How to use thermistor to suppress surge current circuit analysis

As the reliability requirements of electronic products continue to increase and energy resources become increasingly tight, highly reliable and energy-efficient electronic products will be a direction for the development of electronic products in the future. Therefore, in the power supply design of products, they must be fully considered. Reliable performance and power usage efficiency.

This article first analyzes why electronic products have power-on surges, then uses a typical power circuit as an example to analyze how to use thermistors to suppress surge currents, and finally introduces how to select thermistors in practical applications.

Causes of startup surge current

Figure 1 is a simplified circuit of a typical electronic product power supply. C1 is a filter capacitor connected in parallel with the load. At the moment of power-on, the capacitor voltage cannot change suddenly, so a large charging current will be generated. According to the first-order linear non-homogeneous equation established by the first-order circuit zero-state response model, the initial value of the current can be calculated to be equivalent to the current value obtained by short-circuiting the filter capacitor. This current is what we often call the input surge current. It is generated when the filter capacitor is initially charged. Its size depends on the amplitude of the input voltage when starting up and the voltage formed by the bridge rectifier and electrolytic capacitor. The total resistance of the loop.