Reverse Recovery characteristics of diode

Reverse Recovery characteristics of diode:

A diode when functioning in its forward bias condition has its depletion region shrinked to almost nothing. That is, the external supply voltage applied will be used by the device to overcome the barrier potential which gets imposed on it due to the presence of immobile charge carriers in its depletion region. Now, imagine that one reverse biases this voltage by inverting the polarities connected to the terminals of the diode. Ideally, the act of doing so should bring the diode from its ON state to OFF state immediately. That is, the diode which is conducting current in its forward direction is expected to stop conducting instantly.

However, practically, this cannot be experienced as the flow of majority charge carriers through the diode does not cease right at the moment of reversing the bias. They will, in fact, take a finite amount of time before stopping and this time is known as reverse recovery time of the diode.

During this reverse recovery time of the diode, one can see that there will be fairly large amount of current flowing through the diode, but in the opposite direction (I_rr in Figure 1). However its magnitude reduces and gets saturated to a value of reverse saturation current, once the time-line crosses reverse recovery time (t_rr) of the diode.

Graphically one can describe the reverse recovery time of the diode as the total time which starts from the instant at which the reverse current starts to flow through the diode to the time instant at which it reaches to zero (or any other pre-defined low level, say 25% of I_rr in the figure) while decaying (t_d), on reaching its negative maxima (t_p).

The ratio of these two time factors (viz., t_p and t_d) is known as the softness factor.

In the case of a normal diode, the time is taken by the current to decay (t_d) will be smaller in comparison to the time taken by the current to reach its negative peak (t_p).

On the other hand, for a soft recovery diode, the situation will be the reverse. That is, here, t_d will be larger in comparison to t_p.

We can see that the softness factor gives a measure of semiconductor losses incurred during switching. Greater is this ratio; greater will be the switching loss.

From this, one can conclude that when we use soft-recovery diodes, the losses experienced by the semiconductor switching are more than those encountered when we use normal diodes.