The Cause of Surge Current in Switching Power Supply
Among all kinds of power supplies used in the past and now, switching power supply is very popular and can generally meet any design requirements. This kind of power supply is economical, but there are also some problems in industrial design. This is that many switching power supplies (especially high-power switching power supplies) have an inherent disadvantage: they draw a large current at the moment of power-on. This surge current may reach 1O times to 100 times of the static working current of the power supply. As a result, there may be at least two problems. If the DC power supply cannot supply enough starting current, the switching power supply may enter a locked state and cannot be started. This surge current may cause the voltage of the input power supply to decrease, which is enough to cause other power equipment using the same input power supply to lose power instantly.
The traditional input surge current limiting method is series negative temperature coefficient thermistor (NTC). However, this simple method has many shortcomings, such as the current limiting effect of NTC resistor is greatly affected by the ambient temperature, the current limiting effect can only be partially achieved when the input power grid is interrupted for a short time (about several hundred milliseconds), and the power loss of NTC resistor reduces the conversion efficiency of switching power supply. In fact, the two problems mentioned above can be solved by a "soft start circuit", which is introduced in detail below.
1 switching power supply surge current causes
The input circuit of switching power supply mostly adopts capacitor filter rectifier circuit. At the moment when the incoming power supply is closed, because the initial voltage on the capacitor is zero, a large surge current will be formed at the moment when the capacitor is charged. Especially for high-power switching power supply, the filter capacitor with large capacity is used to make the surge current reach more than 100A A. Such a large surge current at the moment when the power supply is turned on will often lead to the burning of the input fuse or the contact of the closing switch, and the overcurrent of the rectifier bridge will be damaged; The lighter one will also make the air switch fail to close the brake. All the above phenomena will cause the switching power supply to fail to work normally, so almost all switching power supplies are equipped with soft-start circuits to prevent inrush current, so as to ensure the normal and reliable operation of the second-hand robot power supply.
2 electrical working principle of soft start circuit
If the "soft start circuit" is used to eliminate the surge current when the switching power supply is started, the shortcomings of the above traditional surge current limiting method can be well avoided. Controlling the start of switching power supply by "soft start" to eliminate surge current includes two design principles: removing load at the moment of power-on and limiting useful current at the same time. If the load is not driven, the switching power supply usually has a small current when it is started. In many cases, the starting current may actually be smaller than the steady-state working current maintained by this method.
