Note for Power Electronics - PE By JAMEEL AHMAD
- Power Electronics - PE
- Note
- UET Lahore - UET
- Electrical Engineering
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Thyristor Most important type of power semiconductor device. They can operate as a bistable switch that can be turned on by a gate signal, but can not be turned off by the gate. Proposed in 1950s at Bell Lab. The first thyristor was developed in 1957 by General Electric (GE) Have highest power handling capability up to ~4500 Volt, up to ~5000 Amperes Maximum switching frequency for typical thyristors is around 1kHz. But it can rise up to 20kHz for some special types. It is inherently slow switching device compared to BJTs or MOSFETs. EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014 3
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Structure It is a four-layer semiconductor device of pnpn structure with three pn-junctions; e.g. J1, J2 and J3. Terminals; Anode (A), Cathode (K) and Gate (G) Forward blocking mode: When Anode voltage is made positive with respects to Cathode, the junctions J1 and J3 are forward biased, and J2 is reverse biased, and therefore a small leakage current can flow through the device. Reverse blocking mode: When Cathode has positive voltage than the Anode, J1 and J3 are reverse biased, and J2 is forward biased, and therefore a small leakage current can flow through the device. For both blocking modes the gate terminal is assumed as open circuit. EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014 4
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Avalanche breakdown IT Forward leakage current is very small. If the VAK is increased to a sufficiently Latching current large value, the leakage current Forward breakdown increases and the junction J2 voltage IL breaks down. This voltage is called as forward breakdown voltage (VBO). VAK VBO Forward leakage current Since the other junctions J1 and J3 are already fwd biased, then the device will be in conducting state or on-state. The anode current must be more than a value known as latching current to keep the thyristor is on-state. Otherwise the device will revert to the blocking condition as the VAK is reduced. Latching current is the minimum anode current to maintain the thyristor in the on-state immediately after it is turned on (no gate signal) EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014 5
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Holding current IT Once the thyristor conducts, Forward voltage it behaves as a conducting diode drop Latching and there is no control over the current Forward device. breakdown voltage IL Therefore the device current IH must be limited by external Holding current components, such as resistors. VAK VBO The forward voltage drop is due to the ohmic drop in the four layers and it is small, typically 1V. If the fwd anode current is reduced below a level known as holding current IH, a depletion region is developed around junction J2 due to the reduced number of carriers, and the thyristor will be in blocking state. The holding current is very small in the order of milliamperes, and it is less than the latching current. EE328 Power Electronics, Dr. Mutlu Boztepe, Ege University, 2014 6
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