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I never knew that USB-C extensions are not allowed for a reason
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this post was submitted on 26 Dec 2024
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Anybody care to sum this up for people who can’t watch videos?
So a standard cable needs to be chipped to show its rating to the device, its not that the device can pull what it wants or can get, but the cable itself tells it what it can supply. Extension cables can’t do that, because it doesn’t know what it’s plugged into, and that would be if they even bothered to put a chip in. They instead piggy back off the chip for the main cable. The problem comes when you you have a 240 watt cable hooked up to a cheap 120 watt cable, with the device being told it can push 240, and starts to super heat the extension cable
This sounds solvable, doesn't it? Have the extension cable have a chip saying it can do X at maximum, then compare with whatever is to be extended and communicate the minimum of both upstream. Might not become a sleek cable-like design, but would extend the 240W cable with the extender safely staying at 120W
That's an active extension cable, which is essentially a single port USB hub.
Shouldn't it be possible to only do the negotiation part and otherwise bridge everything? Not having to do anything high-bandwidth actively should keep the silicon costs down.
Yes, and such cables already exist, like this splitter cable:
https://www.amazon.de/dp/B0CRZ6JJ6D (not an affiliate link)
It's not an extension cable, but it does exactly what you are suggesting. It gets the available PD profiles from the charger and then intelligently negotiates a profile that will work best to split the power to the 2 devices connected to it. The charger thinks it's just connected to 1 device, and the connected devices think they are directly connected to a charger.
Doing the same for with a USB C extension would be trivial, but it's probably hard to market such a cable when passive USB c extension cables are available at a fraction of the cost, even if those aren't compliant to the USB standard
I wish there was a clearer explanation or nomenclature for this. With things like cables and converters everything always seems to have a black box layer.
I don’t understand why there are so many PD profiles either. Maybe Cat-1 USB-C, Cat-2 USB-C, etc? Maybe just having a smaller set of voltage-defined profiles that have a safe maximum current rating? Maybe that’s already how it is? I don’t know
There are technical reasons for why so many PD profiles exist.
In fact they were not enough, which is why the USB Standard was extended with the "PPS" extension recently, which let's the attached device freely choose a voltage between 3V and 21V in steps of 20mv, and more importantly it let's the device freely change this voltage without interrupting the charge process. This change makes it possible for devices to bypass their own but in charging electronics and just directly forward the voltage coming from the charger to the device, improving efficiency and significantly decreasing how much the device hears up during charging
Sadly PPS is not found on many devices or chargers yet, and makes the already complicated USB C charging situation even more complicated for consumers
I am highly sceptical of anything that would connect USB voltage, no matter how finely negotiated, directly to the battery terminals. Finely tunable voltage over USB is useful for keeping the buck/boost converter on the device side small, though, or just efficient because it doesn't have to do as much work. If you can charge over standard PD extending to charging over PPS should only be a software change as your hardware is already more than capable.