Introduction to how to prevent input surge current in switching power supplies
Usually, when starting a switching power supply, it may be necessary for the main power grid at the input end to provide a short-term high current pulse, which is commonly referred to as an "input surge current". The input surge current first causes trouble in the selection of main circuit breakers and other fuses in the main power grid: on the one hand, circuit breakers need to ensure that they melt during overload to provide protection; On the other hand, it is necessary not to melt when there is an input surge current to avoid misoperation. Secondly, the input surge current will cause the input voltage waveform to collapse, resulting in a deterioration of power supply quality and subsequently affecting the operation of other electrical equipment.
The reason for the occurrence of input surge current
In a switching power supply, the input voltage is first filtered by interference, then converted into DC through a bridge rectifier, and finally smoothed by a large electrolytic capacitor before entering the true DC/DC converter. The input surge current is generated during the initial charging of this electrolytic capacitor, and its magnitude depends on the amplitude of the input voltage at startup and the total resistance of the circuit formed by the bridge rectifier and electrolytic capacitor. If it happens to start at the peak point of the AC input voltage, a peak input surge current will occur.
The series connected negative temperature coefficient thermistor current limiting resistor NTC is undoubtedly the simplest method to suppress input surge current so far. Because NTC resistors decrease with increasing temperature. When the switching power supply is started, the NTC resistor is at room temperature and has a high resistance, which can effectively limit the current; After the power is started, the NTC resistor will quickly heat up to about 110 º C due to its own heat dissipation, and the resistance value will decrease to about one fifteenth of that at room temperature, reducing the power loss during normal operation of the switching power supply.
advantage:
The circuit is simple, practical, and cost-effective
Disadvantages:
The current limiting effect of NTC resistors is greatly affected by environmental temperature: if the resistance is too large and the charging current is too small during low temperature (minus zero) starting, the switching power supply may not be able to start; If the resistance of the resistor is too small during high-temperature starting, it may not achieve the effect of limiting the input surge current.
