Power Electronics, A First Course, 2nd Edition

13 THYRISTOR CONVERTERS

13.1 INTRODUCTION

Historically, thyristor converters were used to perform tasks that are now performed by switch-mode converters, discussed in previous chapters. Thyristor converters are now typically used in utility applications at very high power levels. In this chapter, we will examine the operating principles of thyristor-based converters.

13.2 THYRISTORS (SCRs)

A thyristor is a device that can be considered a controlled diode. Like diodes, they are available in very large voltage and current ratings, making them attractive for use in applications at very high power levels.

Thyristors, shown by their symbol in Figure 13.1a, are sometimes referred to by their trade name of silicon controlled rectifiers (SCRs). These are 4-layer (p-n-p-n) devices, as shown in Figure 13.1b. When a reverse ( ) voltage is applied, the flow of current is blocked by the junctions pn1 and pn3. When a forward ( ) polarity voltage is applied, and the gate terminal is open, the flow of current is blocked by the junction pn2, and the thyristor is considered to be in a forward-blocking state. In this forward-blocking state, applying a small positive voltage to the gate with respect to the cathode for a short interval supplies a pulse of gate current that latches the thyristor ...