Causes of surge current in switching power supply

In the past and today's various common switching power supplies, power transformers are very popular. Generally, all design schemes can be considered. This power supply is very economical, but there are also some problems in industrial design. This is an inherent disadvantage of many switching power supplies (especially high-power switching power supplies) absorbing high current when powered on.

This surge current may reach 10 ~ 100 times of the static working current of the power supply. Therefore, at least two problems may arise. First, if the DC power supply cannot provide sufficient starting current, the switching power supply may enter the locked state and cannot be started; Second, this surge current may reduce the input power supply voltage.

The traditional method of limiting the input surge current is the series negative temperature coefficient thermistor power resistor (NTC). However, this simple method has many defects: for example, the actual effect of overcurrent protection of NTC resistor is greatly threatened by the operating temperature. In fact, the above two problems can be solved by a "soft start circuit", which will be described in detail below.

The input circuit of switching power supply mostly adopts capacitor filter rectifier circuit. When the input power is turned on, since the initial voltage on the capacitor is zero, a large surge current will be formed when the capacitor is charged. Especially for high-power switching power supply, large capacity filter capacitor is used to make the surge current reach more than 100A.

Such a large surge current will often cause the input fuse to burn out or the closing switch contact to burn out at the moment of power on, and the rectifier bridge to be damaged by overcurrent; The light will also prevent the air switch from turning off. The above phenomenon will cause the switching power supply to fail to work normally. Therefore, almost all switching power supplies are equipped with soft start circuits to prevent surge current, so as to ensure the normal and reliable operation of the second-hand robot power supply.