POWER SEMICONDUCTOR DIODES

Objectives

1. Identify different important features and steady-state and switching characteristics of power semiconductor diodes
2. Understand the importance of forward and reverse recovery times
3. Know the applications of different power semiconductor diodes in Power Electronic Circuits
4. Get idea on characteristics of a commercial diode

Power Diode: Features
Power diodes have larger power-, voltage-, and current handling capabilities than that of ordinary signal diodes.
Power diodes have an additional lightly doped drift region (n-) which helps to increase the voltage capability of the device.
However, the frequency response or switching speed is low compared to that of signal diodes.
Applications: 1) as uncontrolled rectifiers to convert AC to fixed DC voltages and 2) as freewheeling diodes to provide a path for the current flow in inductive loads.

I-V Characteristics (at steady state) of Power Diode

Forward-biased region, where v_D > 0. I_D is very small if v_D < V_TH (threshold or cut-in or turn-on or knee voltage). I_D is large if v_D >V_TH. The diode conducts fully and I_D is limited externally.

Reverse-biased region, where v_D < 0. The reverse leakage current, I_R (or I_S) is very small, in the range of micro- to milli-ampere due to the flow of the thermally generated carriers.

Breakdown region, where v_D < -V_BR (breakdown voltage). V_BR is avalanche or zener voltage, where avalanche breakdown begins to occur. The reverse current increases rapidly with a small change in reverse voltage beyond V_BR. If the current is limited external, the diode is not spoiled as long as the power dissipation (I_D*V_BR) < P_D,max.

Comparisons of Power and Signal Diodes
VTH

Power diodes: ~1V
Signal diodes: ~0.7V

Forward i-v Characteristics

Power diodes: linear
Signal diodes: exponential

Ron (ohmic resistance)

Power diodes: >0Ω
Signal diodes: ~0Ω

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