JPH0242247B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an overvoltage protection device for protecting a semiconductor switch device from overvoltage applied to the semiconductor switch device.

Conventionally, there has been a device of this type as shown in FIG. In the figure, 1 is a DC power supply, 2 is a load including a reactor, 3 and 5 are current balancing reactors, and 4 and 6 are semiconductor switch circuits, each including a gate turn-off thyristor (hereinafter abbreviated as GTO), 41, 61, and a snubber. capacitors 42, 62,
It is composed of diodes 43, 63 and discharge resistors 44, 64. 7 is a voltage clipper circuit.

Next, the operation will be explained with reference to FIGS. 2 and 3. Second
As shown in figure a, when the currents I _A 1 and I _A 2 flowing through GTO 41 and 62 are cut off at time t ₀ , GTO 4
The voltages v1 and v2 across the terminals 1 and 61 rise at a voltage increase rate dv/dt as shown in FIG. 2c. i _S 1, i _S 2
represent the currents flowing into the snubber capacitors 42 and 62 after I _A 1 and I _A 2 are cut off, respectively. When the time constant of the load 2 is very large, v1 and v2 increase linearly until the time t1 when the clipper voltage V _M of the voltage clipper circuit 7 is reached. The rate of increase in voltage at this time is
It depends on the current at the time of interruption and the capacitance C of the snubber capacitors 42 and 62, and as shown in Fig. 2a, the load current Id at the time of interruption is divided into the GTOs 41 and 61 in exactly the same way (Id/2 each). Voltage increase rate when
dv/dt is dv/dt=(Id/2)/C (1). Here, Id is the load current, and C is the capacitance of the snubber capacitors 42 and 62.

When the voltages v1 and v2 across the GTO further rise and become larger than the clipping voltage V _M of the clipper circuit 7, the currents i1 and i2 flowing into each switch circuit begin to decrease, and the energy of the reactors 3 and 5 for current balance increases. , continues to flow until time t2 when it is absorbed by the snubber capacitors 42, 62. At this time, the current i _M of the voltage clipper circuit 7 increases so that the sum of i1+i2 and i _M of the currents i1 and i2 flowing through the switch circuit becomes equal to the load current i. That is, i=i1+i2+i _M
becomes. During this period t1-t2, the snubber capacitors 42, 62 are overcharged and rise up to Vp.
Vp can be expressed by the following formula.

Vp=V _M +ΔVp=V _M +√・(1d/2)
...(2) However, V _M : Clipping voltage of the voltage clipper circuit L: Inductance of balance reactors 3 and 5 C: Capacity of snubber capacitor Id/2: Current flowing through each GTO before shutoff (2) ), ΔVp=√・(Id/2)
is an equation calculated assuming that the energy of the reactors 3 and 5 is absorbed by the snubber capacitors 42 and 62, respectively. That is, L/2・(Id/2) ² =C/2・(ΔVp) ² ...(3) After time t2, the voltages v1 and v2 applied to the semiconductor switch circuits 4 and 6 are controlled by the voltage clipper circuit 7. While the current is flowing through the circuit, that is, while the main circuit current i (load current) is flowing, the clip voltage V _M becomes
When the load current i stops flowing, the power supply voltage becomes E.

Therefore, the maximum voltage Vp applied to the semiconductor switch circuits 4 and 6 is, for example, power supply voltage E = 1.500 volts, clip voltage V _M = 3.000 volts, snubber capacitor capacity C = 2.0 _μF , and balance reactor inductance L = 20.0μ _H , and assuming that the current Id/2 flowing through the GTO before shutoff is 1.000 amperes,
A voltage of ΔVp=1.000×√202=3.162 volts is applied to the snubber capacitor 42 by the reactors 3 and 5.
It is overcharged to 62. Also, the peak voltage is Vp=
3.000+3.162=6.162 volts. Even if only the capacitor capacity is C=2.0μ _F , ΔVp=1.000
volt, Vp=4.000 volt. Therefore,
When selecting a semiconductor switch circuit, it is necessary to select a switching element with a much higher withstand voltage than the clip voltage V _M of the overvoltage protection device, or to select a very large snubber capacitor.

