JPS6315816B2 - - Google Patents

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 figure, 1 represents two DC voltages E ₁ and E ₂ (E ₁
= E ₂ ), which is a variable voltage source 2 whose rectifying element is a thyristor whose conduction angle is controlled.
, a chiyoke coil 3, and smoothing capacitors 4 and 5. 6 is four thyristors 6a~
6d is a first switch circuit (circuit opening/closing means) connected by a bridge, and this switch circuit 6
A first resonant capacitor 7 (capacitance C) is inserted between the connection point between the thyristors 6a and 6b and the connection point between the thyristors 6c and 6d. Further, 8 is a first resonant inductance (the inductance is L) including a wiring inductance and an inductance of a commutation induction coil. In the above portion, the smoothing capacitor 4 (DC power supply of voltage E ₁ ), the switch circuit 6 , the resonant capacitor 7 , and the resonant inductance 8 constitute a first series resonant circuit 12 . Further, 9 is a second switch circuit corresponding to the switch circuit 6, 10 is a second resonant capacitor (capacitance C) corresponding to the resonant capacitor 7, and 11 is a second resonant inductance corresponding to the resonant inductance 8. (inductance is L), and these switch circuit 9, resonant capacitor 10, resonant inductance 11, and the smoothing capacitor 5 (DC power supply of voltage _E2 ) constitute a second series resonant circuit 13. A load 16 is inserted between both terminals 14 and 15 of the series resonant circuits 12 and 13.

In this configuration, the thyristors 6a, 6d, thyristors 9c, 9b, thyristors 6c, 6b, and thyristors 9a, 9d are operated at times t ₁ , t ₂ , by gate currents as shown in A, B, C, and D of FIG. _t3 ,
t ₄ . . . , a half-sine wave-shaped resonant current I 1 is applied to the first series resonant circuit 12, and a resonant current I ₁ of a half-sine wave is applied to the second series resonant circuit 13, as shown in E of the figure. A half-sine wave resonant current _I2 flows, and as a result,
A high-frequency output current Il that is a combination of both of these resonant currents I ₁ and I ₂ flows through the load 16 . In this case, the voltage between each of the resonant capacitors 7 and 10 is
Vc ₁ and Vc ₂ change as shown in F and G of FIG.

By the way, in such a high frequency inverter, when the load 16 is short-circuited, the series resonant circuit 1
Since the Q of 2 and 13 is almost infinite, the voltages Vc ₁ and Vc ₂ across the resonant capacitors 7 and 10 are equal to the load 16.
Every time a commutation operation is performed from the time when the voltages are short-circuited, the voltages twice as high as the voltages E ₁ and E ₂ are accumulated and increased. That is, as shown in FIG. 3, if the load 16 is short-circuited immediately before time t ₁ , the voltage Vc ₁
is time t ₁ (firing time of thyristors 6a and 6b),
Each time a commutation operation is performed at time t ₂ , t ₃ , t ₄ ......
Vc ₁ +2E ₁ , Vc ₁ +4E ₁ , Vc ₁ +6E ₁ , Vc ₁ +8E ₁ ,
It increases as follows. 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 thyristors and other circuit components used will be destroyed by the high voltage. There is sex.

As a protection circuit for preventing abnormal increases in the resonant capacitor voltage due to sudden changes in load impedance as described above, a protection circuit provided with a voltage clamping diode as shown in Figure 4 has been proposed. There is.

In FIG. 4, 17 and 18 are each a voltage
This is a diode for clamping Vc ₁ and Vc ₂ to a predetermined voltage. Here, the diode 17 is inserted between the connection point of each cathode of the thyristors 6b and 6d and the load terminal of the smoothing capacitor 5, but in this case, generally approximately 40% of the inductance L of the resonant inductance 8 The wiring inductance 8a corresponding to the smoothing capacitor 5 - smoothing capacitor 4 - thyristor 6a, 6c - resonance capacitor 7 - thyristor 6d, 6b - inductance 8a
- diode 17 - smoothing capacitor 5 (diode circulating current path). Similarly, the diode circulating current path including the diode 18 includes a wiring inductance 11a.
will exist.

