On 17/01/2013 5:22 PM, RussellMc wrote:

> At nominal 12V x 3A your LEDs are dissipating 36 Watt (as you know) so
> the 6W is only an extra 16% efficiency wise - and the on PCB resistors
> are probably dropping 1.5V to 3V at 12V in.

Yes, I realise that.  The issue is more of the constraints of having the=20
6W dissipated in a small area.  The LED dissipation is spread out and=20
has been deemed (by the client) not to be an issue.

> A simple buck regulator will easily do what you want. This could be
> 90-95% efficient with due care, but even with little care 85%+ should
> be easy enough.

For some reason I was under the impression that a buck converter can't=20
have the output voltage equal to the input voltage.

> There are many of these available from ebay and elsewhere and it is
> very likely not worth building your own. Without looking I'd guess
> they may cost $US3 or less each. If you get a higher current one you
> can parallel more strips if desired.

The form factor comes into play here.  We need to make it fit a specific=20
space and have it very clearly marked as we are not installing them.

> If you use linear PWM and regulate v average or current average then
> the LEDs will be exposed to the peak currents and may get unhappy. If
> you add a series inductor to limit the peaks and smooth things out you
> are back at a buck converter.

I should have a sample of the LED strip soon and will look into the=20
configuration and limiting resistor values.

> If you arrange for continuous operation then Vout/Vin is =3D to PWM
> on/total time ratio (ie high% x Vin) and you can run it open loop.
> Closed loop control is very easy. You could use a PIC BUT you could
> also use a 555 :-).

The input voltage will typically be around 14V, but could be anywhere=20
from 12V to 14.5V I expect.  Client has asked for up to 18V input.

> If there is ALWAYS some Vin drop then adding a series R such that VLED
> is OK when Vin is at Vin_min will move dissipation into the resistor.

No, there may be times when Vin - Vout approaches zero.

Thanks for your reply.
David...

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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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