Converter In-Rush Thermistors etc, UPDATE -4

Salvo wrote:

ken white wrote:


I know how soft-start works, I am interested in seeing it implemented in an RV type converter.

If you know how soft start works then you also know how it's implemented in a RV type converter. There's no difference.

So the NTC is not applied at the AC power input?

Does it matter? I don't know where they put it. It could be before or after the diodes. From the looks of the OP's converter it's located before the diodes.

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.

Yes, the thermistor protects the diodes as well as the caps.

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.

The thermistor resistance is sized according to the capacitance of the cap bank. The greater the capacitance, the greater the thermistor resistance. If the cap bank loses capacitance then we can get by with a lesser thermistor resistance, not a larger one as you claim.

Lots of possibilities in the failure analysis, but not much data...

If you know what you're doing then it's easy to eliminate the goose chases. A *few* years ago I was doing failure analysis on space hardware.

Sal