CIRCUIT DESIGN

Inrush Current

Overview

Power supplies are responsible for a large number of inrush current issues due to their large amounts of input and output capacitance. Advanced switch-mode power supplies will limit the inrush current to the output appropriately (either fixed or set by a resistor to a dedicated pin on the SMPS controller), but you still have the input capacitance to deal with.

Standard Resistors

Standard resistors are one of the simplest ways of limiting inrush current. They are typically used in low-power situations, as unlike many of the more complex inrush current protection methods, their resistance does not reduce after start-up, meaning they dissipate significant power continuously and can cause large voltage drops.

NTC Thermistors

NTC thermistors can be used for reducing the amount of inrush current.

They will always be some measurable amount of voltage drop when using a NTC, as some continuous thermal dissipation is required to keep the thermistor warm enough to be in it’s low resistance state.

The total energy $E$ provided by the inrush current is given by:

$$E = \frac{1}{2} C V_{in}^2$$

where:
$$C$$ = the downstream capacitance, in Farads
$$V_{in}$$ = the input voltage, in Volts