Converter In-Rush Thermistors etc, UPDATE -4
UPDATE on 17 Nov, Update 2 on 22 Nov, Update 3 on 16 Jan 14, Update 4 on 19 Oct 14. --------------- In a recent thread there was mention of how you can wreck your gizmo (in my case a converter) ...
Forum Discussion
Salvo wrote:
Soft start is a standard function. It is virtually in every dc/dc converter. I've used them over 25 years ago.
LT3580
Soft start does not limit current to the input capacitors. You got an ac source (120Vac) connected to a bridge rectifier which in turn is connected to a 4,200uF capacitor. That's where the problem is. Once you turn on the ac power there's a huge inrush of current into the capacitor as it charges up to about 170Vdc. That current needs to be tamed. That's the function of the NTC thermistor. The thermistor resistance is *supposed to* be sized according to the size of the capacitor. For the OP's converter, a 1 ohm thermistor will do just fine. There's absolutely no reason to go to 5 ohms. I think that could be part of the problem.
Sal
I know how soft-start works, I am interested in seeing it implemented in an RV type converter.
So the NTC is not applied at the AC power input?
Are you saying the NTC is applied in series between the rectifiers and energy storage filter capacitor? If that is the case, the additional resistance provided by the NTC is used to protect the rectifier diodes from high peak currents during the initial charge cycle.
A lower impedance hot state device may not fix the problem and may fail earlier and be worse. This would occur if the energy storage capacitor has lost capacity, then the ripple voltage will increase, which will reduce the average voltage. This would force the NTC to drop drop more voltage in steady state, and if a lower hot resistance device used, it would consume more power due to (Vripple)^2/Rntc.
Lots of possibilities in the failure analysis, but not much data...
