Technique for Limiting DC Power Supply Current Switching
1. Series resistance method
If the resistance is large and the pulse current is small, but the power consumption on the resistance is large, a compromise resistance value should be selected so that the pulse current and power consumption on the resistance are within an acceptable range.
When connected to a pulsed DC power supply, the series circuit resistance must be able to withstand high voltages and high currents. In this application, a resistor with a high current rating is reasonable. Wirewound resistors are generally accepted by DC power supply manufacturers, but in high humidity conditions resistors should not be wirewound. Due to the electrical resistance of wire windings in high humidity conditions, the instantaneous thermal stress and expansion of the windings will degrade the performance of the protective layer and may damage the electrical resistance due to moisture intrusion.
2. Thermal resistance method
In small power switching power supplies, when the switching power supply starts, the thermistor has a higher NTC resistance value, which can limit the peak current. As the NTC heats up, its resistance value decreases, which reduces power consumption under operating conditions.
The thermistor method also has disadvantages: During start-up, the thermistor needs time to reach its resistance value under operating conditions. If the input voltage is close to the small value at which the power supply can work, there will be a large voltage drop at first start-up due to the large thermistor. The power supply can run in hiccup mode. When the switching power supply is turned off, the thermistor needs a cooling time for its resistance to increase to normal temperature. Cooling time is typically 1 minute, depending on equipment, installation method and ambient temperature. After the switch is turned on after a power failure, the thermistor has not cooled down, and the inrush current has lost its limiting effect at this time, so the power supply that controls the inrush current in this way cannot be turned on after a power failure.
3. Active inrush current limiting method
For high-power circuit breakers, the inrush current limiter must be short-circuited during normal operation, which can reduce the power consumption of the inrush current limiter.
In this triac start circuit, the thyristor is only powered through a coil on the main breaker transformer. The delayed start-up of the SCR is provided by the slow start-up of the switching power supply, allowing the input resistor R1 to fill up the input capacitance before the power supply starts up.