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 Il), the output In the case where it is necessary to make the pulse width of the current Il extremely short, for example, if the load 16 is short-circuited, the voltages Vc ₁ and Vc ₂ will change across the diodes 17 and 18.
(this voltage is E ₁ + E ₂
Even after reaching (), the energy in the wiring inductances 8a, 11a continues to rise until it is consumed via the diode circulating current path. In such a case, the voltages Vc ₁ and Vc ₂ will rise to, for example, more than 10 times the voltages E ₁ and E ₂ , so there is a risk that circuit components such as the thyristors 6a to 6d and 9a to 9d will be destroyed. Extremely high. Furthermore, in a high frequency inverter equipped with such a protection circuit, the voltages Vc ₁ and
Since Vc ₂ always rises to the clamp voltage by diodes 17 and 18, if the wiring inductances 8a and 11a are, for example, 40% of the resonant inductances 8 and 11, the rated output is actually
As much as 20% of the power is consumed in the diode freewheeling current path, which is impractical. In addition, when such a protection circuit is provided, even when the frequency of the output current Il is low, the pulse width of the output current Il widens due to the diode circulating current, which reduces the power factor and prevents the series resonant circuit. Since the Q of is increased to the same value, the voltages Vc ₁ and Vc ₂ will be balanced in a state where they have 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 b represents the voltage across the series resonant circuit 12. , 13, and the broken line d shows the currents in the series resonant circuits 12 and 13 when the diodes 17 and 18 are not provided, respectively.

This invention was made for the purpose 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 ends of this rectifier circuit and that is A clamping capacitor having a capacitance 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. This is a characteristic feature.

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

FIG. 6 is a circuit diagram showing an embodiment of the protection circuit according to the present invention and a main part of the series resonant circuit 12 of a high frequency inverter equipped with this embodiment. In addition,
The series resonant circuit 13 is also provided with a protection circuit having a configuration similar to that shown in this figure. In Figure 6,
19 is a rectifier circuit that performs full-wave rectification of the voltage Vc ₁ between both ends of the resonant capacitor 7, and diodes 19a to 1
9d are bridge-connected. Further, 20 is a clamping capacitor having a capacitance C ₀ sufficiently larger than the capacitance C of the resonance capacitor 7 (for example, 10 to 30 times the capacitance C); 21 is a capacitor for clamping;
It is a charging power source that includes an AC voltage source 22 and a rectifier circuit 23 and outputs a voltage E ₀ . The output of the rectifier circuit 19 is passed through a resistor 24 (a damping resistor with a small resistance value), and the voltage _E0 output from the charging power source 21 is passed through a resistor 25 to a clamping capacitor 20. It is now being supplied to Also, clamp capacitor 2
A discharge path consisting of a resistor 26 is provided between both ends of 0. 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 V ₀ (that is, the clamping voltage) across the clamping capacitor 20 is a constant value determined by the voltage E ₀ and the resistance values of the resistors 25 and 26. voltage. This clamp voltage
V ₀ is set to be slightly higher than the voltage Vc ₁ in the steady state, so in the steady state the diodes 19a to 1 of the rectifier circuit 19
9d are all reverse biased. And now,
If the load 16 is suddenly short-circuited, the voltage Vc ₁ will rise rapidly, but if this voltage Vc ₁ exceeds the clamp voltage V ₀ , the voltage Vc ₁ will pass through the rectifier circuit 19 and the resistor 24 in turn. The voltage is then supplied to the clamp capacitor 20. In other words, according to this protection circuit, when the voltage Vc ₁ exceeds the clamp voltage V ₀ , the capacitance C of the resonant capacitor 7 becomes equivalently
It will increase to C ₀ or 10C to 30C. Therefore, the voltage Vc ₁ hardly increases from the clamp voltage V ₀ . In this case, the clamp voltage V ₀ is the voltage Vc ₁ of the clamp capacitor 20.
However, this increased amount is discharged through the resistor 26, so if the voltage Vc ₁ falls below the clamp voltage V ₀ again, the clamp voltage V ₀ also returns to its original value.

