EEE421: POWER ELECTRONICS Lecture 5 Power Semiconductor Devices: Thyristor
INTRODUCTION Thyristors are electronic switches used in some power electronic circuits where control of switch turn-on is required. The term thyristor often refers to a family of three-terminal devices that includes the silicon-controlled rectifier (SCR), the triac, the gate turnoff thyristor (GTO), the MOS-controlled thyristor (MCT), integrated gate commutated (IGCT)and others. Thyristor and SCR are terms that are sometimes used synonymously. Thyristors are capable of large currents and large blocking voltages for use in high-power applications, but switching frequencies cannot be as high as when using other devices such as MOSFETs. Hence, these devices are traditionally employed in applications such as DC transmission lines.
OPERATION The device is a three-junction PNPN type device, which can be represented as two bipolar junction transistors connected through a regenerative feedback. The transistor Q1 is formed by the n, p, and n- regions, while the second transistor Q2 is formed by the p, n- and p layers. The SCR is known for its symmetric voltage blocking nature, due to its ability to block voltages in both the forward and reverse directions. The SCR can conduct current only when a positive voltage is applied to the anode terminal, relative to the cathode terminal. However, the device starts to conduct only when a positive gate current is supplied. This current causes the transistor Q1 to turn on.
OPERATION Since Q1 and Q2 are connected in a regenerative feedback manner, the collector current of Q1 provides the necessary base current to Q2, thereby turning on Q2. Hence, a positive feedback mechanism is created, causing minority charge carriers to be injected into all four semiconductor layers. As a result, due to conductivity modulation, the turn-on resistance of the device is lowered considerably and the device is latched to the on-state. When IG is greater than the latching current, the current flowing through the SCR becomes independent of IG. It behaves similarly to a conventional p-n junction diode and with the current varying exponentially with VA. In order to turn off the SCR, a reverse current can to be applied at the anode or a negative anode-to-