POWER ELECTRONICS Instructor: Eng.Moayed N. EL Mobaied The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department بسم الله الرحمن الرحيم EELE 5450 — Fall Lecture 6

Chapter Three Thyristors Two Transistor Model of Thyristors When Driving I B2 using I G and I C1 then α 1 + α 2  1 then IE will increase Via I A And positive feedback will occur. Thus SCR will turn –on.

Chapter Three (α1 + α2 >= 1) This condition will turn on the SCR and it will happen when: If the temperature of the device is very high,the leakage current through it will increases, and this increase in current causes the alphas to increase. When the voltage between anode and cathode exceed V BO (break down of J2). If there is gate current I G.

Chapter Three SCR transient characteristics

Chapter Three SCR Trigger circuits Firing angle circuits involves three considerations: Choice of a suitable circuit which will supply the firing signal under the required circuit conditions. Maximum voltage and current. Minimum voltage and current. Less trigger current is required at high temp. Less trigger current is required when V AK is high. Sharp pulse is required in senstive applications.

Chapter Three SCR turn-off circuits (Commutating circuits) AC state. DC state. Positive cycle in AC state. Circuit (b) is seldom used except at low power levels. Many methods for these switch circuits will be discuss in chapter 8 End of Lecture **** Eng.moayed