Reverse-Current Phenomenon in Synchronous Rectifiers

Focus:

In power converters that use synchronous rectification, a reverse current can occur in the synchronous rectifiers under various conditions because a MOSFET, unlike a diode, cannot block a reverse current. As this article explains, a reverse current can occur during turn off of the power supply with no-load or light-load condition, pre-bias turn on, shutting down of the power supply during an overvoltage protection (OVP) or overcurrent (OCP) condition or during dynamic load change from full load to no load or light load and during sudden load removal. Whenever synchronous rectification is used, the reverse current must be controlled carefully, or the reverse current can cause an abnormal turn on/turn off condition and may damage the MOSFET. This article explains the causes of reverse currents in synchronous rectifiers and describes techniques for preventing reverse currents in nonisolated and isolated dc-dc converters. These techniques involve use of a comparator to sense output voltage, a soft start method, sensing of the inductor current, or monitoring the pulse width of the rectifier MOSFET-and using this information to control turn on or turn off of the synchronous MOSFET.

What you’ll learn:

• How to understand causes of reverse currents in synchronous rectifiers
• How to prevent reverse currents in dc-dc converters using synchronous rectification

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Author & Publication:

Suresh Kariyadan, International Rectifier, El Segundo, Calif., How2Power Today, Feb 19 2010

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