The NTC thermistor is useful for protecting a power system from overheating or cooling system failures because of the following features:
• Low cost
• More sensitive response than thermocouples
• Easy to use
• Immune to noise
• Temperature range is well matched to power module operating temperature range
An NTC thermistor has a time constant in the range of a few seconds, meaning that it takes at least a few seconds to detect the change in temperature in a module. The thermistor time constant is temperature dependent. At 100℃ the thermistor time constant is half as long as at room temperature.
Due to its slow response, the NTC thermistor is not suited to detect rapid changes in temperature and therefore can only be used to protect the system form slow changes in temperature. The NTC thermistor cannot be used for short circuit or over current protection.
The response of the NTC thermistor is exponential. In spite of its nonlinearity, the NTC thermistor is useful for module temperature measurements because:
1. A simple threshold circuit can be used to indicate an over-temperature condition, which will be discussed
2. The exponential response can be processed by analog circuitry or by software in a digitally controlled system.