Reasons for Inrush Current in Switching Power Supply Design.adjustable power adapter

　　Among the various power supplies commonly used in the past and present, switching power supplies are very popular and can generally meet any design requirements. This kind of power supply is economical, but also has some problems in industrial design. This is why many switching power supplies (especially high-power switching power supplies) have an inherent disadvantage: they must draw a large current at the moment of power-on. This surge current may reach 10 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 enough starting current, the switching power supply may enter a locked state and cannot start;

　　Second, this inrush current may cause a drop in the input power supply voltage enough to cause a momentary loss of power to other power equipment using the same input power supply.

　　The traditional input inrush current limiting method is to connect a negative temperature coefficient thermal current limiting resistor (NTC) in series. However, this simple method has many disadvantages: for example, the current limiting effect of the NTC resistor is greatly affected by the ambient temperature, and the current limiting The effect is only partially achieved during brief input mains interruptions (on the order of hundreds of milliseconds), the power loss of the NTC resistor reduces the conversion efficiency of the switching power supply... . In fact, the two problems raised above can be solved by a "soft-start circuit".

　　Reasons for Inrush Current of Switching Power Supply

　　The input circuit of the switching power supply mostly adopts the capacitor filter type rectifier circuit. At the moment when the incoming power supply is closed, since the initial voltage on the capacitor is zero, a large surge current will be formed at the moment of charging of the capacitor, especially for high-power switching power supply. Filter capacitors with larger capacity can make the surge current reach 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 or the contacts of the closing switch to burn out, and the rectifier bridge will be damaged due to overcurrent; The above phenomena will cause the switching power supply to fail to work normally. 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.

　　Soft start circuit electrical working principle

　　If a "soft-start circuit" is used to eliminate the inrush current when the switching power supply starts, the shortcomings of the traditional inrush current limiting method mentioned above can be well avoided. Controlling the startup 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 switching power supply generally has a small current when it starts up. In many cases, the start-up current may actually be lower than the steady-state operating current maintained by this method.

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