•  
• EDN Europe MAGAZINE
•  
• Free SUBSCRIPTION
• Subscription Renewal
• View Media Kit

• EDN
• EDN Australia
• EDN Asia
• EDN China
• EDN Japan

Synchronous rectifier controller

Zetex has introduced a rectifier controller to drive MOSFETs acting as diodes—synchronous rectifiers, replacing Schottky diodes—in flyback converters in the range 50 to 150W. In a 60W notebook adapter, reduction in losses is typically 20%. The ZPDD incorporates a high-voltage differential amplifier stage and a high-current driver in an SM8 package. It monitors MOSFET drain-source reverse voltage to detect conduction in the body diode, at which point it produces a positive gate voltage to completely turn the MOSFET on. Being proportional to the MOSFET drain-source reverse voltage, the drive voltage to the gate progressively reduces, thereby ensuring rapid MOSFET turnoff at the zero-voltage switching point: hence the description of the device as a ZPDD (zero-point-detector driver), accurately sensing the point at which secondary current reaches zero. This ensures that reverse conduction does not occur. The driver needs only three external passive components and produces source and sink currents of up to 2.5A. As the chip needs no timing information from the primary side, and there are no timing components on the secondary side, the ZXGD3101 is simple to implement. A future device in the series will support OR-ing of power supplies for redundant connection. For evaluation in existing adaptor designs, the designer can replace a Schottky with a MOSFET, and plug onto it a small board that carries the rectifier controller and an independent battery power supply. The chip is priced at $1.05 (1000).

Zetex, www.zetex.com