How to prevent input surge current in switching power supplies
Usually, when a switching power supply is started, it may be necessary for the main power grid at the input end to provide short-term high current pulses, which are commonly referred to as "input surge currents". The input surge current first causes trouble for 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 fuse during overload to play a protective role; On the other hand, it is necessary not to fuse when the input surge current occurs to avoid misoperation. Secondly, input surge current can cause input voltage waveform collapse, resulting in poor power supply quality and 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 then smoothed through a large electrolytic capacitor before entering a true DC/DC converter. The input surge current is generated during the initial charging of the electrolytic capacitor, and its magnitude depends on the amplitude of the input voltage during 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 negative temperature coefficient thermistor ntc is undoubtedly the simplest method to suppress input surge current so far. Because NTC resistors will 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 supply is started, the NTC resistor will rapidly 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.
Advantages:
Simple and practical circuit with low cost
Disadvantages:
1. The current limiting effect of NTC resistors is greatly affected by environmental temperature: if the resistance is too high and the charging current is too low during low temperature (below zero) starting, the switching power supply may not be able to start; If the resistance value of the resistor is too small during high-temperature starting, it may not achieve the effect of limiting input surge current.
2. The current limiting effect can only be partially achieved during a brief interruption in the main power grid (about a few hundred milliseconds). During this brief interruption, the electrolytic capacitor has been discharged, while the temperature of the NTC resistor is still high and the resistance value is small. When the power supply needs to be restarted immediately, the NTC cannot effectively achieve current limiting effect.
3. The power loss of NTC resistors reduces the conversion efficiency of switching power supplies.
Option 2
When making low-power switching power supplies, directly use power resistors to limit surge currents.
Simple circuit, low cost, and almost unaffected by high and low temperatures in terms of limiting surge currents
Disadvantages:
Only suitable for small power switching power supplies
● Significant impact on efficiency
Option 3
The NTC thermistor is connected in parallel with an ordinary power resistor to limit surge current
When starting at room temperature, the resistance value of the power resistor and thermistor in parallel is used to limit the surge current. When starting at low temperature, the resistance value of the NTC thermistor increases sharply, but the resistance value of the power resistor remains basically unchanged, which can ensure low-temperature starting. However, during high-temperature experiments, the surge circuit is also large.
Advantages:
Simple and practical, with good results for starting at room and low temperatures
Disadvantages:
● Significant impact on efficiency
High temperature surge current
Option 4
A series fixed resistor is used in conjunction with a thyristor to limit the input surge current. When powered on, Vs is cut off, and the current passes through R1, which acts as a current limiting device. When certain conditions are met, VS conducts and opens R1 circuit. The efficiency loss is greatly reduced.
Advantages:
Low power consumption
The limitation of surge current is almost unaffected by high and low temperatures
Disadvantages:
Large volume and high cost
