In the AL9910 LED driver datasheet it shows a part, which I assume is a varistor, is it so? Varistors are shown to be connected across L and N lines, but here it is connected in series. Please explain the operation principle.
This is an NTC current limiting resistor. When plugging in the circuit, the NTC is cold and a bad conductor - this is limiting the charge current to the input capacitor. When the NTC is hot from the power dissipation it will become a good conductor with low losses.
It’s likely a negative temperature coefficient device used to limit inrush current into the bulk storage capacitor and prevent fuses blowing or breakers tripping. It’s quite a common thing to implement.
Once the device warms up its resistance lowers and thus the peak current at initial switch on is significantly reduced.
It is a negative temperature coefficient thermistor (NTC). It maintains constant resistance at room temperature. It's resistance will Decreases as temperature rises.
These are commonly used in inrush current limiting to the input capacitors, protects the fuses and breakers from tripping.
Always the most confusing part while referring to these kind of circuits is the symbol, many people represent both (varistor and thermistor) in same way. so we have to decide the device whether it is MOV or NTC based on its operation and application in the circuit (i mean whether it is series, parallel).