Forward controller simplifies high-voltage control in vehicles

March 20, 2014 // By Christoph Hammerschmidt
Reducing both size and complexity of the power supply circuitry, the LT3752 synchronous forward controller from Linear Technology is ideal for automotive HV / HEV applications. An optocoupler is not required.

The device 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 complexity and size of the main power transformer by avoiding the need for extra windings to create bias supplies. The LT3752 operates over a broad input voltage range from 6.5V to 100V. For up to 400V inputs and greater, Linear provides the LT3752-1 which allows RC start-up from the input voltage with the maximum voltage limited only by the choice of external components.

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 realized. The LT3752/-1 sends a control signal via a pulse transformer to a secondary-side MOSFET driver for the synchronous rectification timing. In specific cases - in particular when operating over a narrow input voltage range - it can also be used in self-driven applications 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 VOUT 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 VOUT if the opto path breaks open or, alternatively, defines VOUT 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 100kHz to 500kHz operating switching frequency and can also