Re: Design idea for low voltage SMPS

From: Steve (nospam_at_microsoft.com)
Date: 11/03/04 

Date: Wed, 03 Nov 2004 09:54:17 +0000

On 2 Nov 2004 15:31:44 -0800, Winfield Hill
<whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
> You can easily do this if you design a two-stage converter, as we
> discussed here on s.e.d. a few months back, IIRC.

Can you remember what sort of subject line it was Win, I'd like to browse
back?
I'll take a look at the two stage approach, thanks.

and On Tue, 02 Nov 2004 21:28:39 GMT, Joerg
<notthisjoergsch@removethispacbell.net> wrote:

>>1. The supply is a single rechargeable cell of only a nominal 1.2v which
>>rules out many of the usual IC SMPS control devices.
>>
>That is right. Your idea to use a blocking oscillator approach is a good
>one. Can't use FETs either since there isn't much to turn on the gates
>hard enough so it will most likely have to be bipolar transistors
>
>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.

>>2. The unit should need no user power switch and only consume a maximum
>>of perhaps a few 100uA when the load is disconnected.
>>
>There is the challenge. First you'll have to detect a load which can be
>done simply via a load sensing resistor and a transistor. That can
>trigger the converter. Afterwards this resistor needs to be bridged in
>order not to waste much power.
>
>Then you have to detect when the load current drops below a given limit
>and that's not so easy. You don't want to drop 700mV somewhere on a load
>sense resistor so it will have to be done in the millivolt range. Here
>you'd have to look at devices made for hearing aid applications. Don't
>expect a large variety since hearing aids are nearly all done with ASICs
>these days.
>
>Also, whatever you use, if it is for mass production you need to make
>sure there are enough of other markets for the parts you design in. Else
>they may be obsoleted and then you would be back to square one.

You've got that right! I've been bitten by that before with many components,
such as MCUs and specialist display controllers. The problem is often one of
availability rather than obsolescence.

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.

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.

Meanwhile, thanks everyone for the responses so far. 

-- 
Steve

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