TEXT 12

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Thyristors too, can misbehave as switches

I.

Thyristors also have switching problems that can be improved by snubbing networks, usually of the simple RC type. In contrast to the switching problems of bipolar transistors and power MOSFETs, it is the extremely rapid turn on of thyristors that tends to cause troubles. For example, the RFI (radio frequency interference) and EMI (electromagnetic interference) generated by thyris­tors originates primarily from this characteristic. Such interference might not only play havoc with communications equipment, but all too often causes false turn on of other thyristor control circuits. Another source of false turn on in thyristors emanates from the so-called dv/dt effect. In so many words, if the voltage across the thyristor rises too rapidly following turn off, the thyristor might be internally retriggered, thereby losing control. This retriggering occurs because of capacitive feedback from the anode to the gate and is one of the factors limiting the frequency at which proper control can be maintained. High frequency is tantamount to high dv/dt and there is more internal capacitive feedback to the gate. In actual applications, snubbing is very effective in preventing erratic performance of this nature. Of course, the proper thyristor must be selected for the frequency involved.

Another way of improving dv/dt immunity is to use a low value of gate-cathode resistance to divert much of the internal anode-gate feedback current. This is a very effective approach, but it is at the expense of increased drive power. A negative bias of about 1 V at the gate can also be used to extend immunity from the dv/ dt effect. Such reverse bias is more easily applied to SCR than to triac circuits.

Whether false triggering is attributed to RFI, line transients, or dv/dt effects, keep in mind that thyristors trigger with less gate charge as temperature increases. A basic requirement of a satisfactory driver is to adequately trigger the thyristor over its entire range of operating temperature. This sometimes imposes a problem at the cold end of the range where it is found the thyristor will not turn on. Conversely, at the high temperature region of operation, there may be unanticipated vulnerability to false turn on. (1900)

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II.

False turn on problems are made worse by inductive loads; the thyristor tries to turn off while there is high voltage across it because of the phase displacement of voltage and current caused by the inductive load. That is, zero load current no longer coincides with zero load voltage as is the case with a resistive load. This is essentially a high dv/dt situation tending to retrigger the thyristor as previously described. This type of dv / dt, known as commutating dv/dt can exist in 60 Hz and low frequency control systems, but otherwise acts as ordinary dv/dt. The solutions to this type of false triggering are. for the most part, those already mentioned- snubbing for reducing dv/dt and gate circuit modifications for reducing vulnerability to the internal anode-gate feedback current.

As if all this is not enough, thyristors can be damaged from too high a rate of load current rise during turn on. Turn on can be likened to the ignition and propagation of a flame, which is not instantaneous. Making a high current demand before the junctions are fully aflame can create localized hot spots in the junctions not unlike those causing secondary breakdown in bipolar transistors. Obviously, this type of damage is not likely to occur with inductive loads where the initial rate of rise of the current is impeded. By the same token, a guard against di/dt destruction when using resistive loads is to insert a small amount of inductance in the load circuit. Sometimes a delay reactor in the form of a nonlinear inductance is used. The square-loop core permits initial slow-down of di/dt. A straightforward way of improving the di/dt capability of a thyristor is to drive it hard from a rapid-rise trigger source. This speeds up the propagation process so that full turn on capability is quickly attained. Conversely, di/dt failures are usually found in circuits where the gate drive is minimally sufficient to reliably trigger the thyristor.' Within reasonably limits, the only drawback to overdriving the gate is to slightly lower overall switching efficiency. (2000)

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