Switching Characteristics of Power Diodes

Voltage and current waveforms for a power diode driven by currents with a specified di_F/dt and a specified di_R/dt.

1. Finite times are required for fully turn-on (t₁+t₂) and turn-off (t₃+t₄+t₅).
2. This is due to the excess carriers, space charges, and the bulk semiconductor material.

Two Types of Reverse Recovery Characteristics
t_rr = reverse recovery time, measured as the time between the initial zero crossing of the diode current to the time when this current reaches 25% of the peak reverse current. I_RR = maximum reverse current t_a = time between zero crossing and the maximum reverse current and it is due to the charge stored in the depletion region of the junction t_b = time between maximum reverse current IRR and 25% of the of the maximum reverse current IRR and is due to charge stored in the bulk semiconductor material The reverse recovery time is measured from the initial zero crossing from forward conduction to reverse blocking condition of the diode current to 25% of the maximum reverse current I_RR. Its magnitude depends on:

1. junction temperature
2. rate of fall of forward current
3. forward current prior to commutation

From the graph
 
Reverse Recovery Charge
This is the amount of charge carriers that flow across the diode in the reverse direction due to changeover from forward conduction to reverse blocking condition. Its value is determined from the area enclosed by the path of the reverse recovery current (Recall Δ Q = (Δ I)(Δ t)). That is

or   or
From above equations we get




Ideally diodes should not have a reverse recovery time, and it is possible to construct such a diode. However, the manufacturing cost of such a diode would be quite high for such a feature which in most cases has minor consequences
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