It is a transient condition. We obtain this characteristic when the diode enters the reverse biased state (i.e. the off-state) from the forward Biased state (i.e. the on-state). It is exactly the reverse process of Turn ON Transient.
- Reverse Recovery Characteristics of Diode is actually the Turn-off transient portion of the Switching Characteristics.
- Here, mainly the excess charges stored due to the Double Injection in the Drift Region are removed first so the depletion region could be formed and the diode can be reversed biased (i.e. Off-state). This is the important process of the turn-off transient which is known as the Reverse Recovery.
Steps of the Reverse Recovery Process
- First, the stored excess/minority charge in the drift region is removed so that the junction could be Reverse Biased. Second, the sweeping and recombination lead to the expansion of the depletion region.
- During Forward Biased/ON state, the drift region contains excess/minority charge carriers due to double injection. Until this stored excess/minority charge carriers decay, the forward current flows with a decaying nature during time period ‘t3’ as shown in the figure.
- After the time period t3, the reverse current begins to flow.
- As the stored excess carriers from the Drift Region decreases, ohmic resistance ‘R’ of Drift Region increases which causes VON to decrease quickly after t3.
- Reverse current increases to its maximum when all charges in the depletion region flow out represented by Irr as shown in the figure. After maximum reverse current at the end of t4, when very few excess charge carriers are left then reverse current decreases and the depletion region increases, Voltage across the diode attains negative value when depletion region expands further (flow of Reverse current in the negative direction causes the negative voltage drop).
The Terminology used in Reverse Recovery Characteristics
- t4 : Time is taken by the reverse recovery current to become maximum Irr.
- t5 : Time is taken by the reverse recovery current to decreases to 25% of Irr from the maximum value Irr.
- S = t5 / t4, where S = snappiness or softness factor
- Reverse Recovery Time trr : Total time taken by the reverse recovery current to reach 25% of the maximum value i.e. Irr. It is denoted by trr. From the above figure,
- Reverse Recovery Charge QRR : Amount of charge carriers that flow across the diode during the flow of reverse current. The area enclosed by Reverse Recovery current (from the figure)
Note: Here we have assumed that the graph is triangular to calculate the area.
Derivation for the reverse recovery time (trr) and reverse recovery current (Irr)
From the above waveform;
From equation (i) we get,
If snappiness ‘S’ is neglected i.e. S = 0
Which implies t5 << t4 (from S = t5 / t4) which then leads to trr = t4.
Substituting the value of t4 = trr in equation (iii) gives
From equation (ii) and (iv), we get
- Trr and Irr both depend on Qrr.
- And Qrr is dependent on IF therefore everything depends on max forward current.
Importance of Reverse Recovery Characteristics
- When a diode takes a transition from forwarding bias to reverse bias condition then due to the delay caused to the flow of reverse recovery current the blocking time increases. Therefore, it affects the switching action in the diode which reduces switching speed.
- It affects the rectification action of rectifiers.