THYRISTORS

OBJECTIVES

1. Understand the different types of thyristors and their characteristics.
2. Examine the methods of turning on and turning off thyristors.
3. Demonstrate the techniques for di/dt and dv/dt protection of thyristors.
4. Examine thyristor firing circuits.

Thyristor, is a switch that conduct only in one direction when:

1. The voltage between anode and cathode is positive
2. The gate is triggered, by a short pulse

Types of Thyristors

Thyristors are manufactured almost exclusively by diffusion. Thyristors can be classified into nine categories depending on the physical construction, turn-on time and turn-off time. These categories are:

1. Phase Control Thyristors (SCRs)
2. Bi-directional phase-controlled thyristors (BCTs)
3. Fast Switching Thyristors (SCRs)
4. Light Activated Silicon Controlled Rectifiers (LASCRs)
5. Bidirectional Triode Thyristors (TRIACs)
6. Reverse Conducting Thyristors (RCTs)
7. Gate Turn-off Thyristors (GTOs)
8. FET Controlled Thyristors (FET-CTHs)
9. MOS-turn-off thyristors (MTOs)
10. Emitter turn-off (control) thyristors (ETOs)
11. Integrated gate-commutated thyristors (IGCTs)
12. MOS Controlled Thyristors (MCTs)
13. Static Induction Thyristors (SITHs)

 

Thyristor/SCR Symbol

1. A thyristor is a four layer pnpn semiconductor device consisting of three pn junctions. It has three terminals: an anode a cathode and a gate. Fig. shows the thyristor symbol and a sectional view of the three pn junctions.
2. When the anode voltage is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse biased. The thyristor is said to be in the forward blocking or off-state condition. A small leakage current flows from anode to cathode and is called the off state current.

previous Types of Power Diodes
next Characteristics of Thyristors