Synchronous forward controller generates secondary bias

March 24, 2014 // By Graham Prophet
LT3752/LT3752-1 is a high input voltage capable synchronous forward controller with active clamp transformer reset. A controlled VOUT start-up and shut-down is maintained using an integrated housekeeping controller to bias both the primary and secondary IC’s.

The internal bias generation also reduces the complexity and size of the main power transformer by avoiding the need for extra windings to create bias supplies. The LT3752 operates over 6.5V to 100V input voltage range. For up to 400V inputs and greater, the LT3752-1 allows RC start-up from the input voltage with the maximum voltage limited only by the choice of external components, making it suitable for HV/HEV automotive applications.

An output voltage regulation of ±5% can be attained without the use of an optocoupler. When an optocoupler is used, ±1.5% regulation can be realised. The LT3752/-1 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. With the LT3752/-1, secondary side ICs no longer require start-up circuitry to operate when the output voltage is 0V, allowing a controlled V OUT startup.

The LT3752/-1 contains an accurate programmable volt-second clamp. When 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 ratios applications, each IC 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 LT3752/-1 has a programmable 100 kHz to 500 kHz operating switching frequency and can also be synchronised to an external clock allowing the use of a wide range of output inductor values and transformer sizes.

The LT3752/-1 is available in a TSSOP-38 package with several pins removed for high voltage spacing. The