4.1: 4.2: 4.3: 4.4: 4.5: 4.6: 4.7: 4.8: 4.11: 4.12: 4.13:

Introduction. Thyristor Characteristics. Two-Transistor Model of a Thyristor. Thyristor Turn-On. di/dt Protection. dv/dt Protection. Thyristor Turn-Off. Thyristor Types. Thyristor Firing Circuits. Unijunction Transistors. Programmable Unijunction Transistors.

IntroductionA 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 IGBTs 2 kv

MCTs Current BJTs Hz 1k 10 kHz MOSFET s 100 kHz 1 MHz 3000 A

1 kv

Frequency

500 A

1000 A

1500 A

2000 A

2500 A

4.12: Unijunction Transistors. 4.13: Programmable Unijunction Transistors.

T = 1 / f RC ln (1 / 1- )where 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