100W at 5V means 20A. We usually recommend 4 A/m2 max for copper conductor sections, so it would require two 5mm2 wires in the cable here. It would feel more like a rod than a cable IMO. Anyone got an idea how they plan to address this? Higher voltages?
Well they're pretty limited in what they can do. Most things you'd plug a USB cable into are not thing you'd want to plug higher voltages into, so that would require device-side voltage conversion which is wasteful in energy, space and complexity.
I suspect that the 100W figure is actually just a pulsed maximum specification. The thing is that current ratings for wires are actually specified as a max. continuous current for a given temperature rise in the conductor, per unit length. So blowing 3-4x the current through a conductor for a very short time (think of flashing a bulb or moving a servo) is not a big deal. You just get a transient heat rise. Also, most applications simply don't require 20A. Even microwaves and kettles stay below the 15A residential fuses. (Although they do come close. I once had a shitty basement suite with an underrated fuse. If I ran my toaster and my kettle at the same time the breaker would flip!)
Almost all your USB-powered devices have voltage converters with varying inefficiencies.
It should also be noted that a switched mode voltage converter can have well over 90% efficiency, even with large changes in voltage.
You should also remember that those microwaves and kettles are getting up to 15A @ 120VRMS continuously, which works out to 1800W. You can verify the actual power output of a kettle by timing how long it takes to boil a liter of water, and calculate power from this time and the specific heat capacity of water.
There are some practical limits to voltage conversion if you want to keep that high efficiency. Probably most important is that your switching frequency shouldn't be as high as it is in most small devices (because high freq. allows you to use smaller components).
In any case, as dfox mentions, most internal voltage level conversions won't be switched, because it adds complexity. They will be some form of linear regulation s.t. they can move between logic levels. That's different than moving from whatever high voltage is on your 150W USB line into a level that won't fry CMOS circuitry. There's a reason that the wall-->DC plug conversion usually happens in a brick on your power cable. Switched mode will be used as sparingly as possible, such as when you also need AC signals rectified, if you need both buck and boost depending on a battery or something, or if you need to be able to modify the control loop dynamically.
> those microwaves and kettles are getting up to 15A @ 120VRMS continuously, which works out to 1800W
Well that's kind of my point. Even at 1.8kW those devices don't need to draw 20A continuous (or even pulsed, because of the fuse). Basically no matter what you're doing, the copper losses are roughly fixed by the hardware. What you can control are heat dissipation and current levels, and it's a lot more fun to play with Ohm's law than try to fight against thermodynamics.
USB delivers 5V power and has 3.3V signaling levels, so any reasonable bus-powered device has to contain some kind of voltage conversion. Most common solution is LDO, voltage limits in USB specification even seem to imply that solution. Designing switched mode power supply that meets all the USB 2.0 (don't know about 3.0 but it is probably mostly similar) requirements on minimal voltage on input, current draw in various states and so on is not exactly trivial.
Who usually recommends 4A/mm? Using the wire parameter calculator below, a 2mm diameter wire is capable of carrying 20A, with a 0.1V drop over a 1m (3") cable. A 2mm diameter stranded copper wire is not entirely unwieldy.
The reason those cables get hot is the same reason that you got them so (relatively) cheap: metal is a significant fraction of the cost of a cable. The manufacturer who produced the cable knew exactly what their customer was looking for, and provided a low cost cable with thin conductors.
Yeah, higher voltages. Can't remember off-hand if this tops out at 12V or 20V at the higher charge wattages, but they're definitely not using 5V for them.
