JPH0134524Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention provides a semiconductor element having a self-extinguishing function,
In particular, it relates to a protection circuit for gate turn-off thyristors.

For example, in a gate turn-off thyristor (hereinafter simply referred to as GTO), in order to reduce the rate of rise in off-voltage, the so-called dv/dt value (usually 100 to 200 V/μs) when the gate is turned off, A snubber circuit 3 as shown in FIG. 1 is connected to. That is, in FIG. 1, a capacitor 3c is connected in series to a diode 3a having a discharge resistor 3b connected in parallel, and all of these are connected in parallel between the anode and cathode of the GTO 1 as shown. The on/off state of GTO1 is controlled by the gate pulse from the gate control circuit 2. Figures 2a and b show the voltage V _AK between the anode and cathode of GTO1 at gate turn-off, and the voltage between the anode and cathode of GTO1 when the gate is turned off. This diagram qualitatively shows the relationship between the current I _AK and the current _ID flowing through the diode 3a. In addition, detailed changes in the anode-cathode voltage V _AK and anode-cathode current I _AK of GTO1 at gate turn-off are explained in the fourth section.
Shown as shown. According to this, first GTO
When an off-gate voltage is applied to GTO 1, the anode current flowing through GTO 1 is commutated to the snubber circuit 3 side. Then, a spike voltage Vsp with a large peak value is generated in the snubber circuit 3 at the beginning of the rise of the off-voltage of the GTO 1. The equivalent circuit when GTO1 is off is as shown in Figure 5, and the spike voltage
Vsp is expressed by the following formula.

Vsp=(Ls+Lcs)dI _AK /dt+1/C ∫ ^t2-t1 _t1 I _AK dt+v _fr ...(1) However, Ls is the wiring inductance of the snubber circuit, Lcs is the internal inductance of the capacitor 3c, C is the capacitance of the capacitor 3c, v _fr
is the forward recovery voltage of the diode 3a (Ds).

In other words, the spike voltage Vsp is determined by the inductance of the snubber circuit 3 and the anode-cathode current _IAK
This is generated by the voltage of the capacitor 3c charged during the period when the voltage decreases (between _t1 and _t2 ) and the forward recovery voltage of the diode 3a. This spike voltage Vsp
is one of the major factors in determining the maximum current (controllable current) of the GTO. If the spike voltage Vsp is large, the GTO may be destroyed even if the current below the controllable current is turned off. Therefore, it is necessary to reduce the spike voltage Vsp to prevent destruction of the GTO.

Also, although the GTO has a short turn-off time,
As the value of the current that is cut off increases, a spike current I _D with a large peak value will flow through the diode 3a, and the current increase rate di/dt may be as high as 400 A/μs in some cases. As a result, the diode 3a may be damaged. Although the cause of damage to the diode 3a can be found in switching loss, the reality is that damage cannot be prevented even if a diode 3a with a large current capacity is used.

An object of the present invention is to provide a protection circuit for a GTO that is configured so that the GTO is not destroyed by a spike voltage with a large peak value.

To this end, the present invention replaces conventional diodes by connecting a plurality of small current capacity diodes with the same characteristics in parallel.

The present invention will be explained below with reference to FIG.

As shown in the figure, the GTO protection circuit 5 according to the present invention is obtained by connecting the diode 3a in FIG. 1 in parallel with a required number of small current capacity diodes 5a ₁ , 5a ₂ , etc. having the same characteristics. Of course, the discharge resistor 5b and capacitor 5c are each connected to the first
This corresponds to the discharge resistor 3b and capacitor 3c in the figure. The number of small current capacity diodes to be connected in parallel should be individually determined in consideration of the allowable rise in junction temperature. When the required number of small current capacity diodes are connected in parallel, even if their total current capacity is small, they will function as diodes without being destroyed, and this has been experimentally confirmed in many cases. It is a fact.

The reason why the intended purpose is achieved by connecting small current capacity diodes in parallel is considered to be as follows. That is, since the break-down speed of the GTO is 1 μs or less, it is clear that a current with a steep rise (di/dt 400 A/μs in the experimental example) flows through the diode. By the way, in general, the narrower the pulse width, the wider the junction that carries the current, the smaller the conduction area, the locally high current density, and the greater the loss generated.
This is thought to be the cause of the damage to the diode mentioned above. However, for such a steep di/dt, the entire area cannot be effectively conductive regardless of the current capacity, but if diodes are connected in parallel, the conduction area will be expanded and the conduction area will be reduced. It is possible to reduce the current density at Therefore, the voltage (v _fr ) generated in the diode can be reduced,
It becomes possible to reduce the spike voltage Vsp in equation (1) described above. This can prevent GTO1 from being destroyed. Furthermore, damage to the diodes 5a ₁ , 5a ₂ , . . . is also effectively prevented.

As explained above, in the present invention, the diode connected in parallel to the discharge resistor and connected in series to the capacitor is composed of multiple parallel connections of small current capacity diodes with the same characteristics.
It is possible to suppress the voltage generated in the diode when the GTO turns off. For this reason GTO
It is possible to reduce the spike voltage that occurs between the anode and cathode of the GTO, and prevent damage to the GTO element. Another advantage is that the diode is not thermally damaged.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a GTO protection circuit according to the prior art, Figures 2a and b are voltage and current waveform diagrams at the main parts at turn-off, and Figure 3 is a diagram of the GTO protection circuit according to the present invention. The configuration diagram and Figure 4 are voltage and current waveform diagrams showing detailed changes in turn-off operation.
FIG. 5 is an equivalent circuit diagram during turn-off operation. 1...GTO, 5a ₁ , 5a ₂ ...Diode, 5
b...Discharge resistance, 5c...Capacitor.

Claims (1)

[Scope of utility model registration request] A diode, a discharge resistor, and a capacitor are connected in parallel and series, respectively, and are connected in parallel between the anode and cathode of a gate turn-off thyristor to suppress a sudden rise in the voltage between the anode and cathode when the thyristor is turned off. A protection circuit for a gate turn-off thyristor, characterized in that the diode is constituted by a plurality of diodes having the same characteristics connected in parallel.