Lec 5 Thyristors.pptx - THYRISTOR Thyristor symbol three PN junctions Background On Thyristors \u2022 The name Thyristor comes from two similar device

Lec 5 Thyristors.pptx - THYRISTOR Thyristor symbol three PN...

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THYRISTOR
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Thyristor symbol & three PN junctions
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Background On Thyristors • The name Thyristor comes from two similar device names ‘Thyratron’ and ‘Transistor’ • Thyristors are useful due to their ability to handle large current in power applications and fast switching • The most common thyristor is the SCR which stands for “Silicon Controlled Rectifier”
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Thyristor circuit & static characteristics
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Thyristor Operating Modes Thyristors have 3 operation modes Forward blocking mode: Anode is positive w.r.t cathode, but the anode voltage is less than the break over voltage (VBO) . Only leakage current flows, so thyristor is not conducting . Forward conducting mode: When anode voltage becomes greater than VBO, thyristor switches from forward blocking to forward conduction state, a large forward current flows If the gate is triggered, thyristor can be turned ON even when anode voltage is less than VBO. • The current must be more than the latching current (IL). • If the current reduced less than the holding current (IH), thyristor switches back to forward blocking state.
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Thyristor Operating Modes Reverse blocking mode: When cathode is more positive than anode , small reverse leakage current flows. However if cathode voltage is increased to reverse breakdown voltage , Avalanche breakdown occurs and large current flows
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Thyristor Turn-ON Methods Thyristor 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 : • High voltage triggering • dv/dt triggering • Thermal triggering • Light triggering • Gate triggering
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High Voltage Triggering • When breakover voltage (VBO) across a thyristor is exceeded than the rated maximum voltage of the device, thyristor turns ON. • Breakover 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).
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