We've build 3 copies of a SMPS to output 28 VDC up to 1 Amp. The PS has 2 possible DC sources 36V or 45V that are switched between depending on conditions. It is based on the design from SwitcherPro with a TPS40200 chip. The first copy functioned initial, but demonstrated an instability when switched from the 36 to the 45 V source. This was solved by changing the type on capacitor on the voltage input to the TPS. This unit after some more rework to change other parameters of the PS locked the FET on and does not switch. This was no real surprise because of all the rework it is possible we overheated something. A second copy was built incorporating the desired changes to the design and has been running continuously for the week and shows no issues when switched between the 2 source. A third copy has now been assembled in the same configuration as the second, but will not switch the FET so it won't start.
I am looking for some guidance on troubleshooting and on what to look for beyond the obvious replacement of possibly damaged components when the chip does not switch.
Thanks,
KS