JPS5866524A - Circuit for protecting high frequency inverter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high frequency inverter used for example in induction heating, and in particular to a protection circuit for a high frequency inverter that protects circuit elements from destruction when a load is short-circuited. .

First, the operating principle of this type of high frequency inverter will be explained based on the basic circuit shown in FIG. In this diagram,
1 is λ DC voltage E 1, E'2 (E 1-E 2
), a variable voltage source 2 whose rectifying element is a thyristor whose conduction angle is controlled, and a choke coil 3.
and a smoothing capacitor 4.5. 6 is μ
A first switch circuit (circuit opening/closing means) in which thyristors 6a to 6d are bridge-connected, and between a connection point of thyristors 6a and 6b and a connection point of thyristors 6c and 6d in this switch circuit 6. A seventh resonant capacitor 7 (capacitance C) is inserted. Further, 8 is a seventh resonant inductance (the inductance is L) including the wiring inductance and the inductance of the commutation induction coil. In the above part, smoothing capacitor 4
(“α-flow power supply of voltage El”), a switch circuit 6, a resonance capacitor 7, and a resonance inductance 8 constitute a seventh series resonance circuit 12. Also, 9 is a second switch corresponding to the switch circuit 6. A circuit, 10 is a first resonant capacitor (capacitance C) corresponding to the resonant capacitor 7, 11
is a second resonant inductance (inductance is L) corresponding to the resonant inductance 8, and these switch circuit 9, resonant capacitor 10, resonant inductance 11 and the smoothing capacitor 5 (DC power supply of voltage E2) are the second resonant inductance (inductance is L) corresponding to the resonant inductance 8. A series resonant circuit 13 is configured. A load 16 is inserted between each terminal 14.15 of the series resonant circuit 12.13.

In this configuration, thyristors 6a, 6d, thyristors 9c, 9b, thyristors 6c, 6b, thyristors 9a,
9d at time t1. t2゜t3. t4...
As shown in FIG. A wavy resonant current generator 2 flows, and as a result, a feeder frequency output current It, which is a combination of both of these resonant current generators 1 and 2, flows through the load 16.

In this case, the voltages VC1 and vC2 across each of the resonant capacitors 7.10 change by 1° as shown in FIG. When short-circuited, the Q of the series resonant circuit 12° (13) becomes almost infinite, so the resonant capacitors 7, (10)
The voltages VC1, (VC2) between both ends of the voltage E1. (
g2) increases in a cumulative manner. That is, as shown in FIG. 3, suppose that the load 16 is short-circuited immediately before time t1, but each time it is short-circuited, V(! 1+uE1
, VC1-+4E1, Vc 1-11EI, Vc 1-
1-J'EI, 17). therefore,
In such high-frequency inverters, if the load is short-circuited, the voltage across the resonant capacitor will rise abnormally, and there is a danger that the thyristor circuit components used will be destroyed by the high voltage. There is sex.

As a protection circuit for preventing the abnormal rise in the resonant capacitor voltage caused by sudden changes in the load impedance as described above, it has conventionally been proposed to provide a voltage clamping diode as shown in Figure μ. There is.

In the figure, 17 and 18 are voltages vC1 and 18, respectively.
This is a diode for clamping VC2 to a predetermined voltage. Here, the diode 17 is connected to the thyristors 6b and 6d.
is inserted between the connection point of each cathode of and the negative terminal of the smoothing capacitor 5, but in this case, generally, a wiring inductance 8a corresponding to approximately the third of the inductance L of the resonant inductance 8 is Smoothing capacitor 5 - Smoothing capacitor 4 - Thyristor 6a (6c) - Resonance capacitor 7 - Thyrisk 68 (6b) - Inductance 8a -
The diode 17-smoothing capacitor 5 exists in a path (diode circulating current path). Similarly,
A wiring inductance 11a exists in the diode circulating current path including the diode 18.

