Michael Allen
BEAR Power Supplies

In AC/DC power converters above a few watts, a large inrush current flows when the input capacitors are suddenly charged during start-up. If unrestricted, this current can easily exceed 50 A at the peak of the ac cycle. This high inrush current severely stresses the converter's fuse, input rectifiers and power switch. It can significantly reduce the reliability and life expectancy of the modules. Universal power supplies are particularly subject to high inrush current, since their input capacitors must be large enough to handle the currents at line voltages as low as 110 Vac, but also able to handle high voltages of up to 240 Vac at turn on. This paper compares several methods of inrush current limiting and when to use them.