1
\$\begingroup\$

I would like to know if it is necessary (or recommended) to add fuses at the inputs when using multiple DC-DC converters. I have attached a quick schematic to illustrate what the design would look like if fuses are involved.

I guess in principle the fuses are there to protect the other DC-DC converters in case one becomes shorted.

However the DC-DC converters I plan to use seem to advertise "continuous short protection" and various other protections as well (ex: PYB20-Q48-S12 from CUI Inc.).

Are fuses really necessary if there is already protection in place? Would I be doing potentially more harm than good by adding them to the circuit? (ie: if I end up selecting fuses with incorrect thresholds)

Cheers!

enter image description here

\$\endgroup\$
8
  • \$\begingroup\$ If you don't know start surge currents and use a fast fuse, they can become unreliable like light bulbs. So if redundant, no need. If you expect installation failures from damaged goods, with hot service down time not permitted, maybe useful with slowblow. But not needed in PC PSU's. \$\endgroup\$ Jul 3 '18 at 18:16
  • \$\begingroup\$ What is at the load side of all your DC-DC converters? \$\endgroup\$ Jul 3 '18 at 18:59
  • \$\begingroup\$ @TonyEErocketscientist That's a good point, I think it makes sense to use slow fuses in this case, particularly since I will have some capacitors at the DC-DC converter inputs \$\endgroup\$
    – Vlad
    Jul 4 '18 at 13:28
  • \$\begingroup\$ @AliChen I had the loads hooked up and then I measured the steady input currents to all the DC-DC converters and they are in the ~0.100A range for each one. The total current drawn from the battery in steady state is about 0.450A \$\endgroup\$
    – Vlad
    Jul 4 '18 at 13:33
  • \$\begingroup\$ I mean what is happening with loads? Are they securely attached via reliable connections, or some user can shuffle with power leads and can accidentally short them? Why do you assume the case that "one becomes shorted"? \$\endgroup\$ Jul 4 '18 at 15:38
1
\$\begingroup\$

The answer depends on what you are trying to accomplish. The fuse should help protect the battery from driving a short circuit and protect the harness wiring, both of which could cause a fire.

At a minimum, a single fuse on the +28V coming from the battery before it branches out to the individual feeders is sufficient.

Branch fuses, as in your schematic, still achieves the goal of protecting the battery and wiring while providing additional redundancy since one blown fuse won't take down the other branches.

Although the DC/DC converters may have 'continuous short' protection, there are other reasons to have the fuse (wiring and battery protection).

\$\endgroup\$
1
  • \$\begingroup\$ My main focus was on protecting the other branches in case one DC-DC converter fails (by shorting). It's not really clear from the data sheets if the short protection on these DC-DC converters have enough circuitry to prevent damage to the surrounding input circuit (ie: the battery and other branches). I didn't consider the wiring and battery protection so much, it makes sense to worry about that too. Thanks \$\endgroup\$
    – Vlad
    Jul 4 '18 at 13:44
1
\$\begingroup\$

You must always add at least a fuse after the battery to protect the cabling.

You could use fuses for each load, but only if these fuses are lower rated or faster than the main battery fuse. Since you'd want these to blow first.
The advantage of these fuses is that one failed DC-DC does not take the entire system offline. And it reduces the let trough energy, reducing fire hazard of the failed DC-DC.

The short-circuit protection on the DC-DC converters will be on their secondary side, and of no use protecting the primary side.

\$\endgroup\$
0
0
\$\begingroup\$

You must always add a single fuse after the battery to protect the cabling.

You could use fuses for each load, but only if these fuses are lower rated or faster than the main battery fuse. Since you'd want these to blow first.
The advantage of these fuses is that one failed DC-DC does not take the entire system offline. And it reduces the let trough energy, reducing fire hazard.

The short-circuit protection on the DC-DC converters will be on their secondary side, and of no use protecting the primary side.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.