Reverse Recovery Characteristics of Power Diode

During the switching-off of a forward conducting diode, the diode won’t stop conducting immediately after its forward current decays to zero, but continues to conduct in reverse direction for a certain period of time called reverse recovery time t_rr. This is due to the presence of minority charges stored in the diode. This stored charge is present both at the depletion region and in the semiconductor layers. Actually an ideal diode should always prevent the flow of current in reverse direction, but due to the presence of these charges the diode continues to conduct in reverse direction until these charges are removed. This current is called as reverse recovery current as shown in the below figure.

Reverse Recovery time of a Diode, t_rr

Reverse recovery characteristics for soft recovery diode

Reverse recovery characteristics for abrupt recovery diode

The time taken by the diode’s reverse recovery current to decay to 25% of its peak measured from the instant when the forward current is reached zero is called reverse recovery time. A diode should not have any reverse recovery time and though it is possible to construct such a diode with no reverse recovery time it is usually not done due its high manufacturing cost.

The reverse recovery time can be seen from the above graphs. It is composed of two segments. The first segment lasts for time t_a, being measured from the instant of forward current zero to instant when the reverse current reaches its peak, I_RR. During this time the charge stored in the depletion region in removed. The second segment is time t_b, measured from the instant of peak reverse current to the instant when the reverse current decays to 25% of its peak value. During this time the charge stored in the semiconductor layers is removed. The recovery time is very important while designing the power electronic circuits because, the forward voltage must only applied only after the stored charge in the diode is completely removed.

Softness factor

The softness factor is also called as S-factor is a measure of voltage transients during reverse recovery period. It is the ratio of t_b/t_a. If the softness factor is equal to 1 then such a diode is called soft recovery diode and it indicates low oscillatory response. Whereas, the diodes having low value of S-factor will have high voltage oscillations. Such diodes with low S-factor are called as fast recovery or abrupt recovery diodes. The above graphs shows the characteristics of diodes with both soft recovery and abrupt recovery. As can be seen from the graph the abrupt recovery diode has oscillations at the end.

Derivation of reverse recovery time and Reverse recovery current

From the graph it can be observed that,

t_rr = t_a+t_b

I_RR = t_a(di/dt)                               ----(1)

The Charge stored in the diode due to minority charge carriers is given by,

Q_RR = [1/2(I_RR/t_a)] + [1/2(I_RR/t_b)] = 1/2(I_RR/t_rr)

⇒ I_RR = 2QR/t_rr                           ----(2)

From equations (1) and (2)

2Q_RR/t_rr = t_a(di/dt)

⇒t_at_rr = (2Q_RR)/(di/dt)                  ----(3)

If t_b is negligible with respect to t_a, then t_rr ≅ t_a,

From (3) we get reverse recovery time as,

⇒t_rr² = (2Q_RR)/(di/dt)

∴ t_rr = [ (2Q_RR)/(di/dt) ]^1/2           ----(4)

From (2) and (4) we get the reverse recovery current as,

I_RR = [(2Q_RR)(di/dt)]^1/2

Thus it can be inferred from the above equations that the peak reverse recovery current and reverse recovery time depend on the charge stored in the diode Q_RR and rate of change of current. The charge Q_RR is in-turn dependent on the diode forward current. Hence the reverse recovery current depends on the diode forward current.

Also Read

V - I characteristics of power diode
Structure of power diode
Types of power diodes