# Re: [RESEND PATCH v3 1/2] power: supply: add input voltage limit property

From: Pavel Machek
Date: Thu May 02 2019 - 17:13:21 EST

Hi!

> On Tue, Apr 16, 2019 at 10:42:30AM +0200, Enric Balletbo i Serra wrote:
> > On 16/4/19 9:19, Pavel Machek wrote:
> > >> This patch exposes a new property, similar to input current limit, to
> > >> re-configure the maximum voltage from the external supply at runtime
> > >> based on system-level knowledge or user input.
> > >
> > > Well, and I suspect it should expose input power limit, not input
> > > voltage limit.
> >
> > Oh, ok, I thought we were agree that input voltage had sense after had some
> > discussion in v3. Seems that no, let me try to give you another example...
> >
> > > DC-DC convertor efficiency normally does not much depend on input
> > > voltage....
> >
> > As we said we have a heat "problem" due the internal voltage conversions.
> >
> > Lets assume you have a linear regulator instead with a Vin range from 5V to 9V
> > and we want an output of 3.3V/1A
> >
> > For 9V:
> > Input power : P(in) = 9V x 1A = 9W
> > Output power: P(out) = 3.3V x 1A = 3.3W
> > Regulator power dissipated: P(reg) = P(in) - P(out) = 9W - 3.3W = 5.7W
> >
> > For 5V:
> > Input power : P(in) = 5V x 1A = 5W
> > Output power: P(out) = 3.3V x 1A = 3.3W
> > Regulator power dissipated: P(reg) = P(in) - P(out) = 5W - 3.3W = 1,7W
> >
> > In the first case the regulator needs to dissipate more power, hence the
> > temperature is greater than the second case.
>
> I would be surprised, if a linear regulator is being used in this
> place :) That would basically render functionality of higher voltage
> completley useless and a good reason to always limit to 5V. For the
> generic case I agree with Pavel, that control over the power (voltage
> * current) is the better choice. Still I believe it makes sense to
> have a control knob for the voltage available, since some hardware
> designs suck.
>
> For example the bad hardware design might be the remote side,
> that has issues providing some voltages and this would make it
> possible to debug that.

Ok, I agree it might be useful _somewhere_, mostly for hardware
debugging. But before if we add voltage_limit, lets add power_limit,
too; and for problems that can be solved using power_limit, lets use
power_limit...

Best regards,
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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