Re: Design idea for low voltage SMPS

From: Joerg (notthisjoergsch_at_removethispacbell.net)
Date: 11/03/04 

Date: Wed, 03 Nov 2004 19:06:43 GMT

Hi Steve,

>>LTC has some PWM controllers that are claimed to work down to 1V. Check
>>their web site to see if any can be used with external devices large
>>enough to handle your current requirement.
>>
>>
>
>Thanks, I'll take a look at those. Actually I prefer a discrete design for
>this for reasons of cost and parts availability.
>
>
That's the way to go, especially when it is manufactured abroad. For an
even better deal it pays to obtain a preferred parts list with pricing
from your prospective manufacturer.

>I've been toying with a self oscillating design with a FET and an
>autotransformer, with an overwinding for gate drive. Since I needed no
>output components imparting a load, it does indeed stop oscillating when the
>load is removed. I can re-trigger it with a simple periodic oscillator for
>low consumption when unloaded. I haven't yet dreamt up a satisfactory
>constant current characteristic though.
>
>It IS an interesting exercise though, and does prove I can use a low loss FET
>rather than a saturated NPN. I keep remembering the constant current
>requirement only needing to hold to 10% and wondering if I can achieve it
>within some magnetics without high gain active feedback. The requirement is
>for a low cost design probably to be manufactured in the far east.
>
>
You can achieve proper limits and range via the properties of the
magnetics but this is playing with fire. The materials tolerance needs
to be very tight. Also, you'd have to make absolutely sure that the
contract manufacturer won't change horses some day because they could
get a better deal from another ferrite manufacturer. How often did you
hear from a purchaser "oh, but the rep assured us that this is a 100%
second source"? There would have to be a very tight ECO control on the
transformer, or the whole unit for that matter.

It would be better if you could provide some kind of feedback to
regulate the output current. If you can post a schematic we could shoot
off ideas.

The two-step approach that Winfield and Graham suggested can get you out
of this bind but it would mean a cost increase. So maybe the first thing
would be to see if your existing idea could be furnished with feedback
at a lower cost.

>Mook:
>Yes it's a valid concern regarding supply volts drop. The cell is a 8A/hr
>unit capable of delivering large currents. I estimate it needs to operate
>down to around 1.1v or just under, under max load.
>
>
That would already be close to a 10% drop, even more reason for a
feedback strategy.

Regards, Joerg

http://www.analogconsultants.com

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