Instructor: Eng.Moayed N. EL Mobaied بسم الله الرحمن الرحيم The Islamic University of Gaza Faculty of Engineering Electrical Engineering Department POWER ELECTRONICS EELE 5450 — Fall 2009-2010 Instructor: Eng.Moayed N. EL Mobaied Lecture 5

Chapter Three Thyristors The SCR (Silicon-controlled rectifier) is the most important member of the family of power semiconductor devices.

Chapter Three Applications

Chapter Three Thyristors family There are other power semiconductor devices of the thyristor family which share the latching characteristics of the SCR. Triac. Bidirectional diode switch. Silicon controlled switch. Silicon unilateral and bilateral switches( SUS,SBS). Light activated devices Like LASCR and LASCS. Complementary SCRs. Programmable unijunction transistors( PUT).

Chapter Three Thyristor (SCR) If the anode-to cathode voltage Vak is increased to a sufficiently large value, the reverse-biased junction J2 will break. This is known as avalanche breakdown and the corresponding voltage is called forward breakdown voltage VBO. Since the other junctions J1 and J3 are already forward biased, there will be free movement of carriers across all three junction, resulting in a large forward anode current. The device will be in a conducting state or on-state. When the anode voltage is made positive with respect to the cathode, the junction J1 and J3 are forward biased. The junction J2 is reverse biased, and small leakage current flows from anode to cathode. ( Forward blocking off-state) ID A Thyristor (SCR) is a four-layer semiconductor device of pnpn structure with three pn junctions. It has three terminals :Anode ,Cathode, Gate.

SCR (V-I characteristics) Chapter Three If the forward anode current is reduced below a level known as the holding current IH, thyristor will be in blocking state. When Cathode is positive with respect to anode (reverse blocking state) IR The voltage drop would be due to the ohmic drop in the four layers and it is small, Typically, 1V. In the on-state, the anode current is limited by an external impedance or a resistor , RL. The anode current must be more than a value known as latching current IL , in order to maintain the required amount of the carrier flow across the junction. SCR (V-I characteristics)

End of Lecture **** Eng.moayed Chapter Three IG effects End of Lecture **** Eng.moayed