hmm
What if I put just a single zener across the load? Something like a 10 W one? I guess thats the easiest and the cheapest way...

Yes, but don't forget the fuse.

But there are some caveats, such as whatever is causing the overvoltage must have the power to actually blow the fuse.

 

If you can find one with the needed voltage and tolerance.

https://eu.mouser.com/ProductDetail/Central-Semiconductor/CMZ5339B-TR13-PBFREE?qs=u16ybLDytRaULopxxdaF2A==

I guess this is the one... 5.6V zener voltage with 5% tolerance, so 5.32-5.88 V fluctuation, ISOUSB111 has absolute maximum of6V VCC, I will put 2 of these in parallel just for insurance... I guess this should work...

 

Yes, but don't forget the fuse.

But there are some caveats, such as whatever is causing the overvoltage must have the power to actually blow the fuse.

Yes, the fuse is vital. I decided to put one resettable fuse per USB port

https://eu.mouser.com/ProductDetail...=sGAEpiMZZMt8Ji9PfVTQzCfMRmef/UFQHT/2nsFzSqU=

As I understand, these resettable fuses are auto regenerating which makes it really convenient.

 

One more side question. Since I am designing USB 1.0/1.1 interface (on a 2 layer board), I faced the need of differential pairs for the first time. I did my research and found out that I have to use special calculators to calculate the differential impedance (my target is 90 Ohm). But Altium calculator and Saturn PCB impedance calculators are telling my different stories... It seems all the parameters I have entered are the same. How can 2 programs result in such different numbers? Which one should I trust?