What causes surge current in a power supply?

This can happen when you overuse extension cords, plug in too many appliances, or use an appliance above the circuit’s amperage levels. Electrical overloads can lead to power surges as the overwhelmed circuit may receive a massive spike in current due to the excessive power being drawn.Today qihe smt pick and place machine sharing what causes surge current in a power supply .

Among various power supplies commonly used in the past and present, switching power supplies are very popular and can generally meet any design requirements. This power supply is very economical, but there are also some problems in industrial design. This is the inherent shortcoming of many switching power supplies (especially high-power switching power supplies): they must draw a large current at the moment of power-on. This surge current may reach 1O times to 100 times the quiescent operating current of the power supply. As a result, at least two problems may arise:

First, if the DC power supply cannot supply sufficient starting current, the switching power supply may enter a locking state and fail to start;

Second, this surge current may cause the input power supply voltage to drop, enough to cause other power equipment using the same input power supply to instantly lose power.

The traditional input surge current limiting method is to connect a negative temperature coefficient thermistor current-limiting resistor (NTC) in series. However, this simple method has many shortcomings: for example, the current-limiting effect of the NTC resistor is greatly affected by the ambient temperature. The effect is only partially achieved during brief input main grid interruptions (on the order of a few hundred milliseconds). The power losses in the NTC resistor reduce the conversion efficiency of the switching power supply… In fact, the two problems raised above can be solved by a “soft start circuit”, which is introduced in detail below.

Causes of surge current in switching power supply

Most of the input circuits of switching power supplies use capacitor filter rectifier circuits. When the incoming power supply is turned on, since the initial voltage on the capacitor is zero, a large surge current will form at the moment when the capacitor is charged. Especially for high-power switching power supplies, which use Larger capacity filter capacitors can increase the surge current to more than 100A. Such a large surge current at the moment when the power is turned on will often cause the input fuse to blow out or the contacts of the closing switch to burn out, causing overcurrent damage to the rectifier bridge; in the worst case, it will also cause the air switch to fail to close. The above phenomena will cause the switching power supply to fail to work properly. For this reason, almost all switching power supplies are equipped with a soft-start circuit to prevent inrush current to ensure the normal and reliable operation of the second-hand robot power supply.

Electrical working principle of soft start circuit

If a “soft start circuit” is used to eliminate the inrush current when the switching power supply is started, the shortcomings of the traditional inrush current limiting method mentioned above can be well avoided. Controlling the start-up of the switching power supply to eliminate inrush current through “soft start” includes two design principles: removing the load at the moment of power-on and limiting the useful current at the same time. If the load is not driven, the current of the switching power supply is generally very small when it is started. In many cases, the starting current may actually be less than the steady-state operating current maintained using this method.

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