Reverse Recovery Transient
In most switching applications a diode is switched from forward conduction to a reverse-biased state, and vice versa. The resulting stored charge transient
is somewhat more complicated than for a simple turn-off transient, and therefore it requires slightly more analysis. An important result of this example is that a reverse current much larger than the normal reverse saturation current can flow in a junction during the time required for readjustment of the stored charge.
Reverse Recovery Transient:
- Let us assume a p - n junction is driven by a square wave generator that periodically switches from E to - E volts (Fig. a).
- While E is positive the diode is forward biased, and in steady state the current If flows through the junction.
- If E is much larger than the small forward voltage of the junction, the source voltage appears almost entirely across the resistor, and the current is approximately i = If = E/R.
- After the generator voltage is reversed (t > 0), the current must initially reverse to i = Ir = -E/R.
- The reason for this unusually large reverse current through the diode is that the stored charge (and hence the junction voltage) cannot be changed instantaneously.
- Therefore, just as the current is reversed, the junction voltage remains at the small forward-bias value it had before r = 0. A voltage loop equation then tells us that the large reverse current - E/R must flow temporarily.
- While the current is negative through the junction, the slope of the ∂p(xn) distribution must be positive at xn = 0.
- As the stored charge is depleted from the neighborhood of the junction (Fig. b), we can find the junction voltage again.
- As long as Δpn is positive, the junction voltage v(t) is positive and small; thus i = -E/R until Δpn goes to zero.
- When the stored charge is depleted and Δpn becomes negative, the junction exhibits a negative voltage.
- Since the reverse-bias voltage of a junction can be large, the source voltage begins to divide between R and the junction.
- As time proceeds, the magnitude of the reverse current becomes smaller as more of -E appears across the reverse biased junction, until finally the only current is the small reverse saturation current which is characteristic of the diode.
- The time tsd required for the stored charge (and therefore the junction voltage) to become zero is called the storage delay time.
- This delay time is an important figure of merit in evaluating diodes for switching applications.
The storage delay time is equal to