As described above, according to this embodiment, the voltages Vc ₁ ,
It is possible to limit Vc ₂ so that it does not rise above the clamp voltage V ₀ .

By the way, in the above embodiment, when the protection circuit operates, _it is desirable to reduce the power supply voltages E ₁ and _{E 2} accordingly. FIG. 7 shows an example of a circuit that issues a command to reduce the .
In FIG. 7, 28 is a current transformer for detecting the current flowing through the clamping capacitor 20, and includes a ferrite core 29 through which the lead wire of the clamping capacitor 20 is inserted as a primary winding; It consists of a secondary winding 30. The output obtained from this secondary 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, FIG. 8 is a waveform diagram illustrating the process until a command to reduce the power supply voltages E ₁ and E ₂ is issued based on the voltage Va. In this figure, the voltage Va shown in A is not shown. The first comparator compares the voltage Va with the first reference voltage Vb, and the voltage Va becomes the reference voltage.
Every time Vb is exceeded, that is, at times t ₁ , t ₂ , and t ₃ in the figure, a multivibrator (not shown) is activated, and this multivibrator operates over a period of time slightly shorter than the operating cycle of this high-frequency inverter. outputs a pulse signal Pa (see B in the same figure). The pulse signal Pa obtained in this way is integrated by an integrator (not shown) (eighth
(see c in the figure), the voltage Vd is obtained as the result of this integration. This voltage Vd is determined by the second comparator.
This voltage Vd is compared with the reference voltage Ve at time t ₄
When the reference voltage Ve is exceeded at , the power supply voltage E ₁ ,
E ₂ reduction command is issued. This reduction command may be used, for example, to control the conduction angle of the thyristor (rectifying element) of the variable voltage source 2 described above to reduce the power supply voltages E ₁ and E ₂ .

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 circuit, a clamping capacitor inserted between the output terminals of this rectifying circuit and having a larger capacity than the resonant capacitor, and a predetermined clamping voltage being supplied between both ends of this clamping capacitor. Since it is equipped with a charging power source for
The wiring inductance in the series resonant circuit can be reduced compared to the conventional protection circuit using a clamp diode, and the clamp voltage of the voltage across the resonant capacitor can be set to any value by changing the output voltage of the charging power supply. As a result, current can flow into the protection circuit only when the voltage across the resonant capacitor abnormally increases, so no wasted energy is consumed in the protection circuit. Even if the load is suddenly short-circuited, the voltage across the clamp capacitor will gradually rise, so if the short-circuit is resolved within a short time, it will be possible to continue operating the high-frequency inverter. Since the charging path to the clamp capacitor is formed without going through the circuit opening/closing means (switch circuit using a thyristor), the protection circuit can operate normally even if the circuit opening/closing means is closed. It has many excellent effects.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing the basic circuit of a high-frequency inverter, I to I in Fig. 2 are waveform diagrams to explain the operation of the circuit, and Fig. 3 is a resonant capacitor when the load of the circuit is short-circuited. 4 is a circuit diagram showing a conventional protection circuit for a high frequency inverter, FIG. 5 is a waveform diagram illustrating the operation of the same circuit, and FIG. FIG. 7 is a circuit diagram showing the configuration of one embodiment. 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 same embodiment.
C is a waveform diagram for explaining the operation of the same example. 1...DC power supply, 6,9...Circuit opening/closing means (switch circuit), 7,10...Resonance capacitor, 8,
11... Resonance inductance, 12, 13... Series resonant circuit, 16... Load, 19... Rectifier circuit,
20... Clamp capacitor, 21... Charging power supply.

Claims (1)

[Claims] 1. It has a series resonant circuit formed by connecting a DC power supply, a circuit switching means, a resonant capacitor, and a resonant inductance in series, and the circuit switching means is operated at a high frequency to connect the circuit to 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 full-wave rectifies the voltage between both ends of the resonant capacitor, and a rectifier circuit inserted between the output ends of the rectifier circuit and having a larger capacity than the resonant capacitor. a clamp capacitor having
A protection circuit for a high frequency inverter, comprising a charging power source that supplies a predetermined clamp voltage between both ends of the clamp capacitor.