Guía Técnica


> Back

3.2. Inrush current

When the switched-mode power supply is connected to the mains, the input capacitors should charge through the rectifier bridge. If no current limiting steps are taken, a very high current peak will be produced, which may solder the contacts of a switch, break the rectifier bridge or blow the input fuse. In low-range power supplies, a 5 or 10 ohm resistor in series with the power supply input is enough to reduce the inrush current to acceptable values. For medium power levels (20 to 200W) it is common to use an NTC, which provides a resistance high enough to limit the current to a reasonable level and when it warms up it reduces its resistance thereby avoiding too high losses. In power supplies with a higher power level, the solution normally is provided by one or several resistors, which, once the capacitors are charged, are crossed over by means of a triac or a relay.