3. Two transistor Analogy and Turn on methods.pptx - SCR...

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SCR turn-on methodsTwo Transistor AnalogyTurn-on methodsDynamic CharacteristicsImportant specifications
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Typical ratings-10KV,3000A,30MW-1A,10V,10W
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Two transistor model
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Turn-On process of SCRFor Finite IG^-----IA^1^ with IA^->IE12^ with IK^>IE2 (IK=IA+IG)So,( 1+ 2)^Further IA^Regenerative or Positive f/b…. ,( 1+ 2)~1IAis very large ….SCR ON.Under Transient conditions , SCR can turn ONCapacitance of PN jn influence the thyristor.SCR in Blocking state(IG=0)High dv/dt------Current ^ in The jn capacitorsSo,High dv/dt---- iC^--CB01,CB02^--IA^-- ,( 1+ 2)^=1---IAvery largeSo, if dv/dt is high….Undesirable turn on can take place
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Thyristor turn-ON methodsThyristor turning ON is also known as Triggering. With anode positive with respect to cathode, a thyristor can be turned ON by any one of the following techniques : – Forward voltage triggering Gate triggering dv/dt triggering Temperature triggering Light triggering
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Forward Voltage TriggeringWhen break-over voltage (VBO) across a thyristor is exceeded than the rated maximum voltage of the device, thyristor turns ON. • At the break-over voltage the value of the thyristor anode current is called the latching current (IL) .• Break-over voltage triggering is not normally used as a triggering method, and most circuit designs attempt to avoid its occurrence. • When a thyristor is triggered by exceeding VBO, the fall time of the forward voltage is quite low (about 1/20th of the time taken when the thyristor is gate-triggered).• However, a thyristor switches faster with VBO turn-ON than with gate turn-ON,so permitted di/dt for breakover voltage turn-on is lower.
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Gate TriggeringTurning ON of thyristors by gate triggering is simple and efficient method of firing the forward biased SCRs. In Gate Triggering, thyristor with forward breakover voltage (VBO), higher than the normal working voltage is chosen. – This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode with gate open. Whenever thyristor’s turn-ON is required, a positive gate voltage b/w gate and cathode is applied. With gate current established, charges are injected into the inner p layer and voltage at which forward break over occurs is reduced. Forward voltage at which device switches to on-state depends upon the magnitude of gate current.Higher the gate current, lower is the forward break over voltage . When positive gate current is applied, gate P layer is flooded with electrons from cathode, as cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these electrons reach junction J2. As a result, width of depletion layer around junction J2 is reduced. – This causes junction J2 to breakdown at an applied voltage lower
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