Converter In-Rush Thermistors etc, UPDATE -4

DryCamper11 wrote:

The thermistor has no time to cool down during the transient.

It's not the entire thermistor that cools down. It is the tiny active region(s) that must transfer heat from that region into the adjacent material. That transfer has a time constant that is much much less than the bulk time constant of the entire device. You can damage the thermistor without producing significant heating of the entire device if enough current is passed through it in a short time.

Yes, we're only talking about the actual thermistor, not the adjacent material. It has a large time constant. Take a look here.

Encased thermistors have 2.5 s max time constant. The transient is only 4 ms. There is no cooling during the transient. It doesn't matter if you got a 1 ohm or 10 ohm thermistor, both will get to the same temperature following a turn-on transient.

That means your argument is not correct. Resistance has no effect on how hot the device gets during the transient.

We disagree. The resistance of the thermistor controls how much heat (energy=1/2 CV**2) is dissipated in the semiconductor active region(s) of the thermistor per unit time. Given the small size of the semiconductor region, the total energy dissipated as heat has sufficient time to diffuse out of that region before the region is damaged even though the entire device does not have time to cool significantly.

Again, I've provided a link showing thermistor time constant is max 2.5s. The greater the thermistor thermal capacity, the greater the time constant. That's something I was eluding to quite a few pages ago. The big capacity thermistors do not respond fast enough in lowering their resistance. 2.5s is an eternity when the converter is up and running drawing 13A. This is the time when we see the tremendous heat stress. The greater the thermistor resistance the greater the stress.

Resistance does have an effect on steady state use. The greater the resistance the hotter the device gets.

No, the greater the resistance, the longer the time it takes to dissipate the 1/2 CV**2 energy (during the initial charge cycle of the capacitor). Power is energy divided by time, and the energy remains constant, while increasing resistance increases the time over which that energy must be dissipated.

You're not reading my comment correctly. I'm not talking about turn-on, but rather steady state operation. The converter is drawing 13A continuous.

In steady state, we are no longer interested in the initial (zero power) resistance - we are interested in the steady state resistance, which is close to zero.

The thermistor resistance gets close to zero after one time constant. Until then, there's some major cooking going on.

Sal