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“The LTC4417 gate driver pulls down on G1, G2 and G3 with a strong P-channel source follower and a 2µA current source. When the clamp voltage is reached, the P-channel source follower is back biased, leaving the 2µA current source to hold G1, G2 and G3 at the clamp voltage. To minimize inrush current at start-up, the gate driver soft-starts the first input supply to connect VOUT, at a rate of around 5V/ms terminating when any channel disconnects or 32ms has elapsed. Once slew rate control has terminated, the gate driver quickly turns on and off external back-to-back P-channel MOSFETs as needed. A SHDN low to high transition or VOUT drooping below 0.7V reactivates soft-start.”
Root Cause:
The root issue is that the chip is not soft-starting again when the input is switched on. Discharging the capacitors triggers the soft-start again. Our supplies have large series resistance due to the wires attached to them and thus the voltage drop caused by the inrush current (even for capacitors that are not fully charged) triggers an Under Voltage Fault.
Possible Solutions:
So yes - we need to drain the capacitors to below 0.7V in order to reactivate soft-start. One solution would be to just turn on a load to drain them quickly (but the controller board will only be able to activate the load for so long, but a better solution would be a discharge resistor on the board. We will need to make sure that the caps can be fully discharged within a given amount of time in order to properly activate the soft start.
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