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Synchronous rectifier IC boosts flyback, resonant half-bridge designs

Posted: 14 Mar 2006 ?? ?Print Version ?Bookmark and Share

Keywords:IR1167 SmartRectifier chipset? HEXFET MOSFET? International Rectifier? Vince Biancomano?

International Rectifier (IR) touts its SmartRectifier chipset, which comprises the IR1167 with external HEXFET MOSFETs, as the first high-efficiency solution that's viable from the standpoint of technology and cost. Use of a standard diode in the usual flyback design (i.e., secondary-side rectification) is simple and adequate for 60W and below power applications (IR cites the cutoff point in terms of current, at about 2.5A), but it's a low efficiency solution with clear drawbacks at powers out to 600W.

More specifically, system components will tend to be large, and power dissipation generally high. Replacing the diode with a MOSFET offers one way of cutting losses, but the designer needs to implement control circuitry to switch the MOSFET. Often, the search for a better solution migrates to a synchronous rectifier controller, which by its very nature leads in turn to the use of current-transformers to detect current polarity. In these circuits, though, there's generally significant overhead, and it cuts into overall system efficiency.

In contrast, the IR1167 relies on a proprietary voltage sensing circuit, high-voltage IC technology and high-output gate drive capability to address the high-efficiency issue, as well as time-to-market. IR, citing maximium efficiency for its chip under all operating conditions and compliance with CEC 80-Plus and 1W standby specifications, says the IR1167-based solution will provide the highest efficiency of any synchronous rectifier working in the flyback mode. The company also says the IR1167 is the first viable device suitable for use in resonant half-bridge topologies (which come into use at about the 200-300W level).

The device's HVIC design allows the designer to use any (external) MOSFET rated from 30-200V. The IR1167's high-speed gate driver provides high output (7A), ultimately contributing to 1 percent higher efficiency, which has a trickle-down effect that's significant in this class of circuits at higher powers.

Just as important, says IR, the chip cuts external components by up to 80 percent versus traditional current-transformer designs. A typical a 120W laptop adaptor requires just the IR1167, two external MOSFETs (the IRFB4110, IRF7853 and IRFB4227 are recommended), and a total of three resistors and capacitors in a heat sink free design, versus the 21 parts in a traditional current-transformer design. Further, PCB area will be cut in half and designers can expect a cut in system costs by more than 20 percent.

Design tools and support for the chip/chipset include the AN1097 application note, daughtercards (characterized as a fast "drop in" solution that allows designers to become acquainted with the IR1167 in an actual design) compatible with MOSFETs in TO-220 and SO-8 versions, and an educational demo board.

- Vince Biancomano

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