Reasons for surge current generated by switching power supply
Among various commonly used power supplies in the past and present, switching power supplies are very popular and can generally meet any design requirements. This type of power supply is very economical, but there are also some problems 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 on. This surge current may reach 10 to 100 times the static working current of the power supply. As a result, there are at least two possible issues that may arise. Firstly, if the DC power supply cannot provide sufficient starting current, the switching power supply may enter a locked state and cannot be started; Secondly, this surge current may cause a decrease in the input power supply voltage, which is sufficient to cause instantaneous power failure of other power equipment using the same input power supply.
The traditional method of limiting input surge current is to connect Negative temperature coefficient thermistor current limiting resistor (NTC) in series. However, this simple method has many disadvantages: for example, 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 main 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 issues mentioned above can be solved through a "soft start circuit", which will be explained in detail below.
Reasons for the generation of surge current in 1 switch power supply
The input circuits of switching power supplies mostly use capacitor filtering rectifier circuits. 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 at the moment of charging the capacitor. Especially for high-power switching power supplies, larger filtering capacitors are used to achieve a surge current of over 100A. At the moment of power connection, such a large surge current can often cause the input fuse to burn out or the contact of the closing switch to burn out, resulting in overcurrent damage to the rectifier bridge; Even light ones can cause the air switch to fail to close. All of the above phenomena will cause the switching power supply to fail to work normally. For this reason, almost all of the switching power supplies are equipped with soft start circuits to prevent current surges, 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 a "soft start circuit" is used to eliminate the surge current during switching power supply startup, it can effectively avoid the drawbacks of the traditional surge current limitation method mentioned above. Controlling the start of the switching power supply through "soft start" to eliminate surge current includes two design principles: removing the load at the moment of power on and limiting 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
