Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

A key consideration affecting the selection of fuses, filters and switchgear is the inrush current at the point of application of the AC power. The maximum inrush current is usually specified in the data sheet however there is more to this than a simple maximum value.

In lower power, lower cost products the inrush limiting is typically via a simple NTC thermistor which will provide protection from a “cold start” but may take a minute or more to return to its initial value following the initial switch on. This results in an inrush current many times the specified maximum during an off/on cycle even with a relatively long delay. The inrush current is also likely be specified at 25 deg C and may be significantly higher even for a cold start in higher ambient temperatures.

In products above a few hundred watts, this thermistor is likely to be taken out of circuit once the output voltage is established, to remove unnecessary power dissipation, removing this as an issue.

Inrush current is an important factor to be considered to avoid nuisance tripping of fuses and circuit breakers as well as reliability of switches and filtering. The fuse rating given in some data sheets is usually the rating of the internal fuse which is designed to operate only in the event of catastrophic failure and is not user replaceable. Determining the value of the equipment fuse needs to take into account the normal maximum running current, the inrush current and the effects of aging. The equipment fuse should also only be required to operate under catastrophic failure conditions as the electronic power supply overload protection will cater for any problems on the DC side.