuit for an induction heating device that operates at a frequency higher than the resonant frequency of the series resonant circuit.