Synchronous Forward Controller Shrinks 25- To 400-W Power-Supply Footprints

A programmable volt-second clamp built into Linear Technology’s LT3753 primary-side, current-mode, pulse-width-modulation (PWM) controller allows for transformer and MOSFET optimization, resulting in reduced solution size. When the clamp is set above the converter’s natural duty cycle, it provides a duty-cycle guardrail to limit primary switch reset voltage and prevent transformer saturation during load transients. It also limits maximum V_OUT if the optocoupler path breaks open, or it will define V_OUT in applications without an optocoupler. Output voltage regulates to ±5% without the use of an optocoupler. With an opto, it’s possible to realize ±1.5% regulation. The LT3753 operates over an input-voltage range of 8.5 to 100 V, delivers up to 95% efficiency, and targets power levels up to 400 W. The device sends a control signal via a pulse transformer to a secondary-side MOSFET driver for the synchronous rectification timing. It integrates a voltage error amplifier for non-isolated high step-down ratio applications. Programmable operating switching frequency ranges from 100 to 500 kHz. Synchronization to an external clock enables use of a wide range of output inductor values and transformer sizes. All versions of the LT3753 come in TSSOP-38 packaging with several pins removed for high-voltage spacing.

LINEAR TECHNOLOGY CORP.