Power Diode

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What is Power Diode?

A Power diode is two-terminal p-n junction device. They have large power, voltage and current handling capabilities than normal signal diodes. The frequency response(switching speed) is low compared to signal diodes.



Diode acts as switch to perform various functions such as switches in rectifiers, freewheeling in switching regulators, charge reversal of capacitor and energy transfer between components, voltage isolation, energy feedback from the load to the power source and trapped energy recovery.


The diode is said to be forward biased when the anode potential is positive with respect to cathode and reverse biased when cathode potential is positive with respect to anode. Diode conducts when it is forward biased. A conducting diode has small forward voltage drop across it and the magnitude of this drop depends on the manufacturing process and junction temperature. Under reverse biased condition, the diode does not conduct and a small reverse current (leakage current) flows in the range of micro or milliampere and this leakage current increases slowly with the reverse voltage until the avalanche or zener voltage is reached.


Structure of Power Diode


Wd - width of drift layer(depends on breakdown voltage)

The main difference between signal diode and power diode is the presence of drift layer in the power diode. Drift layer increases the voltage handling capacity of the diode. If the width of the drift layer increases, the voltage drop and power loss also increases. The cross section of the drift layer depends upon the magnitude of the current to be handled. For large current, large area is required. Drift layer increases the reverse blocking capacity, on state voltage drop and power loss.
Usually, vertically oriented structure of Power diode is preferred as it,
  • Increase surface area for forward current
  • Reduce forward resistance
  • Reduce on state power dissipation
I-V characteristics



Initially, diode current is zero. From Vs=0 to cut-in voltage, the forward diode current is very small. Beyond cut-in voltage, the diode current rises rapidly and the diode is said to conduct. The cut-in voltage for silicon diode is 0.7 V. When diode conducts, there is a forward voltage drop of 0.8 to 1 V. A small leakage current flows through power diode when it is reverse biased. This leakage current flows due to minority carriers and it is independent of reverse voltage. When the reverse voltage reaches reverse breakdown voltage, avalanche breakdown occurs. The reverse voltage will be constant but the reverse current increases drastically. When voltage and current is high, the device gets damaged.

Switching Characteristics


After the forward diode current decays to zero, the diode continues to conduct in the reverse direction because of the pressure of stored charges in the depletion region. The reverse current flows for a time called Reverse Recovery time(trr). The diode regain its blocking capability until reverse recovery current decays to zero. Reverse recovery time is the time required for the minority carriers to recombine with the opposite charges and gets neutralized and it is the time between the instant forward diode current becomes zero and the instant reverse recovery current decays to 25% of its reverse peak value.

During Turn ON, removal of stored space charge takes place and there will be growth of excess charge carrier distribution in the drift region. By increasing forward current, space charges will be removed from the depletion region. After the removal of space charge, the diode becomes forward biased. During this time, the charge carriers in the p+ and n+ region goes to drift layer and there will be reduction in voltage but forward current remains constant. 

During Turn OFF, removal of charge carriers from drift region takes place. Negative diode current is given to remove excess charge carrier. Then, depletion region will get occupied by space charge. At t3, forward current will be reduced and removal of excess carrier begins. At this time, the voltage will be constant. At the end of t3, current becomes negative and charge carriers will be removed. Hence, the junction becomes reverse biased.

Applications

  • Battery charging
  • DC power supplies
  • Rectifiers
  • Inverters
  • Snubber circuits
References

1] M.D. Singh and K.B. Khanchandani, "Power Electronics", Mc Graw Hill India, 2013.
2] Rashid M.H., "Power Electronics Circuits, devices and Applications", Prentice Hall, India, Third            Edition, New Delhi, 2004.
3] P.S.Bimbra, "Power Electronics", Khanna Publishers, Eleventh Edition, 2003.




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