Controller shrinks 25-W to 400-W power-supply footprints

May 7, 2014. Linear Technology Corp. has announced the LT3753, a primary side, current-mode PWM controller optimized for use in a synchronous forward converter with active clamp reset. The LT3753 operates over an input voltage range of 8.5 V to 100 V, delivers up to 95% efficiency, and is targeted for power levels up to 400 W. 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 optimization, 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 realized. 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. Furthermore, 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 VOUT if the opto path breaks open or, alternatively, defines VOUT in no-opto applications. For nonisolated 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 synchronized to an external clock, allowing the use of a wide range of output inductor values and transformer sizes.

The LT3753 is available in a TSSOP-38 package with several pins removed for high voltage spacing. The LT3753 E- and I-grade versions operate from a -40°C to 125°C junction temperature. The LT3753 H-grade is guaranteed to operate from a -40°C to 150°C operating junction temperature. The LT3753 MP grade is guaranteed to operate from -55°C to 150°C operating junction temperature. The 1,000-piece price starts at $3.19 each.

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