Hi everyone,
this is my 1st post, I hope you can help me.

I placed a NTC thermistor for inrush current suppression in my PCB. The stady state current that flows through the themistor is about 0.4A.
The inrush current is limited as expected by my calculations and this is OK. The max load capacitance constraint is also met.
Also, according to the datasheet, the thermistor has a temperature operating range of -50 +200 °C. In my case, operating at an ambient temperature of 25 °C, it reaches 70 °C. The possible problem is that the value of the thermistor (in ohm) at steady state seems not to be costant. I guess it could be due to the fact that it exhibits a max steady state current of 2.5A that is much higher than the current flowing in my case. So, the current could not be enough to bring the thermistor value low enough -> it dissipates more power, dissipating more power the temperature rises, rising the temperature the value of the thermistor lowers -> it cools and its value rises and so on...

I would know if my supposition is right and if this behavior could damage the thermistor, even operating at higher ambient temperatures. Consider that I already took into account the decrease of the inrush current limiting effect up to an ambient temperature of 75 °C and it is enough even in this case.
Many thanks in advance.

 

Welcome to AAC!

I would know if my supposition is right

I agree with your explanation.

and if this behavior could damage the thermistor

As long as you stay under manufacturer maximums.

The surge limiter I use is Ametherm SL10 10004. It specifies a body temperature of 161C at rated current:

There was no derating information in the datasheet.

 

Welcome to AAC!

The possible problem is that the value of the thermistor (in ohm) at steady state seems not to be constant.

How can that be a steady state if the thermistor value is changing?

 

The current must be changing if the thermistor value is also changing.

 

One possibility that could cause a slow current oscillation is if you are powering a switching power supply.
Such a supply has a negative impedance at its input for a constant output load (input current goes down as the voltage goes up).
That could cause an instability in the input current with the thermistor output voltage going up as the current goes up.
The oscillation frequency would be determined largely by the thermal time-constant of the thermistor.

 

Thank you very much for your replies.
My NTC is the MF72-030D13 by Cantherm. The load is a battery charger and the NTC is used to protect a relay placed at the input of the charger. The abovementioned oscillations do not happen if I use other relays with a maximum rated current of 1 - 1.5 A but they have lower values (in ohm), not enough to protect the relay from the inrush current.
So I would establish if I can keep using the MF72-030D13 safely or if I need to change it.
Thank you very much again.

 

The load is a battery charger

What type of battery charger?
Do you have a part number?

 

What type of battery charger?
Do you have a part number?

Thank you crutschow.
Unfortunately it has no part number and the datasheet reports wrong values. For example, I am quite confident about my inrush current measurements and I obtained a value double than the one reported in the datasheet.