March 4, 2014 – Linear Technology Korea (CEO Hong Sa-gwak) announced the launch of a flyback secondary-side synchronous rectifier driver (product name: LT8309) that replaces the output diode with a MOSFET. This product can deliver up to 10A output current without requiring a heat sink. The maximum output current of a flyback power supply is limited by the power dissipation and heat generated in the output diode when a heat sink is not used.

The LT8309 replaces the diode forward voltage drop with the RDS(ON) of an external N-channel MOSFET. This reduces power losses, increases converter efficiency, and simplifies thermal design. Most previous solutions have used signal transformers driven from the primary side to control the synchronous rectifier. However, the LT8309 senses the drain-to-source voltage of the MOSFET for activation. This technique not only eliminates the transformer but also enables the LT8309 to operate with Linear Technology's optocoupler-free product family required for output voltage feedback BCM (boundary conduction mode)/CrCM (critical conduction mode) flyback converters. These characteristics make it functionally well-suited for various types of applications.

The LT8309 operates within an input voltage range of 4.5V to 40V. Its 500µA quiescent current maximizes light-load efficiency. A strong 1Ω pull-down gate driver provides fast device turn-off, and the minimum on/off time of this product improves noise immunity. Additional features include propagation delay of only 22ns, adjustable and precise trip voltage capability, and the capability to drive 150V-rated MOSFETs.

Photo caption: Flyback synchronous rectifier driver