CODING INTERNATIONAL CO LTD

The LT8309 from Linear Technology Corporation is a flyback secondary-side synchronous rectifier driver that replaces the output diode with a MOSFET, allowing up to 10 A output current without the use of a heat sink. The maximum output current of a flyback power supply has been limited by the power loss and heat generated from the output diode when no heat sink is used. The LT8309 replaces the diode forward voltage drop with the RDS(ON) of an external N-channel MOSFET, thereby reducing the power loss, increasing converter efficiency and easing the thermal design.

Most prior solutions use a signal transformer 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 a transformer, but allows the LT8309 to work with Linear Technology’s growing line of no-opto-coupler products required for output voltage feedback boundary conduction mode (BCM)/critical conduction mode (CrCM) flyback converters, making it highly versatile for many types of applications.

The LT8309 operates from a 4.5 V to 40 V input voltage range. Its 500µA quiescent current maximizes light load efficiency. The strong 1O pull-down gate driver provides fast device turn-off and its low minimum on- and off-times improve noise immunity. Additional features include a mere 22 ns propagation delay, adjustable and accurate trip voltage along with the ability to drive 150 V rated MOSFETs.