Reverse Recovery

The figure shows what happens to the diode in the boost converter presented in Figure 1. When the diode turns on, there's an overshoot due to the higher resistance of the n­ region, relative to the resistance of the n+ region. In the steady state, minority carriers reduce the resistance of the n­ region. But during high-speed turn-on, current increases more quickly than minority carriers can diffuse through the junction.

Such overshoot is significant. But power-supply designers are most concerned about the period of reverse recovery when the diode is trying to turn off. Reverse conduction continues until the stored charge is removed. This is the t_A portion of reverse recovery. When the excess carriers are gone, the junction is reverse-biased. This is the t_B portion of t_RR. The diode voltage now decreases at a rate proportional to dI_F/dt. If that slope is large, the peak reverse voltage also is high and the output voltage may ring, or the diode may fail due to excess voltage.