The causes of inrush current in switching power supplies
Among various commonly used power supplies in the past and present, switch mode power supplies are very popular and can generally meet any design requirements. This power supply is very economical, but there are also some issues in industrial design. This is why many switching power supplies (especially high-power switching power supplies) have an inherent drawback: they need to draw a large current at the moment of power up. This surge current may reach 10 to 100 times the static operating current of the power supply. Thus, there are at least two possible issues that may arise. **If the DC power supply cannot provide sufficient starting current, the switching power supply may enter a locked state and cannot start; **This surge current may cause a decrease in the input power supply voltage, which is sufficient to cause other power equipment using the same input power supply to instantly lose power.
The traditional method of limiting input surge current is to connect negative temperature coefficient thermistor current limiting resistors (NTC) in series. However, this simple method has many drawbacks, such as the current limiting effect of NTC resistors being greatly affected by environmental temperature, the current limiting effect being only partially achieved during brief input main grid interruptions (on the order of several hundred milliseconds), and the power loss of NTC resistors reducing the conversion efficiency of switching power supplies. In fact, the two problems mentioned above can be solved through a "soft start circuit", which will be introduced in detail below.
Reasons for surge current generation in switch mode power supply
The input circuit of switch mode power supplies mostly adopts a capacitor filtering rectifier circuit. At the moment of closing the incoming power supply, due to the initial voltage on the capacitor being zero, a large surge current will be formed during the charging of the capacitor. Especially for high-power switch mode power supplies, larger capacity filtering capacitors are used to make the surge current reach 100A or more. Such a large surge current at the moment of power on can often cause the input fuse to burn out or the contacts of the closing switch to burn out, resulting in overcurrent damage to the rectifier bridge; Mild cases can also cause the air switch to fail to close. The above phenomena can cause the switching power supply to malfunction. Therefore, almost all switching power supplies are equipped with soft start circuits to prevent surge currents, ensuring the normal and reliable operation of the second-hand robot power supply.
2. Electrical working principle of soft start circuit
If a "soft start circuit" is used to eliminate the surge current during the startup of a switching power supply, it can effectively avoid the drawbacks of the traditional surge current limiting methods mentioned above. Controlling the start-up of a switching power supply through "soft start" to eliminate surge currents involves two design principles: removing the load at the moment of power up while limiting the useful current. If the load is not driven, the current when the switching power supply is started is generally very small. In many cases, the starting current may actually be smaller than the steady-state operating current maintained using this method.
