Model of a Power Thyristor

Model of a Power Thyristor



Introduction

Thyristors

A thyristor is one of the most important types of power semiconductor devices. The thyristors are used extensively in power electronic circuits. It is operated as bistable switches, from a non-conducting state to a conductive state. For many applications, it can be assumed that the thyristor switches or switches are ideal, but practical thyristors exhibit certain characteristics and limitations (Trakjovic, 1990).

Thyristor characteristics:

A thyristor is a semiconductor device of four-layer PNPN structure with three PN junctions has three terminals: anode cathode and gate. The fig 1 shows the thyristor symbol and a straight section of three pn junctions. The thyristors are manufactured by diffusion.

When the anode voltage is positive with respect to the cathode, the joints J 1 and J 3 have a bias or positive. The union J 2 is reverse biased, and only a small current will flow from anode to cathode leakage. We then say that the thyristor is in a position to direct or block off state leakage current calling the idle current I D. If the anode to cathode voltage V AK is increased to a value large enough J 2 union enter into the reverse-biased break. This is known as avalanche breakdown and the corresponding voltage is called direct breakdown voltage V BO. Since the joints, J 1 and J 3 and are biased there is free movement of carriers across the three unions will cause a large current of the anode. Then said the device is in conducting state or activated (Christiansen, Donald, Alexander & Charles, 2005).

The voltage drop will be due to the ohmic drop of the four layers and will be small, typically 1V. In the active state, the anode current must be greater than a value called coupling current I L, in order to maintain the required amount of flow of carriers across the junction, otherwise, by reducing the anode voltage the cathode, the device returns to the block condition. The coupling current, I L, is the minimum anode current required to maintain the thyristor in conduction state immediately after it has been activated and the signal is removed from the gate.

Once the thyristor is activated, it behaves like a diode in conduction and, there is no control over the device. The thyristor will continue to lead, because at the junction J 2, there is no depletion layer of life to free movement of carriers. However, if direct current is reduced anode below a level known as the holding current I H, generates a depletion region around the junction J 2 due to the small number of hosts, the thyristor will then be in a state of blockade. Holding current is of the order of milliamps and is less than the current engagement, I L. This means that R L> R H. The holding current I H is the minimum anode current to maintain the thyristor in steady state. The holding current is less than the current hitch (Dorf, Richard, ...