Switching power supply designs typically have AC-DC and DC-DC designs. In power modules we know that resistors are often needed to protect the circuit. The most common of the power modules we have are varistors and thermistors, which usually use resistors to protect the circuit from the periphery of the power module. So what are the functions of these two commonly used varistor and thermistor in the circuit? MICROHM Electronics analyzes for you as follows:
  First, the role of thermistor in the design of power modules
 
Thermistor is a temperature-related device, generally divided into two types, NTC is a negative temperature coefficient thermistor, that is, the higher the temperature, the lower the impedance; PTC is the positive temperature coefficient thermistor, that is, the higher the temperature, the bigger the impedance. The use of impedance-temperature sensitive characteristics plays an important role in circuit design.
 
In the circuit, NTC mainly suppresses the starting current during the starting process of the circuit. During the system startup process, due to the existence of power circuit, capacitive and inductive load inside the system, a very large inrush current occurs at the moment of starting. If the instantaneous resistance of the device is not taken into consideration during the selection process of the circuit device, the system may easily cause breakdown of the device during multiple startup operations, and adding NTC to the circuit is equivalent to when the input circuit is started. Increasing the input impedance reduces the inrush current, and when the system is in a stable state, the NTC generates heat, and according to its negative temperature characteristic, the impedance is reduced, so that the loss on the NTC is also reduced, and the overall loss of the system is reduced.

 Second, the role of varistor in module design
Although the circuit can be protected, the varistor does not provide complete voltage protection. The varistor can withstand limited energy or power and cannot provide continuous overvoltage protection. Sustained overvoltage can damage the protective device (varistor) and cause damage to the device. The parts that the varistor cannot provide protection include: inrush current at startup, overcurrent at short circuit, voltage drop, etc. These situations require other methods of protection.
 
Through the above understanding of the role of varistor and thermistor in the circuit, when designing the power module, they can be used together to ensure the safe and stable operation of the circuit.