Can an NTC (Negative Temperature Coefficient) thermistor be placed before a MOV (Metal Oxide Varistor) for protecting an SMPS from inrush current and voltage spikes?
Here is a drawing to clarify my question:
You really asked two independent questions: Can a series NTC thermistor followed by parallel MOV help with inrush, and can it help with spikes?
Yes, it can help with inrush. The series resistance starts out high due to the thermistor being cold. Presumably, things are sized so that the thermistor is a bit warm during normal operation, meaning it presents less series resistance. This means the device as a whole will draw less current initially until the thermistor warms up. That will decrease initial inrush since that ocurrs when the thermistor is cold.
No, it won't do much about spikes. These can happen at any time. Once the unit is running normally, the thermistor resistance will have stabalized. Spikes are by definition a short-term phenomena, so no time for the thermistor to react by changing temperature. The the thermistor will therefore function no differently than a fixed series resistor or the duration of the spike. The MOV will clamp the spike against this series resistance, and any addition series resistance of the incoming power.
The thermistor will actually make the spike case a little worse. Some fixed energy will be imparted onto the thermistor due to the extra spike current. This will raise its temperature, decreasing its resistance, making the unit more susceptible to subsequent spikes for a short time.
I know it's an old question, but maybe can be useful to somebody...
I think both NTC and MOV can be useful together in your circuit. As @Olin Lathrop said (as I understand it): PTC will protect against inrush and won't do much against spikes. But MOV will protect against spikes. In fact, I've got several switched power supplies that are protected this way. Some standard (or resettable) fuse will add additional protection.