- The MOSFET is used as the switching element of DC-DC converters or as a general-purpose current switch. When the FET is operated at a higher temperature than specified due to overloading or insufficient heat radiation, the FET can become damaged. Damage to the FET can be prevented by detecting whether it is overheating with "POSISTOR" for overheat detection.
- Murata's "POSISTOR" for overheat detection detects the temperature of the FET itself, with the resistance of the "POSISTOR" increasing when the temperature exceeds a desired point. This enables the base current to flow through the transistor and switch it on. The gate current of the FET is then bypassed to decrease the gate voltage, preventing the FET from being damaged.
- An appropriate shape of "POSISTOR" (discrete type, screw
mount type, or surface mount type) can be selected according to the
transistor shape (discrete type or surface mount type).
Circuit example: Protecting the MOSFET from overheating
When the rated temperature of the FET is exceeded because of overload or insufficient heat radiation, the FET may break down. "POSISTOR" is useful to monitor the FET's temperature. When the FET's temperature approaches the maximum rating, "POSISTOR" turns the transistor on. In this way the FET's gate voltage decreases below its threshold level and the FET is turned off. When the FET's temperature returns within the rating, normal operation returns. There is therefore no need to exchange protecting elements like fuses.
Roles of "POSISTOR"
- Detects abnormal temperature rise of the MOSFET, switches on the transistor, and decreases the FET's gate voltage to prevent the FET from overheating.
- Decreases the resistance of the "POSISTOR" and unlike the thermal fuse, recovers its original resistance, when the MOSFET temperature drops.
