Design, performance, and application of new bipolar synchronous rectifier

A bipolar transistor has been developed which is aimed at synchronous rectifier applications. Because of its low on-resistance of approximately 7 milliohms, this device achieves improved efficiency over a conventional Schottky rectifier at currents up to approximately 40A. The emitter-base junction is purposely designed to have a breakdown voltage in excess of 50V rather than the usual 6 to 12V of a conventional transistor. Thus both first and third quadrant blocking capability is achieved. This behavior is not available with a power MOSFET and enables this transistor to be used in a PWM mode where both regulation and rectification can be achieved in the power supply secondary. Circuit problems attributable to the storage time of the bipolar are alleviated by using a novel base drive circuit which gives only a minor increase in circuit complexity and allows operation at switching frequencies up to 250 kHz.