Therefore, in a high-frequency inverter equipped with such a protection circuit, when the load is, for example, a multiplier resonant load (the resonant frequency of the load is n times the frequency of the output current It), the output In the case where it is necessary to make the pulse width of the current generator t extremely short, for example, if the load 16 is short-circuited, the voltage vcl, (VC2) becomes the clamp voltage (
Even after this voltage reaches E10E2), the wiring inductance 8a, (11a
) will continue to rise until the energy is dissipated through the diode freewheeling current path. In such a case, the voltage Vc1, (V(!2) will rise to, for example, more than 10 times the voltage El, (E2), so the thyristor 6
There is an extremely high risk that circuit components such as a to 6d and 9a to 9d will be destroyed. Furthermore, in a high frequency inverter equipped with such a protection circuit, the voltage Vel, V(!2) always rises to the clamp voltage by the diode 17.18 even under normal load conditions, so for example, the wiring inductance 8a, lla) is the resonant inductance 8
, (11), for example, if po%, then the actual rated output is,
It is impractical for as much as 20% of the power to be dissipated in the diode freewheeling current path. Furthermore, when such a protection circuit is provided, even when the frequency of the output current t is low, the same output current I can be reduced due to the diode circulating current.
Since the pulse width of t widens, the power factor decreases, and the Q of the series resonant circuit increases equally parsimoniously, the voltages VC1, ■C
2 will reach equilibrium in a state where it has increased by that amount.

FIG. 5 is a waveform diagram illustrating such a situation when the load 16 is a triple resonant load. In this figure, the solid line a represents the voltage across the load 16, and the solid line represents the series resonant circuit. 12. (13), the broken line d indicates the current flowing through the diode 17. Series resonant circuit 1 when (18) is not provided
2. (13) are shown respectively.

This invention was made with the aim of solving the above-mentioned problems, and includes a rectifier circuit that performs full-wave rectification of the voltage between both ends of a resonant capacitor, and a rectifier circuit that is inserted between the output terminals of the rectifier circuit and A clamping capacitor having a large capacity and a charging power supply supplying a predetermined clamping voltage across the clamping capacitor are provided to prevent the voltage across the resonant capacitor from rising above the clamping voltage. It is characterized by the following.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the protection circuit according to the present invention and a main part of a series resonant circuit 12 of a high frequency inverter equipped with this embodiment. Note that the series resonant circuit 13 is also provided with a protection circuit having the same configuration as that shown in this figure. In the figure, 19 is a rectifier circuit for full-wave rectification of the voltage Vc1 across the resonant capacitor 7, and a diode 1
9a to 19d are bridge-connected. Further, 20 is a clamping capacitor having a capacitance 1ce that is sufficiently large compared to the capacitance c of the resonant capacitor 7 (for example, 10 to 30 times the capacity of 81it C), and 21 is an AC voltage source 22 and a rectifier circuit 23. This is a charging power source that outputs a stain pressure EO. The output of the rectifier circuit 19 is supplied to the clamping capacitor 20 via a resistor 24 (a damping resistor with a small resistance value), and the voltage EO output from the charging power source 21 is supplied via a resistor 25 to the clamping capacitor 20. It is now possible to do so. Further, a discharge path consisting of a resistor 26 is provided between both ends of the clamp capacitor 20. Note that the capacitor 27 is a capacitor for bypassing frequency components higher than the operating frequency of this high frequency inverter.

Next, the operation of this protection circuit having the above structure will be explained. First, in a steady state, the clamping capacitor 20 is charged in advance by the charging power source 21, and the voltage VO (ie, clamping voltage) across the clamping capacitor 20 is the voltage E. and resistance 2
A constant voltage is determined by each resistance value of 5.26. This clamp voltage VO is set to be slightly lower than the voltage C1 in the steady state. Therefore, in the steady state, the diodes 19a to 1 of the rectifier circuit 19
9d are all reverse biased. Now, if the load 16 is suddenly short-circuited, the voltage VCI will rise rapidly, but this voltage Vc1 will be the clamp voltage V
When it exceeds O, the same voltage VC1 is connected to the rectifier circuit 19 and the resistor 24.
It will be supplied to the clamp capacitor 20 through the following sequentially. In other words, according to this protection circuit, the voltage ■C1
When exceeds the clamp voltage vO, the capacitance C of the resonant capacitor 7 will increase equi-frugally to CO, i.e. IOC~30C. Therefore, the voltage Vc1 is the clamp voltage v
There is almost no rise from O.

In this case, the clamp voltage VO is slightly increased by charging the clamp capacitor 20 with the voltage Vc1, but this increased amount is discharged via the resistor 26, so the voltage vC1 When the voltage drops below the clamp voltage VO again, the clamp voltage vO also returns to its original value.

Thus, according to this embodiment, the voltage Vcl.

It is possible to limit vC2 so that it does not rise above the clamp voltage of 70.