The explanation so far has been based on the assumption that the same current is flowing through each of the semiconductor switch circuits 4 and 6, but the currents are unbalanced, and one
If a large current flows through a GTO and it is cut off, an even more excessive peak voltage Vp will occur in that GTO.

The above description has been made on the assumption that the voltage-current characteristics of the voltage clipper circuit are the ideal characteristics shown in FIG.

As mentioned above, in the conventional device, the energy stored in the current balancing reactor must be absorbed by the snubber capacitor, so when selecting the semiconductor switching element for the switch circuit, the clipping voltage of the voltage clipper circuit must be It is necessary to use elements with a much higher withstand voltage than
Furthermore, if the currents that are shunted to a plurality of switch circuits become unbalanced, an excessive voltage will be applied to the semiconductor switching element that has cut off the large current, and in the worst case, the element will be destroyed.

This invention was made in order to eliminate the above-mentioned conventional drawbacks, and even when the peak voltage applied to the switch circuit at the time of interruption is comparable to the clipping voltage of the voltage clipper circuit, and an unbalanced current is interrupted, the semiconductor The purpose of this device is to provide an overvoltage prevention device that does not generate excessive voltage in switching elements, and is characterized by connecting the connection points of each reactor and semiconductor switching element to a common voltage clipper circuit through a diode. do.

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

In FIG. 4, 8 and 9 are diodes, and the current balancing reactors 3 and 5 of each switch circuit 4 and 6 and semiconductor switching elements (in this example,
Each anode is connected to the connection point with GTO), and the cathode is connected to a common voltage clipper circuit 7. The voltage clipper circuit 7 is made of a non-linear resistor such as zinc oxide, and its voltage-current characteristics are as follows.
It has ideal characteristics as shown in the figure. Other symbols are the same components as those in FIG.

Next, the operation of this circuit will be explained using the waveform diagrams of FIGS. 5 and 6. As shown in FIG. 5a, when the currents i _A 1 and i _A 2 flowing through the GTOs 41 and 61 are cut off at time t ₀ , the voltages v1 and v2 of the GTOs 41 and 61 rise as shown in FIG. 5c. .
If the time constant of load 2 is very large, the voltage
v1 and v2 are the clipping voltage V _M of the voltage clipper circuit 7
It rises linearly until the point t1 when . The voltage increase rate dv/dt at this time is that the load current Id is GTO41,61
If the currents are divided completely in the same way (i10, i20), it will be the same as equation 1 above. In the case of a semiconductor switching device, the voltage change dv/dt at turn-off of the GTO is the value (dv/dt) determined for that device.
Must be determined so that it is less than or equal to max.

For example, (dv/dt)max=500v/μs, (Id/2)
= 1.000 _A , the snubber capacitor capacitance C
It only needs to be 2μF, and there is no need to make it unnecessarily large.

At time t1, GTO41 and 6 of switch circuits 4 and 6
When the voltages v1 and v2 of the voltage clipper circuit 7 become equal to the clipping voltage V _M of the voltage clipper circuit 7, the voltages v1 and v2 do not rise any further as shown in FIG. 5c, and as shown in FIG. up to snubber capacitor 42,62
The currents i _S 1 and i _S 2 that had been flowing into the clipper circuit 7 suddenly decrease at time t1, and a current i _M flows into the clipper circuit 7 as shown in FIG. 5b. After time t1,
Since the energy of the load 2 and the energy of the current balancing reactors 3 and 5 are absorbed by the voltage clipper circuit 7, the voltages v1 and 61 of the GTOs 41 and 61 are
v2 does not rise above the clip voltage _VM .

