>> SEPIC has the great advantage of input voltage being allowed to 
>> vary
>> from below to above the output voltage

> SEPICs are cool because they are simple, but the efficiency is not 
> super
> great.

> But you don't need to use a special controller, I used a UC3843 in 
> this one:
> http://bobblick.com/techref/projects/mp3book/mp3book2/mp3book2.html

> You can put both inductors together one one core and gain a little
> efficiency.

Bob's circuit is a good one to study for anyone with a moderate amount 
of understanding of switch mode supplies but no experience of SEPICS. 
It has a number of additions to the basic design whose function can be 
worked out and mentally removed as you strip it back to the basic 
core. There's an on/off circuit, cycle by cycle current limit using 
FET Rdson combined with hold off control while transferring energy and 
more. Well worth "pulling apart".

When one has finished there's the temptation to reach for a 555 or 
perhaps a package of comparators or ... I'll resist that for now 
(should be working) but it would be interesting to see how basic a 
SEPIC one could devise.


    RM

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