Smaller 25W to 400W power supply designs with synchronous forward controller

May 08, 2014 // By Graham Prophet
LT3753 is a primary side, current-mode PWM controller for use in a synchronous forward converter with active clamp reset. The LT3753 operates over an input voltage range of 8.5V to 100V, delivers up to 95% efficiency and is targeted for power levels up to 400W.

A programmable volt-second clamp provides a safeguard for transformer reset that prevents saturation and protects the MOSFET. This function allows for transformer and MOSFET optimisation, resulting in a reduced solution size.

The device output voltage regulates to ±5% without the use of an optocoupler. When an optocoupler is used, ±1.5% regulation can be achieved. The LT3753 sends a control signal via a pulse transformer to a secondary-side MOSFET driver for the synchronous rectification timing. It can also be used in self-driven applications (when operating over a narrow input voltage range) where the secondary-side MOSFETs are controlled by the power transformer pulses. The LT3753 can be used in nonsynchronous designs, typically required for higher output voltage applications.

When the volt-second clamp is set above the natural duty cycle of the converter, it provides a duty cycle guardrail to limit primary switch reset voltage and prevent transformer saturation during load transients. The volt-second clamp also limits maximum V OUT if the opto path breaks open or, alternatively, defines V OUT in no-opto applications. For non-isolated high step-down ratio applications, the LT3753 contains a voltage error amplifier, allowing a very simple non-isolated fully regulated synchronous forward converter.

Additional features include programmable overcurrent protection, adjustable input undervoltage and overvoltage lockout along with built-in thermal shutdown. The LT3753 has a programmable 100 kHz to 500 kHz operating switching frequency and can be synchronised to an external clock, allowing the use of a wide range of output inductor values and transformer sizes. LT3753 is available in a TSSOP-38 package with several pins removed for high voltage spacing, in a selection of temperature ratings. Pricing is from $3.19 (1000),

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