The operating waveform when the current sharing between the switch circuits 4 and 6 is unbalanced (the current i10 flowing through the switch circuit 4 is larger than that flowing through the switch circuit 6) is shown in the sixth diagram.
As shown in the figure. In this case, the voltage of GTO41 at the time of interruption
v1 has a fast rise as shown in Figure 6c. but,
Voltage v1 applied to GTO of both switch circuits 4 and 6,
v2 does not rise above the clip voltage. FIG. 7 shows another embodiment of the invention. When the load 2 includes a power supply and a reverse voltage is generated in the switch circuits 4 and 6, and the reverse withstand voltage of the main switching element such as a GTO is not large, reverse current blocking is used as shown in the figure. Even if the diode 20 is connected, the overvoltage prevention device works effectively. Code 3~
Components 9 are the same as those shown in FIG. FIG. 8 is a diagram showing another embodiment of the present invention, in which the switch device is bidirectional. In the figure,
10, 12 are current balance reactors, 11,
13 is a switch circuit connected in parallel in the opposite direction to switch circuits 4 and 6; 14 and 15 are diodes;
9 are the same components as those in FIG. According to this configuration, bidirectional overvoltage can be prevented and the common voltage clipper circuit 7 can be used effectively. The voltage clipper circuit 7 in this example must have bidirectional characteristics as shown in FIG.

In each of the above embodiments, a zinc oxide element is used as the voltage clipper circuit 7, but other elements having nonlinear characteristics may be used. The characteristics need not be ideal characteristics as shown in FIG. Further, in cases other than the embodiment shown in FIG. 8, the voltage clipper characteristic may be unidirectional rather than bidirectional.

Furthermore, in each of the above embodiments, an example is shown in which a GTO is used as a semiconductor switching element in a switch circuit, but a self-extinguishing element such as a transistor or an electrostatic induction thyristor may be used, or a non-self-extinguishing element such as a thyristor may be used. A forced commutation circuit may be added using an arc element.

The current balance may be a balancing reactor.

Moreover, it goes without saying that the number of parallel circuits of the current balancing reactor and the switch circuit in series may be three or more.

As described above, in the present invention, since the connection points between each reactor and the semiconductor switching element are connected to the common voltage clipper circuit through the respective diodes,
The energy of the current balancing reactor can be absorbed by the voltage clipper circuit, and the voltage generated in the switch circuit when the switch circuit is turned off can be suppressed to the clip voltage of the voltage clipper circuit. Furthermore, even if the currents of the switch circuits connected in parallel are in an unbalanced state, when the switch circuits are turned off, the voltage of the switch circuits can be suppressed to the clip voltage in the same way as described above. Therefore, the semiconductor switching element of the switch circuit can be prevented from being destroyed due to overvoltage at the time of interruption, and the withstand voltage of the switching element itself can be reduced.

Furthermore, there is also an effect that it is no longer necessary to increase the capacitance of the snubber capacitor that protects the semiconductor switching element more than necessary.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional overvoltage protection device, Figure 2
3 is a characteristic diagram of the voltage clip circuit, FIG. 4 is a circuit diagram of an embodiment of the overvoltage prevention device for a semiconductor switch according to the present invention, and FIGS. 5 and 6 are diagrams for explaining the operation of FIG. Operation explanatory diagram of the above embodiment, 7th
8 and 8 are circuit diagrams of other embodiments of the present invention. In the figure, 1...power supply, 2...load, 3,
5, 10, 12... Current balance reactor,
4, 6, 11, 13... switch circuit, 7... voltage clipper circuit, 41, 61... semiconductor switching element, 42, 62... snubber capacitor. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A semiconductor switch device in which a plurality of series circuits of current balancing reactors and semiconductor switching elements are connected in parallel, characterized in that the connection points of each of the reactors and semiconductor switching elements are connected to a common voltage clipper circuit via a diode, respectively. Overvoltage prevention device for semiconductor switches.