Simple Inrush Current Limiting in Power Applications

Switching Mode Power Suppliy
Various Switching-Mode Power Supplies (SMPS), which are compact, light weight and offer high performance, are commonly used as power sources for electronic equipment. When powering up a SMPS, the device is mostly charged with a high peak inrush current to charge the smoothing capacitor. This surge current may have a negative impact on the service life of the capacitor, as well as risking damage to the power switch contacts or the rectifier diodes in your power supply, so you should carefully consider how to mitigate it.

Below is one possible solution to limit the inrush current to your SMPS, by inserting an NTC thermistor. NTC thermistors are widely used as a way of making a low cost and simple circuit for limiting inrush current in power supplies. The same result can achieved even when the NTC thermistor is connected after the rectifier circuit.

AC-DC power module
In general, AC-DC power module is a power supply that combines an AC-DC rectifier circuit and a DC-DC converter. It requires fewer external components to achieve a space saving optimized power system. By inserting an NTC thermistor (power thermistor), it is possible to effectively limit the surge current applied to the input and output capacitors when energized.

DC-DC converter
In a simple DC power circuit such as DC-DC converter, an NTC thermistor is used as a power thermistor and effectively limits the inrush current, and the input and output capacitors are charged when the power is turned on. The resistance of NTC thermistor becomes very low after being energized, which achieves a lower power loss than when a fixed resistor is used.

Inverter systems chiefly consist of a converter stage, an inverter stage and a DC link capacitor (smoothing capacitor) which is placed after the converter stage as below. When powering on, the device is charged with an inrush current which peak is several times larger than the steady current to charge the DC link capacitor. This inrush current may have a harmful effect on the service life of a DC capacitor or damage the semiconductor device. To protect against the inrush current, NTC thermistors are connected.

Three phase inverter:

Sinlge phase inverter:

For more useful information on inrush current and NTC thermistors, check out TDK’s “How to use NTC Thermistors for Inrush Current Limiting” application note here