Diode reverse recovery characteristics

Reverse recovery: when a conducting diode is switched to reverse bias, its current does not stop at zero. The minority carriers stored in the junction must first be removed, so the current falls through zero, flows in the reverse direction for a short time and then decays to zero.

  iD
   ^ IF
   |-------.
   |        \  slope = di/dt
   |         \
 --+----------\--------------------> t
   |           \      .--------
   |            \   .'
   |   - IRR ....\.'   tb
   |          ta  |<->|
   |         |<-->|
   |         |<---trr--->|

• $t_a$: time from current zero to the peak reverse current $I_{RR}$, while charge stored in the depletion region is removed.
• $t_b$: time for the reverse current to decay from $I_{RR}$ to about $0.25 I_{RR}$, while charge in the bulk is removed.
• Reverse recovery time $t_{rr} = t_a + t_b$; softness factor $S = t_b / t_a$.
• Peak reverse current: $I_{RR} = t_a \dfrac{di}{dt}$.
• Reverse recovery (stored) charge $Q_{RR}$ is the area under the reverse current, approximately a triangle:

$$Q_{RR} = \tfrac{1}{2} I_{RR}\, t_{rr}$$

Soft and abrupt recovery

Effects of reverse recovery

• Extra switching loss $\approx V_R Q_{RR} f$.
• Over-voltage spikes $L\,di/dt$ in stray inductances.
• In bridge circuits, the recovering diode draws a current spike through the incoming switch.

Hence power circuits use fast recovery or Schottky diodes, which have small $t_{rr}$ (a few µs down to ns).