- Reverse Recovery Time Definition: Reverse recovery time is defined as the period during which a diode continues to conduct in reverse after the bias is switched from forward to reverse.
- Understanding Reverse Current: During the reverse recovery time, a large reverse current flows through the diode, which eventually decreases to a stable reverse saturation current.
- Softness Factor Explained: The softness factor, a crucial metric in diode performance, compares the time the current takes to reach its peak to the time it decays, affecting the diode’s efficiency.
- Reverse Recovery Characteristics of Power Diode: The characteristics such as doping level, silicon geometry, and junction temperature directly influence the diode’s reverse recovery time.
- Design Implications: Designing power supplies must consider the reverse recovery time to optimize diode performance and minimize power losses.
A forward-biased diode conducts with its depletion region shrunk to almost nothing: the external supply voltage overcomes the barrier potential that immobile ions in that region would otherwise impose. Reversing the polarity at the terminals of the diode should, ideally, switch it from ON to OFF immediately, so that a diode conducting current in its forward direction stops conducting instantly.
In practice, the charge carriers stored near the junction during forward conduction do not vanish when the bias reverses. They keep a finite reverse current flowing for a duration known as the reverse recovery time of the diode.
During this interval a fairly large current flows through the diode in the opposite direction (Irr in Figure 1). Its magnitude falls away and settles at the reverse saturation current once the interval closes at trr. Graphically, the reverse recovery time of the diode spans the whole time from the instant the reverse current begins to flow through the diode to the instant it decays (td) back to zero, or to another predefined low level such as 25% of Irr in the figure, having reached its negative maximum at tp.
The ratio of these two intervals, td to tp, is known as the softness factor. In a normal diode the decay time td is smaller than the time tp taken to reach the negative peak. A soft-recovery diode shows the reverse behaviour: td exceeds tp. The softness factor measures the semiconductor losses incurred during switching: the larger the ratio, the greater the switching loss, because a soft recovery keeps reverse current flowing for longer while the diode already supports reverse voltage. Soft-recovery diodes therefore dissipate more switching energy than abruptly recovering types, although their gentler current decay produces smaller voltage transients and less electromagnetic interference.
Reverse recovery is basically a parasitic effect in diodes. Its value depends on the silicon doping level and geometry, on the junction temperature and on how quickly the forward current falls; the forward current just before the reverse bias is applied also affects it. The larger the reverse recovery time, the slower the diode, and vice versa. For that reason designers prefer diodes with a short reverse recovery time wherever high switching speed matters. Because a sizeable current flows back towards the supply during this interval, the reverse recovery time of the diode is a factor to weigh in every power-supply design.
