Precautions for the application of NTC in switching power supplies.Infrared digital sensor module

　　1. From the analysis of the working principle of the circuit, we can see that under normal working conditions, a certain current flows through the power NTC thermistor, and this working current often causes the surface temperature of the power NTC to reach more than 100 °C.

　　When the product is turned off, the power NTC thermistor must be completely restored from the high temperature and low resistance state to the normal temperature and high resistance state in order to achieve the same surge suppression effect as the last time.

　　The recovery time is related to the dissipation coefficient and heat capacity of the power NTC thermistor, and the cooling thermal time constant is generally used as a reference. The cooling thermal time constant is not the time required for the power NTC thermistor to recover to the normal state, but the larger the cooling time constant, the longer the required recovery time, and vice versa. Therefore, the power NTC thermistor cannot provide a good protection effect in the case of frequent switching.

　　2. Power NTC thermistors are always connected in series in the protection circuit. If a power NTC thermistor cannot suppress the surge current alone, two or more power NTC thermistors can be connected in series in the circuit. middle. It is not advisable to connect two or several power NTC thermistors in parallel because the load is not evenly distributed. If one of the power NTC thermistors passes a larger current than the other parallel power NTC thermistors, it will become hotter until it finally passes almost all the current, which may eventually damage the power type NTC thermistor, while other paralleled power type NTC thermistors remain cool. Therefore, the power NTC thermistor used to suppress the surge current can only be used in series in the protection circuit.

　　3. In practical applications, the power NTC thermistor should be kept within the rated operating temperature range as much as possible. If the temperature exceeds the specified upper and lower limits, it may cause the failure or damage of the power NTC product. Since the power NTC thermistor is greatly affected by the ambient temperature, the maximum steady-state current at normal temperature (0-25°C) is generally given in the product specification. The rated current will derate linearly to zero at maximum or minimum operating temperature. The application conditions of power NTC thermistor products are not at normal temperature (0~25℃), or due to the design or structure of the product itself, such as some devices with large heat generation in the power supply. When the ambient temperature is too high or too low, it must be derated according to the derating curve.

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