Causes of inrush current caused by switching power supply

From: http://www.21ic.com/app/power/201206/126276.htm

In a variety of past and present common power supply, switching power supply is very popular, generally can meet any design requirements. This kind of power supply is very economical, but there are some problems in the industrial design. This is a lot of switching power supply (especially high-power switching power supply), there is an inherent disadvantage: in the power-up moment to draw a larger current. This inrush current may reach 1O times ~100 times the quiescent operating current of the power supply. As a result, at least two aspects of the problem may arise. First, if the DC power supply does not supply enough start-up current, the switching power supply may enter a lock state and cannot be started; second, this inrush current may cause the input supply voltage to be reduced enough to cause an instantaneous power loss of other power devices using the same input source.

The conventional input inrush current limiting method is a series negative temperature coefficient thermal current limiting resistor (NTC), however this simple method has many drawbacks: for example, the current-limiting effect of the NTC resistor is affected by the ambient temperature, and the current limiting effect can only be partially reached when the transient input main power grid is interrupted (about hundreds of milliseconds in magnitude). The power loss of the NTC resistor reduces the conversion efficiency of the switching power supply .... In fact, the above mentioned two problems can be solved by a "soft start circuit", the following detailed introduction.

1 Causes of inrush current from switching power supply

Switching power supply input circuit is mostly using a capacitor filter rectifier circuit, in the incoming power supply closing moment, because the initial voltage on the capacitor is zero, the capacitor charging moment will form a very large inrush current, especially high-power switching power supply, the use of large-capacity filter capacitors, so that inrush current of up to 100A. When the power is switched on for such a large inrush current, the heavy person often causes the input fuse to burn or the contact point of the switch to burn out, the rectifier bridge over-current damage, the light will also make the air switch can not close the brake. All of these phenomena will cause switching power supply does not work properly, for this reason almost all switching power supplies are set to prevent inrush current soft start circuit to ensure that the second-hand robot power supply normal and reliable operation.

2 soft start Circuit electrical working principle

The

Soft-start circuit is used to eliminate the inrush current at the start-up of the switching power supply, which avoids the disadvantages of the conventional inrush current limiting method described above. The "soft start" controls the start-up of the switching power supply to eliminate inrush currents, including two design principles: to remove the load at the moment of power-up, while limiting the useful current. If the load is not driven, the switching power supply normally starts with a small current. In many cases, the starting current may actually be smaller than the steady-state operating current that is maintained with this method.