DC607A: 12V @ 35A, 36 ~ 72V in, Buck

Demonstration circuit 607A is an isolated full bridge converter featuring the LTC3722, LTC4440, LTC3901, and LT1431. This circuit was designed to demonstrate the phase-shifted full bridge power supply to generate 12V at 35A from a typical telecom input voltage range of 36V to 72V. Isolation voltage is 1500VDC. Fixed zero-voltage transition timing was chosen because of the relatively fast bridge leg transition times with a 48V input. The DC607A PCB layout includes options to implement adaptive timing.

The LTC3722 synchronous dual mode phase modulated full bridge controller is used on the primary and works together with the LTC3901 secondary side synchronous driver to provide a synchronous rectified output. When an input voltage is applied, the LTC3722 begins a controlled soft-start of the output voltage. As this voltage begins to rise, the LT1431 programmable reference is quickly powered up via the output voltage. The LT1431 provides feedback via opto-coupler ISO1 to set the output voltage at 12V. The LTC4440 high voltage gate driver is used to level shift the high side primary MOSFETs gate signals. The LTC3722 provides precise control of gate signals to primary MOSFETs and secondary MOSFETs via T3 and U1 (LTC3901). The LTC3901 includes a timer and current sense to limit reverse inductor current.

For large values of input inductance, a 100V, 47μF electrolytic capacitor can be added across the input terminals to damp the input filter and provide adequate stability. See Linear Technology Application Note 19 for a discussion on input filter stability analysis. A recommended part is the Sanyo 100MV47AX.