Selecting Overcurrent rating

Thread Starter


Joined May 3, 2021
I was designing a dc dc converter and decided to use a fuse at its input for overcurrent protection, but had a doubt how do we determine the current rating at which the fuse should trip
The continuous primary side current is 4.9A


Joined Jun 3, 2019
Just measure the input current at the nominal input voltage when you have maximum output current. It depends on you how close you want to place the fuse to the max current. Fuse is a protector, how much you want to leave slack to the fuse to trigger?
Last edited:

Delta Prime

Joined Nov 15, 2019
Hello there! A belated welcome to AAC! :)The basic components of the switching circuit of a (DC to DC converter)can be rearranged to form a step down (buck)converter, a step-up (boost)converter,or an
inverter (flyback).
DC to DC converters can also isolate input voltage from output voltage.
For me to provide you any useful information, how to calculate & where to place your fuse,you would need to elaborate on your design please!


Joined Mar 30, 2018
A few things you need to know about fuses to help you select the correct rating.

A fuse will normally pass 1.6x its rating for a considerable time before operating, at over 2x rating the fuse should operate within a short time (<30 seconds).

A fuse is essentially a thermal device, the heating from the current flow causing the fuse element to melt.

With a circuit current draw of 4.9A, I would recommend a 5A rated fuse, which should avoid nuisance tripping and provide a reasonable level of overcurrent protection. If producing a number of units, then increase this to 6.3A to allow for variability in current draw.

Given the ability of fuses to pass a high fault current before operating, often their operation is not quick enough to prevent semiconductor damage due to the fault current.