Converter In-Rush Thermistors etc, UPDATE -4

BFL13 wrote:

Salvo wrote:
No surprise. Nothing had changed from the previous thermistor failure. We already knew the initial power-up was not the problem. The problem is P = I^2 * R. Where I is the 10A to 13A when converter starts operation and R is the resistance of the thermistor. The resistance is too high! Use 1 or 2 ohms. It probably also helps if a heatsink is attached to the thermistor.

Sal

It was when the thermistor was already warmed up so its R was low that it got hit with the big current from a "too soon" restart and blew, so how could it help to have a lower R thermistor for the cold start too?

I see the cold start with no load gets the thermistor warmed up and the capacitors done up when I then clamp on and the big current hits, so that is like I was trying not to do with a restart too soon. Same thing. So better to clamp on first. It works on a cold start clamping on last, but I can see there would be less chance of a cumulative effect breaking down the thermistor if I clamped on first.

As I am following the story here, the idea is to hit the thermistor with the high current before it gets warm so its R is still up.

Let's ignore the thermistor for a while and step back out of the "forest".


I believe that the main expected operating mode of the converter is to power it up when it is connected to a battery or load. That is how the vast majority of installation cases would be configured. So it should be designed to handle that case troublefree and well. What other cases it is designed to handle, we don't know.

We don't know if it was designed to "gracefully" handle the case of power up with no load and then connecting a discharged battery. In your case it doesn't appear to handle that case well.

continue to "exercise" the converter connected in the manner I think it was designed to handle gracefully, that of a load connected then powered on. Does it continue to operate trouble free?? If yes, operate it that way. Does it continue to blow thermistors??? Well, then something is going on in your use case causing a product failure.


a bit of information that I think would be REAL informative would be
a current probe on converter input and output and voltage probe on the converter output hooked to a scope set to capture a few seconds of data on turn on.

current and voltage probes and scope would need to be a few MHz BW to capture the fundamental and harmonics.


Then look at input and output current and output voltage waveforms when
a) converter on and stable, then "instantly" connected to a deeply discharged battery
b) converter connected to same battery then powered on.

I suspect the input and output current waveforms over time are going to be radically different as is the output voltage waveform.

Would even give a clue to tell us if the converter is going open loop in case (a) and/or oscillating for while.

case (b) should show a very "well behaved" circuit with output voltage and current ramping up monotonically over time.

It would also be interesting to see what happens to voltage and current waveforms if a heavy load (batteries) is "instantly" disconnected from a powered converter. It may not be as well behaved as one would like or expect.

But these measurements take equipment not readily available to you or many on the board, so while it would be interesting, it is outside anything I can reasonably expect. Don't run out and try to find the equipment just to satisfy me and others.