Methods for limiting current in switching DC power supplies
1. Series resistance method
If the resistance is large and the pulse current is small, but the power consumption on the resistance is high, a compromise resistance value should be chosen to ensure that the pulse current and power consumption on the resistance are within an acceptable range.
When connecting a pulsed DC power supply, the series circuit resistor must be able to withstand high voltage and high current. In this application, a resistor with a high rated current is relatively reasonable. DC power manufacturers usually accept resistors wrapped with wires, but under high humidity conditions, resistors should not be wrapped with wires. Due to the resistance of wire windings under high humidity conditions, the instantaneous thermal stress and expansion of the windings will reduce the performance of the protective layer and may damage the resistance due to moisture intrusion.
2. Thermal resistance method
In low-power switching power supplies, when the switching power supply is started, the thermistor has a high NTC resistance value, which can limit the peak current. As NTC heats up, its resistance value decreases, which reduces power consumption under working conditions.
The thermistor method also has its drawbacks: during startup, the thermistor requires time to reach its resistance value under its operating conditions. If the input voltage is close to the small value that the power supply can operate at, the voltage drop will be significant during the first start-up due to the effect of the large thermistor. The power supply can operate in hiccup mode. When the switch power is turned off, the thermistor needs a cooling time to increase its resistance to normal temperature. The cooling time is usually 1 minute, depending on the equipment, installation method, and ambient temperature. After turning on the switch after a power outage, the thermistor has not cooled down yet, and the surge current has lost its limiting effect. Therefore, the power supply that controls the surge current in this way cannot be turned on after a power outage.
3. Active surge current limiting method
For high-power circuit breakers, the surge current limiter must short-circuit during normal operation to reduce the power consumption of the surge current limiter.
In this thyristor start-up circuit, the thyristor is only powered through a coil on the main circuit breaker transformer. The delayed start of the thyristor is provided by the slow start of the switching power supply, allowing the input resistor R1 to fully charge the input capacitor before the power supply starts.
