> In the attached circuit I've overlaid a standard buck-mode 
> converter
> (SW2, CR2, L1), and a boost mode converter (SW1, CR1, L1) 
> that shares
> the same inductor.

Well done.
That is in fact a standard topology implemented by a number 
of switching regulator ICs.
But, it's a good feeling to re-invent things independently - 
I do it all the time :-).

The circuit CAN be run as you say in boost and buck modes 
independently or, for increased simplicity at the cost of 
some performance, you can close BOTH switches at once and 
then open both at once for a boost-buck that runs smoothly 
from Vin below Vout to Vin above Vout. In that case the 
inductor is 'charged' by the both switches on mode and then 
"rung" (some object to that terminology) into the output.

On some boost-buck converters you will see interesting 
efficiency and/or output capacity curves that dip around 
Vout = Vin as the device transits between boost and buck.

Searches database. OK,

        http://focus.ti.com/lit/ds/symlink/tpic74100-q1.pdf

Fig 5 top of page 14  shows a nasty dip in Iout capability 
at Vin.
Not, I would have thought, a necessary "feature".

The time honoured MC34063 can easily be run in 2 switches at 
once boost buck mode with the addition of an extra switch 
transistor.




        Russell






----- Original Message ----- 
From: "William "Chops" Westfield" <westfw@mac.com>
To: "Microcontroller discussion list - Public." 
<piclist@mit.edu>
Sent: Monday, March 24, 2008 7:12 PM
Subject: [EE] Combining Boost and Buck switchmode 
converters...


>
> In the attached circuit I've overlaid a standard buck-mode 
> converter
> (SW2, CR2, L1), and a boost mode converter (SW1, CR1, L1) 
> that shares
> the same inductor.
>
> Is there some reason that this doesn't work as a 
> boost/buck variable
> voltage regulator? (for boost, you leave SW2 ON 
> continuously, for
> buck you leave SW1 off all the time.)  I don't think I've 
> ever seen
> this sort of configuration.  (for that matter, I haven't 
> seen many
> switch-mode variable power supplies at all...)  I can 
> forsee "issues"
> trying to run this with a traditional analog feedback 
> path, but if
> I'm using a microcontroller as the regulating element 
> anyway?
>
> Thanks
> Bill W
>
>


--------------------------------------------------------------------------------


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