What is Inrush current protection?

When input voltage is applied to power supply, inrush current flows to smooth capacitor, then it will cause input switch melting / blown-up.
Therefore, inrush current protection circuit is installed in power supply to prevent it.

1. Resistor limit method (Fig.1)

There are 2 methods available. One is adding resistor in an input line, and the other one is using impedance of line filter. However, the power loss is always generated during operation in these methods. Therefore, these methods are used for the low output power unit.

Fig.1 Resistor limit method

2. Thermistor method (Fig.2)

Thermistor is installed in an input line to reduce the inrush current. This method utilizes thermistor characteristic below.

At low temperature: Thermistor has high resistance value.
At high temperature: Thermistor has Low resistance value.


  1. (1)Inrush current is suppressed by thermistor (represent high resistance value) when input voltage is applied.
  2. (2)Since thermistor temperature increases due to input current, the resistance value becomes low. Therefore, the power loss of thermistor is also low.

If input voltage is re-applied before the power supply gets cool down sufficiently, the inrush current would be higher because thermistor impedance is still low.

Fig.2 Thermistor method

3. Thyristor:SCR method (Fig.3)

SCR and cement resistor are used in this method.


  1. (1)Inrush current flows through cement resistor (R1) when input voltage is applied (SCR1: OFF).
  2. (2)SCR1 will be turned-on after some time delay (Resistor power loss will be reduced).
  3. (3)If C1 has not been charged completely when SCR1 is turned on, the second inrush current will be observed.

Fig.3 Thyristor method

In DC-DC converters, since almost all of product don't have an input smooth capacitor (C1), inrush current protection circuit is not installed. However, if the input voltage in which rise-up time is rapid is applied, or a mechanical switch is used for power-on/off, please note that the inrush current will be observed for a short time (a few micro-seconds).

4.DC-DC converter

 Since DC-DC converter doesn't have large capacitor in input circuit, inrush current protection circuit is not built in. Therefore, when input voltage is rapidly changed or applied by mechanical switch, inrush current would flow at a very short term. In case that external capacitor is used in front of DC-DC converter, inrush current protection circuit in Fig.4 is effective to reduce inrush current.

Fig.4 Example of inrush current limiting circuit for DC-DC converter

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