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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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