Thyristors

4.1:

Introduction.

4.2:

Thyristor Characteristics.

4.3:

Two-Transistor Model of a Thyristor.

4.4:

Thyristor Turn-On.

4.5:

di/dt Protection.

4.6:

dv/dt Protection.

4.7:

Thyristor Turn-Off.

4.8:

Thyristor Types.

4.11:

Thyristor Firing Circuits.

4.12:

Unijunction Transistors.

4.13:

Programmable Unijunction Transistors.

Introduction A thyristor is on of the most important types of power

semiconductor devices. It thyristor can operate as a bistable switch (conducting or nonconducting). A thyristor may be assumed as an ideal switch for many applications but practical thyristors exhibit certain characteristics and limitations. Due to its voltage and current handling capabailities, a thyristor is the most commonly used semiconductor switch in power electronic applications for many years.

A thyristor is turned ON by increasing the anode current in one of the following ways. • Thermal. • Light. • High Voltage. • dv/dt. • Gate Current. A thyristor is turned OFF by reducing the forward current below the IH. • Line Commutation. • Forced Commutation.

• Phase Controlled Rectifiers (SCRs) • Fast-switching thyristors (SCRs) • Gate-turn-off thyristors (GTOs) • Bidirectional triode thyristors (TRIACs) • Reverse Conducting thyristors (RCTs) • Static induction thyristors (SITHs) • Light activated silicon controlled rectifiers (LASCRs) • FET-controlled thyristors (FET-CTHs) •MOS-controlled thyristors (MCTs)

Voltage

Thyristors

5 kv GTOs 4 kv

3 kv

MCTs Current

IGBTs 2 kv

BJTs

1k

Hz

10 kHz

1 kv

100 kHz

MOSFET s Frequency

500 A

1000 A

1500 A

2000 A

2500 A

1 MHz 3000 A

4.12: Unijunction Transistors. 4.13: Programmable Unijunction Transistors.

T = 1 / f ≅ RC ln (1 / 1- η) where η is called the intrinsic stand-off ratio and lies between 0.51 and 0.82.

Half-wave Uncontrolled Rectifier

Half-wave Controlled Rectifier

Full-wave Uncontrolled Rectifier

Full-wave Controlled Rectifier