Generally speaking, two types of rectifying schemes may be adopted in a secondary side of an isolated switching converter: (1) non-synchronous rectifying with a diode (FIG. 1A), and (2) synchronous rectifying with a synchronous rectifier, e.g., an N-MOSFET (FIG. 1B). The power dissipation-current characteristic is plotted in FIG. 2, for a diode (curve 12) and a synchronous rectifier (curve 11). In practical applications, the work area of a low power isolated switching converter always falls into the shadowed area. In the shadowed area, curve 11 is always above curve 12, i.e., the power dissipation of a diode is higher than the power dissipation of a synchronous rectifier. So, compared with a diode, a synchronous rectifier is more preferable because of less power waste and better efficiency in the low power isolated switching converter. Moreover, the temperature characteristic of the synchronous rectifier is better than that of the diode because of less power dissipation.
Synchronous rectifiers have thus found increasingly wide applications in devices sensitive to power efficiency, such as laptop adapters, wireless equipment, LCD power management modules, power over Ethernet, and so on.
In the synchronous rectified switching converter, a voltage across the synchronous rectifier may be adopted to determine the on and off of the synchronous rectifier. However, the ringing of the voltage across the synchronous rectifier after the synchronous rectifier is turned off may cause mis-trigger.