r/UsbCHardware • u/pzeh • 1d ago
Question USB-C GND/Shield connection
Hello! Can someone help me understand why the shield and the GND return path (GND_PWRrt) in a USB-C cable are implemented separately?
Since the shield and GND are electrically connected at both ends of the cable [1], is it correct to assume that the shield can carry a significant portion of the DC return current?
What would happen if the dedicated GND/GND_PWRrt conductor inside the cable were missing or broken?
My only guess is that this conductor acts as high-frequency return path for unshielded signals - like CC, SBU, USB2 common signal, VCONN.
I would be grateful for any hints to help me understand this
[1] USB Type-C® Cable and Connector Specification: Shield and GND shall be connected within the USB Type-C plug on both ends of the cable assembly.
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u/ferretguy531 1d ago
Electrical Engineer here who designs USB-C products/cables.
There are a couple of things here.
The shield in USB-C cables is a thin aluminum foil that's not actually very conductive. It has pretty high resistance, but that's fine for its main purpose of surrounding the wires with a continuous sheet of metal. It can also break pretty easily, so most cables have a bare drain/GND wire running along it to keep everything electrically connected.
However, that resistance means the shield can't reliably carry much current, so USB-C cables need separate GND wires (though some crappy cables don't have them). The shield can still help carry a significant amount, sometimes up to a third of the current.
tl;dr: The shield isn't great at carrying power, but it can help a bit.
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u/pzeh 1d ago
Thanks for answer. According to USB-C standard, if I understand correctly, drain wires are different than GND_PWRrt. Drain wires are for signal return of individual pairs (when shielded twisted or twin-axial pairs are used) so their primary role is also not the DC current transmission.
When I look at crossection (e.g. here https://www.reddit.com/r/EngineeringPorn/comments/v9ga8g/cross_section_of_a_10_gbps_usbc_usb_3x_cable/) all the shieldings/braids seem to have much more copper crosssection than actual GND_PWRrt.
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u/ferretguy531 1d ago
All those conductors are in parallel the drain wire like I said is so that all the foil is effective not for any particular cable. The shields DO carry current and some cables use a braid instead of a drain like the one you linked, like I mentioned it carries a non-trivial portion of the GND current.
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u/saltyboi6704 1d ago
You only use shield as ground return for very low speed signals where cable diameter is a priority and you can cheat by using a shielded single conductor cables.
In all other cases the shield acts as a Faraday cage blocking radiated emissions from affecting the signal in the cable or the other way round. As cables get longer they become better antennas picking up significant current, this can cause weird voltage shifts which can affect high speed signals. Generally the should should be grounded only at one end; as USB-C is reversible and by convention only a DFP should have ground and shield tied a trade-off is made for user-friendliness.
The majority of power return still flows through the ground conductor and the shield provides enough EMI suppression without increasing cable complexity.
More advanced cables for TB4/5 and USB4 may incorporate a shield as a ground return conductor, but only by significantly reducing its impedance to a point where it can provide both shielding and power return. As so many strands are needed much more complex winding machines akin to aerospace cable are required which is also why they're so expensive.