IEC62368 -external DC source with fuse as input for DC-DC Converter - Page 1
Forum section: electronics / general

author: Magnethicc
author_rank: Contributor
timestamp: October 09, 2022, 08:34:00 am
content: Hi Everyone,We are designing a DC-DC Converter where the input is a DC Battery with built-in fuse.according to IEC62368 do we need to put additional fuse in our DC-DC unit?Thank you.

author: TimNJ
author_rank: Super Contributor
timestamp: October 12, 2022, 12:10:39 am
content: I don't have an exact answer, but some food for thought..Is the battery an internal battery or external battery (e.g. car battery)? (Any external energy source is considered "mains".)Will you submit the DC-DC converter for IEC62368-1 certification by itself (without the battery), or will it be submitted with the battery connected?Is the breaking/interruption current of the battery's built-in fuse greater than the prospective fault current of the DC-DC converter if it were to fail?

author: Magnethicc
author_rank: Contributor
timestamp: October 12, 2022, 09:58:40 am
content: Hi Tim thank you for your questions.1. The input is a DC Generator providing 42V to my DC-DC thus the input is considered "mains" according to IEC62368. Yet I can't find a definitive requirement for need of input protection inside the DC-DC.2. I will not submit the DC-DC at-all. It is built-to-meet IEC62368. the customer, if he wants, will submit the system as a whole to get certification. so to answer your question - the DC-DC with the input generator (if chosen to be submitted) will be submitted together3. I've calculated the DC fault loop resistance and yes, the fault current is higher then the fuse rating.Further information:The DC-DC converter comprised of 4 interleaved converters so I am thinking of placing 4 fuses (1 to each converter) to protect the PCB (since my fuse will break before the generator fuse thus the fault current I2T will be smaller) and will call this issue solved.Does this sound safe for all the involved personals?My main concern is not following the IEC but making sure the design is safe and will not cause any hazzard.

author: TimNJ
author_rank: Super Contributor
timestamp: October 13, 2022, 08:45:25 pm
content: Quote from: Magnethicc on October 12, 2022, 09:58:40 amI don't think it explicitly requires a fuse, but think you need to consider the concept of "single fault conditions". (Check Annex B.4 and Ctrl+F "Single fault condition" and you will find some things to consider.) Without even thinking about the standard, I would simply consider what happens if the input of the DC-DC becomes a short circuit...due to insulation failure, component failure, etc. A fuse is usually good mitigation against this because it will disconnect the fault relatively quickly, not allowing significant dissipation which might cause fire.Quote from: Magnethicc on October 12, 2022, 09:58:40 amIf the fault current is higher than the existing fuse rating, you may have a hard time justifying the suitability of the existing fuse. If empirical testing shows that itcanreliably interrupt the fault current, you may be able to get away with it, but it's not something I would rely on...personally.I would probably stick with one fuse, unless you have some other concern which would necessitate separate fuses. If not, what happens and 1 out of 4 phases is not working? Do more bad things happen?