1 Introduction

In-rush current is the peak current that an electric device draws at power up, before the current settles to a normal operating current. The device's integrated circuits (ICs) have varying amounts of capacitance at the IC's pins. A discharged capacitor is essentially a short circuit when a fast-rising voltage is first applied as shown by the equation below:

where i_C is the current through the capacitor

C is the capacitance

dv is the change is voltage across the capacitor

dt is the time for the change in voltage

If the capacitor's voltage slew rate (dv/dt) is high, the capacitor sinks a high current, commonly called inrush current, until the capacitor fully charges. The current spike places significant stress on the power supply/battery that is applying this voltage and/or any other components or PCB traces through which the current flows. This stress potentially results in thermal damage or electrical shutdown if the components are not rated for the surge. For example, if a USB-C source has negotiated only 500mA output current but the inrush current is 1.5A, the USB-C source detects over current and drops to a lower power state. Designers often consider adding soft start circuitry to slowly charge high capacitive nodes to reduce the current spike. If the component preceding a capacitor is a MOSFET, a common method to reduce this current spike is to slow the FET's turn on time or slew rate and limit the downstream capacitor charging current.