Rob Thorpe (rt.delete@this.nowhere.com) on May 15, 2020 6:36 pm wrote:
> Travis Downs (travis.downs.delete@this.gmail.com) on May 13, 2020 3:07 pm wrote:
> > Danjel McGougan (danjel.delete@this.mcgougan.se) on May 13, 2020 6:52 am wrote:
> > > David Kanter (dkanter.delete@this.realworldtech.com) on May 11, 2020 7:37 am wrote:
> > > > Friends, posters, and lurkers,
> > > >
> > > > Power delivery is one of the most significant challenges in modern processors. The power
> > > > delivery network (PDN) must meet the demanding requirements of modern CMOS technology,
> > > > supply power with excellent efficiency, and swiftly respond to changes in power draw.
> > > >
> > > > I just published a new post that goes into detail on power
> > > > delivery: https://www.realworldtech.com/power-delivery/
> > > >
> > > > It includes a brief discussion of system level power delivery, Intel's FIVR, and decoupling capacitors.
> > > >
> > > > As always, please take a look and comments/feedback/questions welcome here!
> > > >
> > > > David
> > > >
> > >
> > > Thank you for a great article!
> > >
> > > Just a minor nitpick:
> > >
> > > You mention lithium-ion batteries in laptops on page 2 and say that they output 3.7V DC.
> > > While it is true that an individual cell nominally outputs 3.7V, the battery pack of a
> > > typical laptop have multiple cells in series delivering a higher voltage. Typically there
> > > are 3 or 4 cells in series delivering ~11V or ~15V nominally from the battery pack.
> > >
> >
> > Yup, and I guess that's why power bricks for laptops usually output 15V - 20V: so they can
> > use the same step-down DC-DC converter for both wall and battery input. The similar voltage
> > range means that you could efficiently use a single converter optimized for that range.
> >
> > Or maybe 15-20V is just a convenient voltage on the safety vs power-loss tradeoff curve.
>
> The other reason is charging the battery pack itself.
>
> The cable from the charger to the laptop is quite long. You don't really want to run ~3.7V though it at
> lots-and-lots of amps. It would be very inefficient. So, that link has to be a fairly high voltage.
>
> Once you've done that you really want the battery pack at around the same voltage, at
> least just below. That way it can be charged from the cable. If the battery pack used
> a much lower voltage then you'd need *another* DC-DC converter for that. That's because
> everyone expects to be able to use their laptop while the battery is charging.

You always need a dedicated DC/DC to charge the battery because you don't want to charge the battery with a constant voltage from the wall-adapter.
But as Travis pointed out everything is simpler if the voltage from the wall-adapter is just a bit higher than the battery's maximum voltage.