Synchronous rectification

The synchronous rectification is a technique for improving efficiency of power converters in power electronics. It consists of connecting a diode and a transistor (usually a power MOSFET) in parallel. When the diode is forward-biased, the transistor is turned on, to reduce the voltage drop. When the diode is reverse-biased, the transistor is turned off, so no charge can flow through the circuit. This way, a rectifying characteristic is obtained, without the forward voltage drop associated with diodes in the on-state.

Applications

In low voltage converters (around 10 volts and less), the voltage drop of a diode (typically around 1 volt for a silicon diode at its rated current) has a very negative effect on efficiency. One classic solution consists of using Schottky diodes, which exhibit very low voltage drops (as low as 0.3 volts). However, when addressing very low-voltage converters, such as the buck converters that deliver power to the CPU of a computer (voltage is around 1 volt), this is no longer an adequate solution for good efficiency.

On the other hand, the transistors used in these very low-voltage converters are usually MOSFETs. These transistors behave like a resistor, so providing their resistance is low enough (for example by paralleling several devices), their voltage drop can be very low. Furthermore, MOSFETs have an intrinsic diode between their source and drain terminals. This makes these transistors useful for synchronous rectification: They can directly replace the freewheeling diode in converters. They behave inherently like a diode, and when they are turned on (via a control circuit), they behave as a low value resistance, yielding lower losses.