Hi Russell,
Yeah, I'm starting to think the UC3906 (or newer  BQ24450) from TI may=20
just be the easiest way to go.

I don't have details yet as to how far down the SLA voltage will be when=20
put on charge. Maybe 8V - 10V at a guess.

A worst case of 8V for the discharged SLA, means 7V (Vsupply - Vbat) @=20
C10 rate (500mA) and about 3.5W dissipated in the series pass transistor.

Once the SLA voltage is up again, the dissipation would be a lot more=20
reasonable.  Maybe fan cooling over the heatsinks will be sufficient to=20
cope with the extra heat during the initial charge stage.
David...

On 22/09/2010 10:58 AM, RussellMc wrote:
> Linear charging is fine heat wise if you start from a "sensible" input vo=
ltage.
> The highest you need charge to is a shade over 14v in some modes and
> for a basic charger you probably don't need over about 13.6.
> So, say you have a 15 V feed. If you charge at 1C max then at 12V
> battery you need 15 Watt total energy and you lose (15-12) x 1 =3D 3
> Watts in the regulator. If each regulator has a TO220 or DPAK size
> device with individual heat sinking this is not too hard to handle -
> and the average dissipation will be lower both per channel and
> overall. 12/15 =3D 80% and at the top end its say 13.5/15 =3D 90% so say
> 85% mean for a linear regulator and in practice end to end many
> switchers may give you no better than tat. A switcher at 92%
> dissipates only half the heat.
--=20
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David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
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