Causes of inrush current in switching power supplies
Of the various power supplies commonly used in the past and present, switching power supplies are very popular and can generally meet any design requirement. Such power supplies are economical, but there are some problems in industrial design. This is that many switching power supplies (especially high-power switching power supplies), there is an inherent disadvantage: at the moment of power-up to draw a large current. This inrush current may reach 1O times to 100 times the power supply static operating current. As a result, at least two aspects of the problem are likely to arise. First, if the DC power supply can not supply enough start-up current, the switching power supply may enter a locked state and can not start; Second, this inrush current may cause the input power supply voltage reduction, enough to cause the use of the same input power supply of other power equipment instantaneous power down.
The traditional method of limiting input inrush current is to connect a negative temperature coefficient thermal current limiting resistor (NTC) in series, however, this simple method has many drawbacks: e.g., the current limiting effect of the NTC resistor is greatly affected by the ambient temperature, the current limiting effect can only be partially achieved during a short period of input mains grid interruption (on the order of a few hundred milliseconds), and the power loss of the NTC resistor reduces the conversion efficiency of the switching power supply. ....... In fact, the above two problems can be solved by a "soft-start circuit", described in detail below.
Switching power supply inrush current causes
Most of the switching power supply input circuit using capacitor filter type rectifier circuit, in the incoming power supply closing moment, due to the initial voltage on the capacitor is zero, capacitor charging moment will form a very large inrush current, especially high-power switching power supply, the use of large-capacity filtering capacitors, so that the inrush current reaches more than 100 A. In the power on moment, such a large inrush current can be generated by a "soft-start" circuit. In the power supply turn on instant such a large inrush current, the heavier will often lead to the input fuse burnt out or the contacts of the closing switch burnt out, rectifier bridge overcurrent damage; the lighter will also make the air switch can not close the gate. All of the above phenomena will cause the switching power supply can not work properly, for this reason, almost all switching power supply are set to prevent the inrush current of the soft-start circuit to ensure that the used robot power supply normal and reliable operation.
Electrical principle of soft-start circuit
If a "soft-start circuit" is used to eliminate the inrush current during the start-up of a switching power supply, the shortcomings of the traditional inrush current limiting methods mentioned above can be avoided. Controlling the startup of a switching power supply to eliminate inrush current through "soft-start" consists of two design principles: removing the load at the moment of power-up, and limiting the useful current at the same time. Switching power supplies generally start up with very little current if they are not driving a load. In many cases, the start-up current may actually be less than the steady-state operating current maintained by this method.
