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LT3753 9V-15V Input to 54V/3A Isolated Forward Converter

Randyco Prasetyo - Applications Engineer
Gabino Alonso - Strategic Marketing Engineer
Feb 26th 2015

The LT®3753 is a primary side, current-mode PWM controller optimized for an active clamp forward converter topology. It features a programmable volt-second clamp that provides a duty cycle guardrail to limit primary switch reset voltage and prevent transformer saturation during load transients. The LT3753 also features a programmable turn-on current spike blanking (adaptive leading edge blanking plus programmable extended blanking) of the main MOSFET that greatly improves the converter’s noise immunity. Other features include programmable overcurrent protection, adjustable input undervoltage and overvoltage lockout along with built-in thermal shutdown.

LT3753 162W Isolated Forward ConverterLT3753 162W Isolated Forward Converter

This application is one example of the use of LT3753 in a non-synchronous closed loop design. The full load efficiency is higher than 93% across the input voltage range and it is possible to run the circuit without forced air cooling.

LT3753 162W Isolated Forward Converter EfficiencyLT3753 162W Isolated Forward Converter Efficiency

 

Thermal Image at 12Vin to 54V @ 3A Without Forced Air (Bottom Side)LT3753 Thermal Image at 12Vin to 54V @ 3A Without Forced Air (Bottom Side)

 

Thermal Image at 12Vin to 54V @ 3A Without Forced Air (Top Side)LT3753 Thermal Image at 12Vin to 54V @ 3A Without Forced Air (Top Side)

 

The LT3753 is a current mode PWM controller optimized for an active clamp forward converter topology, allowing up to 100V input operation. A programmable volt-second clamp allows primary switch duty cycles above 50% for high switch, transformer and rectifier utilization. Active clamp control reduces switch voltage stress and increases efficiency. A synchronous output is available for controlling secondary side synchronous rectification.

The LT1431 is an adjustable shunt voltage regulator with 100mA sink capability, 0.4% initial reference voltage tolerance and 0.3% typical temperature stability. On-chip divider resistors allow the LT1431 to be configured as a 5V shunt regulator, with 1% initial voltage tolerance and requiring no additional external components. By adding two external resistors, the output voltage may be set to any value between 2.5V and 36V. The nominal internal current limit of 100mA may be decreased by including one external resistor.