The latest addition to Murata's PRF family of chip ceramic thermistors is the PRF18xx5RB range of devices designed for overheat detection of power FETs in point of load (POL) DC-DC converters and other power supply applications.

Each part has two temperature sensing points and features characteristic curves that rise very sharply.

The series has a resistance of 470ohm at 25 deg C, rising to 4.7kohm between 65 and 145 deg C, rising further to 47kohm between 80 and 150 deg C, depending on the part number.

The PRF series is available in tiny 0603 packaging, and is also suitable for detecting overheat conditions in battery packs, motors, power devices and other systems.

The ceramic thermistors are designed to increase their resistance very sharply when exposed to temperatures over a certain level. This can be used to provide a simple and reliable overheat sensing circuit.

A change in the conditions around the FET such that the device went from normal levels to overheating, would produce a more than one hundred-fold increase in the resistance of the part.

Parts are available that detect overheat temperatures between 65 and 145 deg C. Maximum permitted operating voltage for the PRF series is 32Vdc.      

See http://www.murata.eu