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Hi everyone. I really have high hopes that someone will have some time to review this topic and came back with some fresh ideas. I build a LM5118 based regulator which works just perfect delivering variable voltage and current up to 54V and 3A (and beyond but I don't need more current for this project). Here is schematic:
Input is 48VAC rectified (~67VDC), Vccx is delivering from aux bias using 10V zener to lowering it to ~7.5V (trying to lowering LM5118 dissipation since I have additional bias supply powered from the same rectified input, namely DC_OUT). R1, P1, R2 resistor network is used as a mean for seting output voltage. So far, so good. And now a nightmare which I still cannot understand: the following "tracking" circuit I planned to use as a mean to control PREG_OUT from external source (part of post-regulator):
The PREG_FB signal is trimmed to deliver 1.23V when post-regulator output voltage is ~1.5V lower then LM5118's PREG_OUT. Unfortunately this is not working and cause LM5118 burnout. If before mentioned aux bias is used (JP1 shorted) that even destroy ZD1! I don't even need to test with oscilloscope that LM5118 is dead (i.e. testing SYNC, HO and LO pins) - it can be simply tested with multimeter because LO pin is in that case "shortcircuited" to the ground.
My question is: what can be a cause of such disaster, especially how is possible that ZD1 connected to the Vccx is destroyed? Max. absolute rating for FB pin is 7V but presented circuit didn't delivered (I even tried to protect FB input with 5V1 Zener but with no success).
Many, many thanks in advance for any type of feedback!
I also made a TINA model of tracking control mentioned above if that can helps. Please find it in attachment: Tracking control.tsc