Several regulatory bodies around the world address efficiency issues in External Power Supply (EPS) and drive to reduce energy consumption in the compliant adaptors. One example of these is the Energy Star® V2.0 which comes into force in November 2008. The standard targets both the issues of active mode efficiency as well as no load power consumption as shown in Tables 1 and 2. This application note addresses the challenge of complying with the voluntary standard in a flyback converter typically employed as EPS below 100W output power. In particular, the paper discusses the efficiency improvement with the aid of the ZXGD3101 synchronous MOSFET controller and studies its practical limit against normal rectification method. Finally, experimental verification is presented to demonstrate efficiency improvement in a universal input 60W (19V 3.2A) flyback adapter.
When a normal, ultrafast diode rectifier is substituted with a low resistance synchronous MOSFET it is possible to achieve the standards set out by Energy Star® V2.0. Using the ZXGD3101 to control and drive the synchronous MOSFET provides efficient conduction path with optimized delay in sensing the sharp drop of current during the turn-off phase ensuring no reverse conduction. The gate driving scheme combined with properly designed magnetic and power stage minimizes circuit losses while the power supply is in active mode and ensures the no-load conditions are satisfied.