Introduction to Methods for Limiting the Current of Switching DC Power Supplies
There are multiple influencing factors for the inrush current of switching DC power supplies, such as input voltage, input line resistance, internal input inductance and equivalent impedance, equivalent series resistance of input capacitors, etc. These parameters change with the layout of the DC power supply system and the variation of each switch, which makes evaluation difficult. These parameters vary according to the different layouts of the DC power supply system. The most accurate method is the actual measurement of the pulse current amplitude. When measuring the pulse current, the amplitude of the pulse current cannot be changed by inserting a sensor specified by a Hall sensor.
Series Resistance Method
If the resistance is large, the pulse current is small, but the power consumption on the resistance is large. A compromise resistance value should be selected to keep the pulse current and the power consumption on the resistance within an acceptable range.
When connecting a pulse DC power supply, the series circuit resistance must be able to withstand high voltage and high current. In such applications, a resistor with a high rated current is more reasonable. DC power supply manufacturers usually accept resistors wound with wires, but in high - humidity conditions, resistors should not be wound with wires. Because of the resistance of the wire winding under high - humidity conditions, the instantaneous thermal stress and expansion of the winding will reduce the performance of the protective layer and may damage the resistor due to moisture intrusion.
Thermal Resistance Method
In low-power switching power supplies, when the switching power supply starts up, the thermistor has a relatively high NTC resistance value, which can limit the peak current. As the NTC heats up, its resistance value will decrease, which reduces the power consumption under operating conditions.
The thermistor method also has disadvantages: during the startup period, the thermistor takes time to reach its resistance value under operating conditions. If the input voltage is close to the minimum value at which the power supply can operate, due to the effect of the large thermistor, the voltage drop is large during the first startup, and the power supply may operate in hiccup mode. When the switching power supply is turned off, the thermistor needs a cooling time to increase its resistance to the normal temperature. The cooling time is usually 1 minute, depending on the equipment, installation method, and ambient temperature. After a power outage, when the switch is turned on again, the thermistor has not cooled down yet, and at this time, the inrush current loses its limiting effect. Therefore, after a power outage, the power supply that controls the inrush current in this way cannot be turned on immediately.