By the way, in the above embodiment, when the protection circuit operates, it is desirable to reduce the power supply voltages El and E2 accordingly, but here, in order to reduce the power supply voltages El and E2 based on the output of the protection circuit An example of a circuit that issues the command is shown in FIG. In this FIG. 7, 28 is
This is a current transformer that detects the current flowing through the clamp capacitor 20, and is made up of a ferrite core 29 inserted as the lead wire/secondary winding of the shift lamp capacitor 20, and a secondary winding 30 of the ferrite core. ing. This 2
The output obtained at the next winding 30 is rectified by a diode 31 and supplied to a resistor 32. Therefore, a voltage Va corresponding to the current flowing through the clamping capacitor 20 is obtained between the terminals 33 and 34.

Next, in Fig. r, the power supply voltage El, based on the voltage va,
This is a waveform diagram illustrating the process until a command to decrease E2 is issued. In this diagram, the voltage Va shown in (A) is compared with a seventh reference voltage Vb by a seventh comparator (not shown), At times t1, t2, and t3 when the voltage Va is the reference, a multivibrator (not shown) is activated, and this multivibrator is connected to the operating layer of the high-frequency inverter ]υ(
Outputs a pulse signal pa with a slightly shorter time width (
(See (b) in the same figure). Then, the pulse signal pa smoothed in this way is integrated by an integrator (not shown) (see (c) in Fig.
d is obtained. This 'flf and voltage Vd are compared with a second reference voltage Ve by a second comparator, and when this γlr and voltage Vd exceed the chatei voltage Ve at time t4, a command to decrease the power supply voltages El and E2 is issued. . For example, the command to decrease the electric current σ・? It may be used to reduce the voltages El and E2.

As explained above, the protection circuit for a high frequency inverter according to the present invention is a high frequency inverter equipped with a series resonant circuit formed by connecting a DC power supply, a circuit switching means, a resonant capacitor, and a resonant inductance in series. A rectifier circuit that performs full-wave rectification of the voltage between both ends of the rectifier circuit, a clamping capacitor with a larger capacity than the resonant capacitor inserted between the output terminals of this rectifying circuit, and a predetermined clamping voltage applied between both ends of the clamping capacitor. Since it is equipped with a charging power supply, it is possible to reduce the wiring inductance in the series resonant circuit compared to the conventional protection circuit using diodes for clamping, and
It is now possible to set the clamp voltage of the voltage across the resonant capacitor to any value by changing the output voltage of the charging power supply, and as a result of ■■, the protection circuit has an abnormal increase in the voltage across the resonant capacitor. Since the current can be made to flow only when If the above-mentioned short termination is resolved within a short time, the high frequency inverter can continue to operate as it is.■ The charging path to the clamp capacitor is the circuit opening/closing means (switch circuit using a thyristor). Since the protection circuit is formed without any negative impact, it has many excellent effects such as the ability of the protection circuit to operate normally even if the circuit opening/closing means is in the closed state.

[Brief explanation of drawings]

Figure 7 is a circuit diagram showing the basic circuit of a high-frequency inverter, Figure 2 (a) to (g) are waveform diagrams to explain the operation of the circuit, and Figure 3 shows the circuit when the load of the circuit is short-circuited. Figure 5 is a circuit diagram showing the protection circuit of a conventional high-frequency inverter; Figure 5 is a waveform diagram to explain the operation of the circuit; 7 is a circuit diagram showing the configuration of an embodiment of the present invention, FIG. 7 is a circuit diagram showing an example of a circuit used when changing the power supply voltage based on the output of the embodiment, and (a) in FIG. ~
(C) is a waveform diagram for explaining the operation on the ipsilateral side. 1...DC power supply, 6.9...Circuit opening/closing means (switch circuit), 7.10...Resonance capacitor, 8゜11...Resonance inductance ,1
2.13...Series resonant circuit, 16...
Load, 19... Rectifier circuit, 20... Clamp capacitor, 21... Song charging power supply. Heh Tachibana (-L)

Claims (1)

[Claims] The circuit has a series resonant circuit formed by connecting a DC power source, a circuit switching means, a resonant capacitor, and a resonant inductance in series, and the circuit switching means is operated at a high frequency and connected in series with the series resonant circuit. A high-frequency inverter that supplies a high-frequency output to a load inserted in the resonant capacitor includes a rectifier circuit that performs full-wave rectification of the voltage between both ends of the resonant capacitor, and a rectifier circuit inserted between the output terminals of the rectifier circuit and that is larger than the resonant capacitor. 1. A protection circuit for a high frequency inverter, comprising a clamping capacitor having a capacitance and a charging power supply supplying a predetermined clamping voltage between both ends of the clamping capacitor.