Among all kinds of common switching power supplies in the past and today, power transformers are very popular. Generally, all design schemes can be considered. This kind of power supply is very economical, but there are also some problems in industrial design. This is an inherent disadvantage of many switching power supplies (especially high-power switching power supplies) that absorb large current at the moment of power on. The surge current may reach 10 ~ 100 times of the static working current of the power supply. Thus, at least two problems may arise. First, if the DC power supply cannot supply enough starting current, the switching power supply may enter a locked state and cannot be started; Second, this surge current may cause the voltage of the input power supply to decrease, which is enough to cause the instantaneous power failure of other power equipment using the same input power supply.
The traditional limiting method of input surge current is to connect negative temperature coefficient thermistor power resistor (NTC) in series. However, this simple method has many defects: for example, the actual effect of over-current protection of NTC resistor is greatly damaged by the working temperature, and the actual effect of over-current protection is only partially achieved when the short-term transient key is connected to the main power network (about hundreds of MS) The output power loss of NTC resistor reduces the conversion efficiency of power transformer. In fact, the two problems mentioned above can be solved through a "soft start circuit", which will be introduced in detail below.
1. Reasons caused by surge current of switching power supply
The input circuit of switching power supply mostly adopts capacitor filter rectifier circuit. At the moment when the incoming power supply is switched on, because the initial voltage on the capacitor is zero, a large surge current will be formed at the moment of capacitor charging. Especially for high-power switching power supply, filter capacitors with large capacity are used to make the surge current reach more than 100A. Such a large surge current at the moment when the power supply is connected will often cause the input fuse to burn out or the contact of the closing switch to burn out, and the rectifier bridge to be damaged by overcurrent; The lighter one will also make the air switch unable to close. The above phenomena will cause the switching power supply to fail to work normally. Therefore, 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 equipment principle of soft start power circuit
If the "soft start circuit" is used to eliminate the surge current when switching power supply is started, the disadvantages of the traditional surge current limiting method can be avoided. According to the "soft start", the power transformer is operated to clear the surge current, including two design principles: removing the load at the moment of power on and limiting the effective current. If there is no driver load, the starting current of the power transformer is generally small. In many cases, the starting current may be smaller than the constant operating current maintained by this method.
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