New 100W USB 3.0 Spec Can Charge Laptops • Hacker News Board

Re: New 100W USB 3.0 Spec Can Charge Laptops

#61

Post by Scene_Cast2 » Tue, Jul 24, 2012, 8:56 PM UTC

K2h wrote: post #57

It is very interesting to see the group move to a switchable voltage, expecting to support 5V, 12V and 20V. My first reaction was 'oh no.. going to fry some 5V electronics with 20V' but if all the negotiation is done right - and not too easy to spoof it should keep people from building cheap adapters to go inline, request the wrong voltage and fry things. The voltages are a nice choice. 5V: backwards compatibility. E…

3.3V and 1.8V are often used as VCC in embedded processors, and they're the ranges for hard drives & RAM. If you're dealing with those low powered devices, there's usually a dirt-cheap and tiny DC-DC converter on-board. And you're not going to see anything high-powered enough to use these voltages, I don't think.

RE: 12V. Your car's voltage supply has huge margins (ripple, DC offset, etc), but in general - sure.

Re: New 100W USB 3.0 Spec Can Charge Laptops

#62

Post by Dylan16807 » Tue, Jul 24, 2012, 10:12 PM UTC

ars wrote: post #56

Dylan16807 wrote: post #53

Earlier quoted context omitted.

Having a centralized efficient 20v transformer could actually lead to better efficiency overall. A low voltage DC-DC converter can be far above 80% efficiency without any fancy components. And when I called 120v adapters messy I meant for the kind of thing I see that runs off 20v. Relatively high-powered computers and game consoles and monitors.

A centralized transformer wouldn't work well. First you would have to run a lot of very fat wires in order to serve all the outlets. But worse, power supplies have an optimum efficiency, usually at around the 75% usage point. If you have a power supply large enough to power everything, it's not efficient enough to power just a few things. > And when I called 120v adapters messy I meant for the kind of thing I see tha…

Those things tend to have bulky external power supplies that transform 120 volts into 20. 'whatever they use' is already 20 volts, with minimal internal components to get any other levels. You'd be able to eliminate a lot of awkward transformers.

It's probably not worth wiring a whole house with fat wires but in theory having nice safe universal plugs without dealing with external transformers would be nice.

I'm sure if someone wanted to design a supply that's efficient at different currents they could. Worst case would be 2-3 different supplies glued together with a management chip.

Re: New 100W USB 3.0 Spec Can Charge Laptops

#63

Post by catonthe » Wed, Jul 25, 2012, 3:40 AM UTC

ars wrote: post #50

Dylan16807 wrote: post #49

Earlier quoted context omitted.

I find that argument suspect when I look at all the laptops that run on 19 volts. Internal adapters are a lot messier for 120v. Most computing things love the 12 volt range, my screen uses 19 volts, a fancy light bulb is either LED and low voltage or CFL and gets no benefit from 120. Higher voltage is better for distribution and for very high power loads (including heaters). But a lot of things like low voltage.

> Internal adapters are a lot messier for 120v. Why do you say that? Look at any USB charging plug - very simple inside. You only need "messy" electronics for high power or high (above 80%) efficiency. Basically you would need 20v -> 5v vs 120v -> 5v - there isn't really a lot of difference. (There is some, but not enough to make changing everything worthwhile.) Also, the double conversion isn't great (you would need…

>> Internal adapters are a lot messier for 120v. >Why do you say that?

Well, for starters, most for-profit enterprises like to avoid liability. 20 V things are considered "low voltage" for regulatory purposes while 120 V things are considered "high voltage." That has certain implications for how you have to electrically insulate your portable device.

Then you also have to consider the difference between AC and DC. A power supply meant to connect to the wall has to have an energy storage device (usually a capacitor) for the 120 times per second that the wall delivers no energy.

Designing and manufacturing a device to power from 20 VDC (+/- 10% or so) is amazingly easier than one that must slurp 120 VAC.

>Basically you would need 20v -> 5v vs 120v -> 5v - there isn't really a lot of difference

This is completely wrong. 120 VAC is 120 V RMS, which has peaks of +/-170 V. Go to digikey and compare the price of 340V capacitors to 40V capacitors. Then consider the fact that your low voltage capacitors only need to filter relatively high-frequency ripple and occasional step-loads, while your high voltage caps need to supply energy when the wall isn't (so they needs to have much more capacity).