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Power tip: Discretes as alternative to integrated MOSFET drivers (Part 2)

Posted: 26 Jan 2012     Print Version  Bookmark and Share

Keywords:synchronous rectifiers  MOSFET  discrete drivers 

In the previous power tip article, we tackled an emitter follower used in MOSFET gate-drive circuits and saw that drive currents in the 2-A range are achievable with small SOT-23 transistors. In this power tip, we look at self-driven synchronous rectifiers and discuss when discrete drivers are needed to protect the synchronous rectifier gates from excessive voltages.

Ideally, you would drive the synchronous rectifiers directly from the power transformer. However, with wide input voltage variations, the transformer voltage can be high enough to damage the synchronous rectifiers.

Figure 1 shows a discrete driver used to control the conduction of Q2 in a synchronous flyback. This circuit gives you controlled turn-on gate current and protects the rectifier gate from high reverse voltage.

Figure 1: Q1 quickly turns off synchronous flyback FET Q2. (Click on image to enlarge.)

The circuit starts with a negative voltage on the outputs of the transformer. The 12-V output is more negative than the 5-V output, causing Q1 to conduct and short the voltage gate-to-source on the power FET Q2, turning it off quickly. Since base current is flowing through R2, there is a negative voltage on speed-up capacitor C1.

During this time, the primary FET is conducting and stores energy in the magnetizing inductance of the transformer. When the primary FET turns off, the transformer output voltage swings positive. The gate-to-source of Q2 is rapidly forward biased through D1 and R1, causing Q2 to conduct. The Q1 base-to-emitter junction is protected by D2 when C1 discharges.

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