Converter In-Rush Thermistors etc, UPDATE -4

KJINTF wrote:
Hi BLF,

I have had an oppertunity to repair as many as 20 to 30 converters over the last couple of years.

90% of the failures have been the input side 250Vdc 1,000ufd caps (typically there are at least three in parallel). Most switching converters ingest the 115VAC then with a full wave bridge rectifier get about 160VDC on the input side of the switching supply.
Yes from what I have seen plugging and unplugging the source AC supply has contributed to the failures. That said many were defective caps in the first place. I would venture a guess that your thermister was previous to the bridge rectifier on the AC side of things. Surge currents do strange things and are NEVER any good for electronics devices.

I worked on switching supplys for many years on my former job.
Most failures are from a defective under rated component or a just plain poor cheap design to start with. Inrush current is always a problem to deal with and with proper design is not an issue.
As you know the current to charge up the caps creates the main surge and has to be limited. Usually a thermister, some .1mfd caps and a transformer/coil or torid is involved.
The caps absorb the very high,quick peaks of the current and the thermister and usually a 5 ohm series resister help limit the current until it is up and running, then the thermister resistance drops to zero. A torid or 1-1 transformer coil is usually in series which has two functions. It's main job is to filter out the aprox 20 khz operating frequency that can go back into the power line. However it also adds a bit of inductance to 60 hertz and helps with the inrush limiting.
If the circuit design leaves some of this out you can expect trouble sometime.
Ever notice that when plugging in you converter/trailer plug you will get an arc/snap sometimes and not others? It just means you caught the 60 hertz cycle at mid point (no current) or at the peak (max current). I know, more than you want to know.