POWER ELECTRONICS

Instructor: Eng.Moayed N. EL Mobaied

The Islamic University of GazaFaculty of EngineeringElectrical Engineering Department

الرحمن الله بسمالرحيم

EELE 5450 — Fall 2009-2010

Lecture 7

Chapter Three

Triacs and Diacs

A power device with four layers conducts in one direction only.Bidirectional device may be obtained by connecting two of these back-to-back.

The five layers, n1,p1,n2,p2,n3 can be combined into a single structure to form a new device.When T1 is positive with respect to T2 by voltage greater than VBO (p2,n2,p1,n1) thyristor will be on.If Reverse polarity (p1,n2,p2,n3) will be on.

Chapter Three

A five-layer device without agate can be designed for various breakdown voltages and current ratings. A diac is a five-layer gateless device.

Adiac is a device used to trigger other semiconductor power switches. Current and voltage ratings are dtermined by the type of device.

Chapter Three

A five-layer device with gate is called triac.( four stages)

Chapter Three

Unijunction Transistor (UJT)Complementary unijunction transistor (CUJT)

Programmable unijunction transistor (PUT)

Are widely used for generation of trigger pulses for thyristorsBetween B1 and B2 the it like resistance.(4.7k-9.1k) (25 cel)

If the emitter voltage ,VE, is less than the emitter peak point voltage ,Vp, the emitter will be reverse biased and only a small reverse leakage current Ied, will flow.When Ve is equal to Vp, and the emitter current Ie, is greater than the peak point current Ip, the UJT will turn on.

In the on condition ,the resistance between the emitter and base-one is very low and the emitter current will be limited primarily by the series resistance of the emitter to base-one external circuit.

Peak votage of UJT VpVp=ηVbb +Vd η is called intrinsic standoff ratio.(0.51 ---- 0.82).Vd equivalent emitter diode voltage( 0.5 at 25 cel)

Chapter Three

Complementary unijunction Transistor (CUJT)

Like standard UJT except that the currents and voltages applied to it are of opposite polarity. Ideal for stable oscillators, timers, frequency dividers.

Chapter Three

Programmable unijunction Transistor (CUJT)

VR is voltage divider (R1 and R2 can be specified RBB, Ip, η)Vc capacitor voltageWhen Vc > VR the PUT will conduct

Chapter Three

unijunction Transistor trigger circuits

Chapter Three

LASCR (Light Activated SCR)

End of Lecture **** Eng.moayed