Synchronous rectification

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The synchronous rectification is a technique of improving efficiency of power converters in power electronics. It consists in connecting a diode and a transistor (usually a power MOSFET) in parallel. When the diode is forward-biased, the transistor is closed, to reduce the voltage drop. When the diode is reverse-biased, the transistor is open.

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Voltage drop across a diode and a MOSFET. At low current levels, the resistive behaviour of a MOSFET generates less losses than the diode, which exhibits a voltage drop even at very low current levels. Paralelling two MOSFET (orange curve) reduces the losses further, whereas paralelling several diodes won't modify their threshold voltage.

In low voltage converters (10 volts and less), the voltage drop of a diode (typically 0.6 volts for a silicon diode) has a very negative effect on efficiency. One classic solution consist in using schottky diodes, which exhibit very low voltage drops (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 a solution.

In an 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 virtually zero. Furthermore, MOSFETs transistors have an intrinsic diode between their source and drain terminals. This makes these transistor useful for synchronous rectification: They can directly replace the diodes in converters. When they are turned off, they behave as a diode, and when they are turned on, they behave as a resistance, yielding lower losses.