AN6-206

www.fairchildsemi.com AN-6206 Primary-Side Synchronous Rectifier (SR) Trigger Solution for Dual-Forward Converter Introduction In any switching converter, rectifier diodes are used to obtain DC output voltage. The conduction loss of diode rectifier contributes significantly to the overall power losses in a power supply; especially in low output voltage applications, such as personal computer (PC) power supplies. The conduction loss of a rectifier is proportional to the product of its forward-voltage drop and the forward conduction current. Using synchronous rectification (SR) where the rectifier diode is replaced by MOSFET with proper on resistance (RdsON), the forward-voltage drop of a synchronous rectifier can be lower than that of a diode rectifier and, consequently, the rectifier conduction loss can be reduced. The highly integrated FAN6210 is a primary-side SR controller for dual-forward converter that provides control signals for the secondary-side SR driver FAN6206. FAN6210 also provides drive signal for the primary-side power switches by using an output signal from the PWM controller. FAN6210 can be combined with any PWM controller that can drive a dual-forward converter. To obtain optimal timing for the SR drive signals, transformer winding voltage is also monitored. To improve light-load efficiency, green-mode operation is employed, which disables the SR turn-on trigger signal, minimizing gate drive power consumption at light-load condition. This application note d...