This is the approx. circuit where this PTC thermister is used. What is the purpose of using a thermister in this circuit?
The company who makes these is called ampron, its a Chinese company which means that documentation is hard to find, and they might have to be contacted directly.
As far as the use of the PTC it is most likely used as a current limiter for C20. As the current through C20 increases the PTC heats up and increases it's resistance. This would also change the filter pole of L1/C20 as temperature increases. Why it's there exactly, I don't know.