Thyristor Surge Protection Devices

Thyristor surge protection devices are based on a pair of intertwined bipolar transistors created by a 4 layer stack of n and p doped silicon regions as shown in Figure 1 . The n doped region N1, p doped region P1, and n doped region N2 form the emitter, base and collector of an npn transistor while p doped region P2, n doped region N2, and p doped region P1 form the emitter, base and collector of a pnp transistor. With this arrangement the collector of each transistor provides the base of the other transistor. In this way any emitter to collector current of one transistor provides the base current for the other transistor. For a positive Anode to Cathode voltage, both emitter-base junctions, J1 and J3, are forward biased. Only the reverse biased junction J2 prevents current flow. If the Anode to Cathode voltage is increased to the breakdown voltage of the J2 junction currents will begin to flow directly into the bases of the two bipolar transistors. This turns both transistors on. With both transistors on the Thyristor's resistance drops, and the voltage across the Thyristor also drops. The resulting I-V curve for forcing a positive current from the Anode to the Cathode of a Thyristor is shown in Figure 2 . A protection element with this form of I-V curve can provide excellent protection; when triggered the voltage drops well below the trigger condition and considerable current can be carried with very little power dissipation in the protection element. In fact protection elements with this type of I-V curve are often called crowbar devices since it is like dropping a metallic crowbar across the terminals to be protected. A caution is that the current or voltage must fall below the Holding Point, as shown in Figure 2 , to return the Thyristor to its high resistance state.

Figure 1 Thyristor physical structure and circuit

Figure 2 Turn on of Thyristor under positive current injection from Anode to Cathode

Under a negative Anode to Cathode voltage the situation is very different. Only the junction J2 is forward biased. This forward bias junction could be considered the emitter base junction of a pair of bipolar transistors as shown in a non-conventional depiction in Figure 3 . Since the same junction is providing the emitter for both transistors no regenerative behavior is possible. For negative Anode to Cathode bias the Thyristor breakdown looks similar a reverse biased diode, as shown in Figure 4 .

Figure3 Consideration of Thyrstor under Negative Anode to Cathode bias conditions

Figure 4 Full I-V curve for a Thyristor

The protection capability for a simple Thyristor is very asymmetrical as shown in Figure 4 . In the positive direction turn on of the Thyristor results in a dramatic decrease in resistance while in the negative direction the Thyristor provides a voltage clamping action, similar to a diode based TVS device. To provide symmetrical crowbar behavior it is necessary to use two anti parallel Thyrisotrs. This can be done with a pair of discrete Thyristors, as in Figure 5 a, or it can be done with an integrated structure on a single piece of silicon including 5 doping levels, as illustrated in Figure 5 b. The integrated device is usually called a Thyristor Surge Protection Device (TSPD) and its I-V characteristic is shown in Figure 6 .

Figure 5 Illustrated versions of anti-parallel Thyristors. a) a pair of anti-parallel Thyristors b) anti-parallel Thyristors integrated into a single silicon device.

Figure 6 I-V curve of a pair of anti-parallel Thyristors

Most product TSPDs are symmetrical bidirectional designs but there are also unidirectional devices with a built in diode, as shown in Figure 7 . Asymmetric bidirectional TSPDs could also be designed with a reduced trigger voltage in one polarity, as shown in Figure 7 .

Figure 7 Examples of Bidirectional and Unidirectional TSPDs

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