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Power ElectronicsDr. Imtiaz HussainAssistant Professoremail: [email protected]URL :Lecture-5Thyristors1
•Oneofthemostimportanttypeofpowersemiconductor device.•Compared to transistors, thyristors have lower on-state conduction losses and higher power handlingcapability.•However, they have worse switching performancesthan transistors.•Name‘thyristor’,is derived by a combination of thecapital letters fromTHYRatronandtransISTOR.Introduction2
•Thyristorsarefour-layerpnpnpowersemiconductor devices.•Thesedevicesswitchbetweenconductingandnonconductingstates inresponseto acontrolsignal.•Thyristors are used in timing circuits, AC motorspeed control and switching circuits.Introduction3
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 characteristicssimilar to of a thyristor were developed.•These semiconductor devices are SCR, SCS, Triac, Diac,PUT, GTO, e.t.c.•This whole family of semiconductor devices is giventhe name thyristors.4
Thyristor/ SCR•SCR is a three terminal, four layers solid statesemiconductor device, each layer consistingof 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 improvedbreakdown voltage .AKG5
Thyristor/ SCR•Thyristor can handle high currents and high voltages.•Typical rating are 1.5kA & 10kV which responds to 15MWpower handling capacity.•This power can be controlled by a gate current of about 1Aonly.•Thyristor acts as a bistable switch.–Itconductswhengatereceivesacurrentpulse,andcontinue to conduct as long as forward biased (till devicevoltage is not reversed).–They stay ON once they are triggered, and will go OFF onlyif current is too low or when triggered off.6
Thyristor/ SCR Operation•Whentheanodevoltageismadepositive with respect to the cathode,junctionsJ1andJ3are forward biasedand junctionJ2is reverse biased.•Thethyristorissaidtobeintheforwardblockingoroff-statecondition.•Asmallleakagecurrentflowsfromanode to cathode and is called theoff-state current.7
Thyristor/ SCR Operation•IftheanodevoltageVAKisincreasedtoasufficientlylargevalue,thereversebiasedjunctionJ2would breakdown.•This is known asavalanche breakdownand thecorrespondingvoltageiscalledtheforwardbreakdown voltageVBO.•Since the other two junctionsJ1andJ3arealreadyforwardbiased,therewillbefreemovement of carriers across all three junctions.