A smart synchronous rectifier for 12 V automobile alternators

A new gate control unit for power MOSFETs is presented in this paper, which is especially suited for synchronous rectifiers in 12 V automobile vehicle-nets to replace inefficient silicon diodes. Unlike known control units for such synchronous rectifiers based on comparator circuits or external timing control, the proposed circuitry is inexpensive and insensitive, as it consists of only one operational amplifier and some passive components per MOSFET. Measurements taken on a prototype synchronous rectifier connected to a standard passenger car´s alternator prove a major increase of the rectification efficiency as well as a higher output current of the alternator compared to a standard diode rectifier. This synchronous rectifier operates at load currents up to 120 A and up to a maximum alternator speed of 15000 rpm. The entire gate drive circuit is described, providing information about the desired properties of its components. The switching processes of the presented circuit as well as the parameters that determine its operational performance are explained in detail.