20/03/2017 1 Thyristors and their application1 •A thyristor is one of the most important types of power semiconductor devices. •Compared to transistors, thyristors have lower on-state conduction losses and higher power handling capability. •However, they have worse switching performances than transistors. •Name ‘thyristor’,is derived by a combination of the capital letters from THYRatronand transISTOR.Thyristor 2 •Thyristors are four-layer pnpn power semiconductor devices. •These devices switch between conducting and nonconducting states in response to a control signal. •Thyristors are used in timing circuits, AC motor speed control and switching circuits. Thyristors…3 Thyristors… •Bell Laboratories were the first to fabricate a silicon-based thyristor. •Its first prototype was introduced by GE (USA) in 1957. •Later on many other devices having characteristics similar to those of a thyristor were developed. •The semiconductor devices are: SCR, SCS, Triac, Diac, PUT, GTO, e.t.c. •This family of semiconductor devices is given the name thyristors. 4 Thyristor/ SCR •SCR is a three terminal, four layers solid state semiconductor device, each layer consisting of alternately N-type or P-type material, i.e; P-N-P-N, •It can handle high currents and high voltages, with better switching speed and improved breakdown voltage . A K G 5 Thyristor/ SCR…•Thyristor can handle high currents and high voltages. •Typical ratings are 1.5kA & 10kV which corresponds to 15MW power handling capacity. •This power can be controlled by a gate current of about 1A only. •Thyristor acts as a bistable switch. –It conducts when gate receives a current pulse, and continue to conduct as long as forward biased (till device voltage is not reversed). –They stay ON once they are triggered, and will go OFF only if current is too low or when triggered off. 6
20/03/2017 2 Thyristor/ SCR Operation •When the anode voltage is made positive with respect to the cathode, junctions J1and J3are forward biased and junction J2is reverse biased. •The thyristor is said to be in the forward blockingor off-state condition. •A small leakage current flows from anode to cathode and is called the off-state current. 7 Thyristor/ SCR Operation…•If the anode voltage VAKis increased to a sufficiently large value, the reverse biased junction J2would breakdown. •This is known as avalanche breakdownand the corresponding voltage is called the forward breakdown voltageVBO. •Since the other two junctions J1and J3are already forward biased, there will be free movement of carriers across all three junctions. •This results in a large forward current and the device is now said to be in a conductingor on-state.