Interesting. Could be that they are mandatory when you have ports sourcing higher currents? That would be reasonable, to avoid damage on shorts. -- But I've never seen such things on "simple" (500mA) ports, only polyfuses.
I'd really be surprised if there would be anything but polyfuses on the "old WinXP PC" my original comments's parent comment referred to.
Common implementation of overcurrent protection on USB ports consists of polyfuse on VBUS and feedback pin from this fused VBUS to controller (to detect tripped polyfuse), on some devices such polyfuse is even shared by multiple ports.
In theory, original USB1.1 expected the trip-point to be programmable, but most USB hosts are not implemented that way, because it requires some kind of low-drop high-side power switch that tends to be expensive.
Re:Should you fear your USB cable?
by omgtehlion on Friday March 04, 2016 @02:27AM
(#11222486)
Yup, these ICs are low-resistance (low drop) mosfets with some comparators in single package (handles 2 usb ports).
Not very expensive ($.5 a piece), but at scale may become costly.
Right here I have a roll (1000pcs) of chips that do exactly this (usb power management ICs) that cut power off and report this incident to controller. Maybe notebook manufacturers do not use them due to cost or space constarints...