Answer 6—In a discontinuous-mode switching regulator, the coil and diode current drops to zero for some time before the next switching cycle begins. The diode must block current flowing from the output filter capacitor back through the coil. Therefore, low reverse leakage current is a key requirement for high efficiency.

In a continuous-mode switching regulator, the coil and diode are still conducting current when the main switch turns on for the next cycle, and the diode is instantly reverse-biased. Fast reverse recovery time becomes a key requirement for high efficiency.

Reverse recovery time is not as important in a discontinuous-mode regulator because the coil helps prevent reverse current from building up before the diode can fully switch off. Similarly, low leakage is not as important in the continuous-mode regulator because the diode is reverse-biased for a smaller fraction of the overall cycle.

Contributor: David Tweed

Published March 2004