3 ms·

It negotiates and then switches to 20V to deliver 100W, so it only needs to run 5A through the cables. Wire gauge requirements are related to current, not powe

by dheera 3y ago

It negotiates and then switches to 20V to deliver 100W, so it only needs to run 5A through the cables. Wire gauge requirements are related to current, not power; that's why high-speed trains usually operate at 25000V or 50000V to allow their megawatts of power to be delivered through a ~1cm diameter overhead line. That said, I still hate USB-C as a power standard. I would have much preferred 2 connectors just supplying 20VDC, no questions asked, with a standard connector, just like 120VAC power works. Pick another connector for the USA low-voltage standard, none of this negotiation fuss, none of this complicated power circuitry, and much harder to get cables wrong. With the complexity of USB-C there are just way too many substandard cables flooding the market, and average Joes don't understand the difference between a 100W-capable cable and a 60W-capable cable and one that is charging-only, one that is USB 3.0, one that is USB 3.2, one that is USB 3.2 + 100W charging and all of that. People just buy "5-star" rated cables online that often don't meet the standard and it's a shitshow.

userbinator 3y ago
There's also the added failure mode of a PSU deciding it needs to supply 20V to a device that can only withstand 5V.
- ilyt 3y ago
  You need to have device side protection for that anyway because broken PSU can happen in many ways you can't really predict, including "someone implemented that monster of a standard wrong"
- userbinator 3y ago
  Care to add any counterarguments?
  - booberbinator 3y ago
    Sure. Since you asked so nicely, here's a datasheet that illustrates a device which integrates overvoltage protection. Plug a malfunctioning 20V USB-C power supply into your phone, and the phone will internally disconnect the USB-C connection to protect itself. https://www.ti.com/lit/ds/symlink/tps65987d.pdf https://www.ti.com/lit/ds/symlink/tps65987d.pdf I incinerated 250 seconds of my time, or about a billionth of my life for this answer for you - for free! Hope the little dopamine hit was enough for you today. I won't be back.
  - Dylan16807 3y ago
    A PSU "deciding" to do that is an ultra rare case, and probably dwarfed by PSUs that simply break internally and deliver the wrong voltage, which can happen to any PSU. The closest thing I've heard of was a cable that pretended it was a device, making it so that unplug events didn't register properly.
lxgr 3y ago
That would force all power adapters to support a fixed maximum current, which in many cases would mean either wasted materials (consider e.g. shipping a 100W adapter with a smartphone) or unsafe conditions (with a device attempting to pull 5A from an adapter supporting only much less than that). I'm really happy with being able to plug my laptop into my tiny 5V, 2A power adapter and slowly charging it, yet being able to fast-charge it at home at 100W using the same cable on a larger power adapter.
- ilyt 3y ago
  > That would force all power adapters to support a fixed maximum current, which in many cases would mean either wasted materials (consider e.g. shipping a 100W adapter with a smartphone) or unsafe conditions (with a device attempting to pull 5A from an adapter supporting only much less than that). No, it would not require any of that. It would require only for cable to signal it's max current capacity. So cheap cable would allow for say 20v/0.5A (10W), while decent one could signal 2A(40W) or max. Simplest way of signal would be as the previous USB buses used, via a specified value resistor on one of the pins. More complex would be signalling the power supply power to the device on the other side, especially if you want dynamic power or reversible charging direction. But USB-PD is generally horrifically complicated for what it does
  - lxgr 3y ago
    > It would require only for cable to signal it's max current capacity. As you say yourself a bit later, you'd need at least two "simple" resistors (one for the cable itself, one for the adapter) – otherwise it's only fixed-cable power supplies (and I really, really hate having to throw away an entire high-quality power supply just because the cable frayed at the laptop end). > But USB-PD is generally horrifically complicated for what it does Which parts would you suggest are safe to omit, in a world where you want to be able to charge both small accessories (Bluetooth headphones etc.), smartphones, and large laptops using a single connector?
    - ilyt 3y ago
      > As you say yourself a bit later, you'd need at least two "simple" resistors (one for the cable itself, one for the adapter) – otherwise it's only fixed-cable power supplies (and I really, really hate having to throw away an entire high-quality power supply just because the cable frayed at the laptop end). You could have end device regulate itself by looking at voltage drop, akin to MPPT. And note that simple (sub-10W) devices could opt to implement nothing whatsoever and just get the minimum 20V/0.5A, making small accessories need nothing whatsoever when it comes to extra chips to handle it. You really need communication only when you start needing features like switchable sink/source function or voluntarily lowering power usage. >>But USB-PD is generally horrifically complicated for what it does > Which parts would you suggest are safe to omit, in a world where you want to be able to charge both small accessories (Bluetooth headphones etc.), smartphones, and large laptops using a single connector? Not talking about featureset but how hideously complex the implementation is. I remember some early adopters tried to implement it using a microcontroller and it ate like 30+kB of code and it was more than half of total product's code The spec (3.0, rev 1.1) is 574 pages. No fucking wonder we have so many devices implementing it subtly wrong. Hell, they even decided "no, none of the existing serial protocols fits our super special use case, need to invent that too. At least they didn't reinvent CRC32...

