How to Prevent Input Surge Current in Switching Power Supplies
Usually, when the switching power supply is started, the main grid at the input end may be required to provide a short-term large current pulse, and this current pulse is usually called "inrush current (inrush current)". The input surge current first caused trouble to the selection of the main circuit breaker (main circuit breaker) and other fuses in the main grid: on the one hand, the circuit breaker must ensure that it will fuse when overloaded to play a protective role; on the other hand, it must be in the input When the surge current occurs, it cannot be fused to avoid malfunction. Secondly, the input surge current will cause the input voltage waveform to collapse, which will deteriorate the quality of power supply and affect the work of other electrical equipment.
Causes of Input Inrush Current
The input voltage is first filtered by interference, then converted to DC by a bridge rectifier, and then smoothed by a large electrolytic capacitor before entering the real DC/DC converter. The input surge current is generated when the electrolytic capacitor is initially charged, and its magnitude depends on the magnitude of the input voltage at startup and the total resistance of the loop formed by the bridge rectifier and the electrolytic capacitor. If it happens to start at the peak point of the AC input voltage, the peak input surge current will appear.
Option One
The most common method of input inrush current limiting: series negative temperature coefficient thermistor current limiting resistor (ntc)
advantage:
● Simple and practical circuit, low cost
shortcoming:
1. The current limiting effect of the ntc resistor is greatly affected by the ambient temperature: if the resistance is too large and the charging current is too small when starting at low temperature (subzero), the switching power supply may not be able to start; if it is starting at a high temperature, the resistance value of the resistor If it is too small, the effect of limiting the input inrush current may not be achieved. Series negative temperature coefficient thermistor current limiting resistor ntc is undoubtedly by far the easiest way to suppress input surge current. Because ntc resistors degrade with increasing temperature. When the switching power supply is started, the ntc resistor is at normal temperature and has a high resistance, which can effectively limit the current; after the power supply is started, the ntc resistor will rapidly heat up to about 110ºC due to its own heat dissipation, and the resistance value will decrease to room temperature About one-fifteenth of the time, reducing the power loss when the switching power supply works normally.
2. The current limiting effect is only partially achieved during brief input mains interruptions (on the order of hundreds of milliseconds). During this brief interruption, the electrolytic capacitor has been discharged, but the temperature of the ntc resistor is still high and its resistance value is small. When the power supply needs to be restarted immediately, the ntc cannot effectively realize the current limiting function.
3. The power loss of the ntc resistor reduces the conversion efficiency of the switching power supply.
Option II
When making a micro-power switching power supply, directly use a power resistor to limit the inrush current.
advantage:
● The circuit is simple, the cost is low, and the limitation of the surge current is hardly affected by high and low temperature
shortcoming:
● Only suitable for micro power switching power supply
● Great impact on efficiency
third solution
Parallel connection of NTC thermistor and ordinary power resistor to limit inrush current
When starting at normal temperature, the resistance of the power resistor and the thermistor in parallel to limit the inrush current. When starting at low temperature, the resistance of the NTC thermistor rises sharply, but the resistance of the power resistor remains basically unchanged to ensure low temperature Start, but the surge circuit is also very large during high temperature experiments.
advantage:
● Simple and practical, good effect for normal and low temperature start-up
shortcoming:
● Greater impact on efficiency
● Large surge current at high temperature
