Looking at this problem again.

Would it not make sense that I use 6 volt battery with a LDO regulator for
the 5 V?  It looks like the drop out voltage for switching regulators is
kind of large, around 1.5 Volts, so I would have problems.  Using a LDO reg
efficiency wise would be better than boosting to say 7 volts (or 12 volts a=
s
you suggest) and then bucking to 5 volts.  I should be able use most of the
battery capacity with a LDO.

Thanks,

Gordon Williams





----- Original Message -----=20
From: "Spehro Pefhany" <speff@interlog.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Sunday, November 20, 2011 11:04 AM
Subject: Re: [EE] Battery Configuration with boost or buck converter


> At 09:01 AM 11/20/2011, you wrote:
> >Hi,
> >
> >I have to create at dual power supply from batteries:
> >
> >5V @ 500 mA
>
> =3D 2.5W
>
>
> >12V @ 250 mA
>
> =3D 3W
>
> So, roughly equal power consumption on each.
>
>
> >I going to be using 2, 6 Volt 5Ahr sealed lead acid batteries and I'm
> >wondering if it is more efficient/ better to connect them in series or
> >parallel.
>
> Why not start with a 12V 5ah SLA (or 6V 10ah)? You'll have
> less worries about balancing during charge/discharge.
> Anyway, I don't think there's a simple single * answer to this question
for
> switching converters in general.
>
> >Therefore is it more efficient to have a 6V battery supply and step it u=
p
to
> >12 V and down to 5 V or start with 12V battery and try and get 12 volts
> >regulated and 5 volts out of it.
> >
> >When I have a buck regulator, what happens when the input voltage goes
below
> >the output voltage.  Is there a dropout voltage similar to a linear
> >regulator? Or, do I just get the unregulated battery voltage at that
time?
>
> You'll get a dropout, but at some point it should cut out entirely so as
not
> to damage the switch by causing it to go linear. Step up regulators
typically
> can't output much less voltage than the input, so if the input goes much
> higher than the desired output you lose regulation on the other side.
>
> That's assuming no transformers. If there are transformers you can have
> whatever ratio you want, within reason.
>
> >I'm also looking for a source of pre-made modules around 25 mm x 50 mm a=
t
a
> >cost <  $10 each in ones.  Anyone have a favourite source?
>
> There are tons of them (though few are < $10 in singles). Try a parametri=
c
> search at Digikey or whatever. Some of the cheaper ones don't have proper
> short-circuit protection, so that should be provided externally if
required.
> Galvanic isolation is nice to have. I've used the TDK CC-E series- they'r=
e
> cheap and small (but non-potted and electrically noisy). < $11. in
> singles.
>
> >I understand the boost/buck idea, but don't have any experience using
these
> >regulators so any insight of there actual operation would be useful.
>
> You can look at SEPIC converters- they require two inductors each. You
could
> also have a 6V-> 12V converter at 5.5+W then buck that down to 5V with a
2.5W
> converter, or a 12V->6V at 5.5+W then boost that to 12V with a 3W
converter.
> You lose efficiency both ways, so if each converter is 80% efficient, you
> end up with end-to-end 64% for the portion that goes through both, and th=
e
> first converter has to be sized for both loads AND the inefficiency of th=
e
> second converter. This works better if one load is much smaller than
> the other.
>
> * "correct" being assumed
>
> >Best regards,
>
> Spehro Pefhany --"it's the network..."            "The Journey is the
reward"
> speff@interlog.com             Info for manufacturers:
http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:
http://www.speff.com
>
>
>
> --=20
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

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