A white paper from Bourns about thermal protection PTCs on flyback converters. Link here (PDF)
This paper examines the use of resettable polymer fuses for protecting offline flyback converters. Using a thermal model of the resettable fuse surrounding solder pads and copper to optimize the trip time so that the converter is protected during overloads, there are two potential positions considered for polymer Positive Temperature Coefficient (PTC) resettable fuses in the circuit. One position is directly on the winding and the other position is beyond the control loop. Results are taken from the converter and compared with a simulation.