Synchronous Rectifiers

What is a synchronous rectifier?

A synchronous rectifier is a circuit that emulates a diode, allowing current to pass in one direction but not the other without the losses associated with junction or Schottky devices. The circuit comprises a pass-element (most often a power MOSFET), a sense element, a sense-signal conditioner, and a driver.

How does a synchronous rectifier work?

There are two broad techniques to implement the synchronous-rectifier function. The first derives a sync signal from the primary controller. But this method often requires the control signal to cross a galvanic barrier, which can be costly, consume a large board area, and limit the converter’s upper frequency limit.

The second method derives the control signal independently by sensing the electrical conditions at the pass element. In the off state, the sense element detects the polarity of the voltage applied to the pass element. The signal conditioner amplifies the sense signal and determines the polarity of the sensed voltage. When the applied voltage is of the correct polarity for forward conduction, the signal conditioner turns on the driver, which provides the necessary signal to operate the pass element.

In the on state, the sense element detects the polarity of the current through the pass element. The signal conditioner amplifies the sense signal, as it did in the off state. But here it detects the current zero crossing, shutting off the driver and, as a result, the pass element as close to the zero crossing as possible.

How does the MOSFET-based synchronous rectifier compare with the Schottky diode output rectifier in a switch-mode power supply?

The synchronous rectifier’s conduction losses go as I²R_DS(ON). Typical R_DS(ON).s are a few tens of milliohms or less. The Schottky diode’s conduction losses are linear in current, V_FI, resulting in typical dissipation rates in the range of 0.7 to 1 W/A depending on the Schottky’s operating temperature. A plot of a typical 18-mW MOSFET and a Schottky diode with V_F = 700 mV shows the significant reduction in power dissipation the synchronous rectifier offers (see the figure).

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