Chip ceramic thermistors protect power supplies

9 July 2008 Circuit & System Protection

The latest addition to Murata’s PRF family of chip ceramic thermistors is the PRF18xx5RB series, 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 470 Ω at 25°C, rising to 4,7 kΩ between 65 and 145°C, or rising further to 47 kΩ between 80 and 150°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.

With the decreasing size of today's power supply assemblies, the resulting increase in thermal density means these systems have higher risk of overheating. Increasing use of the distributed power architecture means there are a greater number of points that are likely to overheat. Murata’s PRF series of ceramic thermistors increase their resistance very sharply when exposed to temperatures over a certain level. This characteristic provides a simple and reliable overheat sensing circuit. A change in the conditions around the FET such that the device goes from normal levels to overheating, would produce a more than one hundred-fold increase in the resistance of the part.

It is possible to sense multiple temperatures in different areas simply by connecting PRF thermistors in series. By applying an appropriate voltage dividing resistor, a circuit with multiple thermistors in series can easily be used to drive a transistor or FET to shut down the power supply. This eliminates the necessity for analog-to-digital converters and control software. Further, PRF thermistors can be added or removed from the circuit without affecting its performance, so it is possible to monitor different parts of the circuit as the design develops with the same number of thermistors – any unused monitoring points can simply be short-circuited.

Parts are available that detect overheat temperatures between 65 and 145°C. Maximum permitted operating voltage for the PRF series is 32 V d.c.

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