Posted on 05 July 2019

Protection of Thyristors with Snubbers








Snubbers for Single Switches

The regular way of protecting semiconductor elements from overvoltage is by arranging the resistors and capacitors directly in parallel to the individual switches. This ensures that the component is always protected regardless of where the overvoltage comes from. This method is however not always ideal. In certain cases this alone will not suffice.

Figure 1. Snubber for a single thyristor


Whereas single-switch snubbers in noncontrollable rectifier circuits are rather the exception, in controllable circuits single-switch snubbers are used as a rule. This boils down to the fact that thyristors have to be protected not only from excessive nonpermissible surges, but also, more importantly, from the steep rising rate. A very steep increase in voltage can unwantedly trigger the thyristor. Furthermore, a thyristor snubber can also be required to facilitate thyristor triggering in case of inductive loads and discontinuous current. This can also be achieved using a DC-side snubber. The use of a single-thyristor snubber is limited, however, since the capacitors discharge through these networks during triggering and thus cause undesired stress with a steep current rate of rise (di/dt stress). In thyristor assemblies for low current densities and high voltages where relatively small capacitance and high ohmic resistances are sufficient, this di/dt stress is still relatively low, meaning that here a single-switch snubber alone is often sufficient. This is to be dimensioned like an AC side snubber With thyristor assemblies intended for higher current densities. However, the capacitances needed to ensure protection from high-energy overvoltages are so high and the optimum resistances so low that a high non-permissible thyristor di/dt caused by capacitor discharge would result during triggering. Thus, sufficient protection cannot be provided by switch-level snubbers alone. In this case, the solution is to use an AC side snubber (if need be, with additional DC side circuitry).

Snubber Circuits Using Auxilliary Diodes

In some cases, it is possible or even necessary to use auxiliary diodes in the snubber circuits when connecting thyristors with RC elements. For example, using the circuit shown in figure 3, the rate of rise of the forward blocking voltage can be ideally attenuated by the components R and C, while for discharge of capacitor C during thyristor triggering, an additional resistance Rz is effective which limits the current surge to a harmless level. In the reverse direction, Rz is effective too, meaning that in this direction the reduction of surge voltages is not ideal.

Figure 2. Snubber circuits for thyristor control: single-switch snubbers based on auxiliary diodes


In self-commutated rectifiers, overvoltage suppression is often not needed in the reverse direction, since it is already suppressed by the regenerative arm. In such cases, the circuit shown in Figure 2 (b), where the snubber is effective in the forward direction only and an optimum layout is possible, can be used. Here, capacitor discharge does not happen during thyristor triggering at all.

If thyristors are connected in parallel, the different trigger delay times would cause the snubber capacitors of all of the remaining thyristors to fully discharge through the first thyristor that fires. This must be avoided at all costs. For this reason, in parallel thyristor circuit arrangements - and in many cases for individual high-power thyristors as well - the single-switch snubber circuit comprising an auxiliary bridge as shown in Figure 4  is used. This may be relatively complex. It offers, however, optimum protection in both voltage directions and removes the shortcomings of a simple RC snubber at the same time.

For assemblies intended for very high power, in particular for parallel thyristor circuits, it would be beneficial to connect the RC circuit via an auxiliary bridge. The additional costs of these auxiliary bridges are compensated to some extent by using a less expensive electrolytic capacitor and a damping resistor with a low power rating. It is important to keep in mind that when an auxiliary bridge is used, the discharge current surge needed for safe thyristor triggering under inductive load is no longer flowing. Recommended resistance and capacitance values for single-switch snubbers are given in the catalogues.

Figure 3. Single-thyristor snubber networks using an auxiliary bridge

Snubbers for AC voltage converters (W1C)

W1C circuits comprise two antiparallel thyristors, which is always equipped with a snubber circuit - in the simplest case made of one common RC element (Figure 4). If each of the antiparallel components has its own fuse protection, then each must also be equipped with its own RC element (Figure 4b). RC elements with one capacitor greater than 1 μF have to be connected using an auxiliary diode.

Figure 4. Snubber for AC controller; right: with individual fuses


Snubbers for parallel thyristors

Owing to the unavoidable variation in trigger delay times which cannot be reduced below a certain value, even with strong trigger pulses with a steep rate of rise, parallel thyristors will never fire simultaneously. For this reason, in parallel thyristor assemblies, suitable measures are needed to prevent the snubber capacitances of all of the remaining thyristors from discharging through the thyristor that fires first since this could lead to destruction. Therefore, series reactors are often used. The main purpose of this is to achieve even current distribution in stationary state and under short-time overload and short-circuit conditions (Figure 5). At the same time, these series reactors prevent the snubber capacitors of the remaining parallel thyristors from discharging via the thyristor that fires first. To attenuate oscillations, a resistor is often connected in parallel to the series reactor.

Figure 5. Single-switch snubber circuit for parallel thyristors using series reactors

Snubbers for series connected thyristors or rectifier diodes

For series connected silicon rectifier diodes or thyristors, single-switch snubbers with RC elements not only have the effects discussed above, but fulfill an additional function as well. Owing to the variation in reverse recovery time, the single switch that blocks first is at risk of having to take up a high voltage that is not permissible for only a single switch. With series connected thyristors, there is an additional risk: directly before firing, the thyristor that goes over into on-state last, might have to take up an inadmissibly high forward voltage that would result in uncontrolled “break-over” and may damage the thyristor. Both of these risks must be ruled out using a snubber circuit with RC elements. Of course, in addition to this “dynamic voltage distribution”, static voltage distribution by means of parallel resistors is needed. These resistors can be selected such that a current roughly the size of the maximum rated reverse current specified in the datasheet for the thyristor or diode flows through each resistor.


For more information, please read:

Overcurrent Protection for Diodes and Thyristors

DC Side Snubber Circuits

Snubber Circuits Based on Silicon Avalanche Diodes

Safe Firing of Thyristors


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