A power diode is a two terminal power semiconductor device which conducts and allows the flow of
current under forward biased condition and blocks the current under reverse biased condition.
A diode is said to be forward biased when its anode is more positive with respect to cathode. The below circuit shows a forward biased diode connected to a
battery of
voltage V and the below figure shows the V - I characteristics of a power diode. Initially when the applied voltage V is zero (V=0) the diode doesn't conduct, and hence
the current I will be zero. As the voltage is increased, ideally the diode should conduct the full current for any V > 0 as shown in the ideal characteristic. Practically, the diode doesn't conduct the full current but allows only a small current until the applied voltage reaches a certain minimum value called
threshold voltage (also called as
cut-in voltage). The threshold voltage for silicon diodes is around 0.7 V and for germanium diode it is 0.3 V. After the anode voltage exceeds this threshold voltage the diode starts conducting and the current increases rapidly. When the diode is conducting there will be a forward voltage of around 1 V across the diode.
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| Diode connected in forward bias |
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V - I characteristics of power diode.
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| Ideal characteristics of a diode |
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A diode is said to be reverse biased when its cathode is more positive with respect to anode. The below circuit shows a diode connected in reverse bias.
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| Diode connected in reverse bias |
A diode will not allow the flow of current in reverse direction but it still allows a very small amount of current in the order of milliamps called as leakage current. The leakage current is almost constant with respect to the reverse bias voltage but as the voltage increased, at certain point called breakdown voltage the diode loses its reverse blocking capability and allows high-current. This reverse breakdown leads to excessive power dissipation at the diode and may destroy the device and must be avoided. To safely operate a diode in reverse bias it must always be operated below a certain specified voltage called maximum reverse repetitive voltage V
RRM.
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