by dheera 3y ago

userbinator 3y ago

There's also the added failure mode of a PSU deciding it needs to supply 20V to a device that can only withstand 5V.

ilyt 3y ago
You need to have device side protection for that anyway because broken PSU can happen in many ways you can't really predict, including "someone implemented that monster of a standard wrong"
userbinator 3y ago
Care to add any counterarguments?
- booberbinator 3y ago
  Sure. Since you asked so nicely, here's a datasheet that illustrates a device which integrates overvoltage protection. Plug a malfunctioning 20V USB-C power supply into your phone, and the phone will internally disconnect the USB-C connection to protect itself. https://www.ti.com/lit/ds/symlink/tps65987d.pdf https://www.ti.com/lit/ds/symlink/tps65987d.pdf I incinerated 250 seconds of my time, or about a billionth of my life for this answer for you - for free! Hope the little dopamine hit was enough for you today. I won't be back.
- Dylan16807 3y ago
  A PSU "deciding" to do that is an ultra rare case, and probably dwarfed by PSUs that simply break internally and deliver the wrong voltage, which can happen to any PSU. The closest thing I've heard of was a cable that pretended it was a device, making it so that unplug events didn't register properly.

lxgr 3y ago

That would force all power adapters to support a fixed maximum current, which in many cases would mean either wasted materials (consider e.g. shipping a 100W adapter with a smartphone) or unsafe conditions (with a device attempting to pull 5A from an adapter supporting only much less than that). I'm really happy with being able to plug my laptop into my tiny 5V, 2A power adapter and slowly charging it, yet being able to fast-charge it at home at 100W using the same cable on a larger power adapter.

ilyt 3y ago
> That would force all power adapters to support a fixed maximum current, which in many cases would mean either wasted materials (consider e.g. shipping a 100W adapter with a smartphone) or unsafe conditions (with a device attempting to pull 5A from an adapter supporting only much less than that). No, it would not require any of that. It would require only for cable to signal it's max current capacity. So cheap cable would allow for say 20v/0.5A (10W), while decent one could signal 2A(40W) or max. Simplest way of signal would be as the previous USB buses used, via a specified value resistor on one of the pins. More complex would be signalling the power supply power to the device on the other side, especially if you want dynamic power or reversible charging direction. But USB-PD is generally horrifically complicated for what it does
- lxgr 3y ago
  > It would require only for cable to signal it's max current capacity. As you say yourself a bit later, you'd need at least two "simple" resistors (one for the cable itself, one for the adapter) – otherwise it's only fixed-cable power supplies (and I really, really hate having to throw away an entire high-quality power supply just because the cable frayed at the laptop end). > But USB-PD is generally horrifically complicated for what it does Which parts would you suggest are safe to omit, in a world where you want to be able to charge both small accessories (Bluetooth headphones etc.), smartphones, and large laptops using a single connector?
  - ilyt 3y ago
    > As you say yourself a bit later, you'd need at least two "simple" resistors (one for the cable itself, one for the adapter) – otherwise it's only fixed-cable power supplies (and I really, really hate having to throw away an entire high-quality power supply just because the cable frayed at the laptop end). You could have end device regulate itself by looking at voltage drop, akin to MPPT. And note that simple (sub-10W) devices could opt to implement nothing whatsoever and just get the minimum 20V/0.5A, making small accessories need nothing whatsoever when it comes to extra chips to handle it. You really need communication only when you start needing features like switchable sink/source function or voluntarily lowering power usage. >>But USB-PD is generally horrifically complicated for what it does > Which parts would you suggest are safe to omit, in a world where you want to be able to charge both small accessories (Bluetooth headphones etc.), smartphones, and large laptops using a single connector? Not talking about featureset but how hideously complex the implementation is. I remember some early adopters tried to implement it using a microcontroller and it ate like 30+kB of code and it was more than half of total product's code The spec (3.0, rev 1.1) is 574 pages. No fucking wonder we have so many devices implementing it subtly wrong. Hell, they even decided "no, none of the existing serial protocols fits our super special use case, need to invent that too. At least they didn't reinvent CRC32...