Synchronous Rectifiers

Jan. 11, 2007

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).

Click here to download the PDF version of this entire article.

About the Author

Sam Davis 2

Sam Davis was the editor-in-chief of Power Electronics Technology magazine and website that is now part of Electronic Design. He has 18 years experience in electronic engineering design and management, six years in public relations and 25 years as a trade press editor. He holds a BSEE from Case-Western Reserve University, and did graduate work at the same school and UCLA. Sam was the editor for PCIM, the predecessor to Power Electronics Technology, from 1984 to 2004. His engineering experience includes circuit and system design for Litton Systems, Bunker-Ramo, Rocketdyne, and Clevite Corporation. Design tasks included analog circuits, display systems, power supplies, underwater ordnance systems, and test systems. He also served as a program manager for a Litton Systems Navy program.

Sam is the author of Computer Data Displays, a book published by Prentice-Hall in the U.S. and Japan in 1969. He also authored the book Managing Electric Vehicle Power. He is also a recipient of the Jesse Neal Award for trade press editorial excellence, and has one patent for naval ship construction that simplifies electronic system integration.

You can also check out additional articles on his other author page.