piclist 2002\10\31\234109a >
Thread: Son of simple step-up SMPS challenge
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face BY : Russell McMahon email (remove spam text)



> Hi, here is an "entry" for Russell's SMPS challenge.
> It's not a high performance circuit (yet) but is MEGA
> CHEAP and will run a PIC from a single 1.5v cell
> (like an AA cell which are the cheapest battery).

These designs look excellent as a starting point.
This basic 2 transistor design will almost certainly fill the  Vpp, Opamp
rail, & FET gate drive applications. For these fixed input voltage,
relatively fixed output voltage applications, efficiency should be able to
be optimised. Due to the low passive parts count, the addition of a third
transistor is no great disaster.

As both transistors in the basic design share grounded emitters this would
lend itself to a surface mount dual transistor package with commoned
emitters.

I can feel a moderate power version coming on using the ULN2803 / 2003 or
variants. Unusual enough an application to cause some real puzzlement on
first encounter. There is enough power handling capability there to drive
significant loads eg possibly automotive cellphone or notebook chargers that
require more than 12 volts. Note that some of the less seen ULN2xxx variants
use higher value input resistors, allowing lower losses in the front ends.
The darlington configuration is a slight shame but not a fatal flaw. Maybe
even the internal catch diodes can be used as the output diode !!! :-) Using
a 3rd section / transistor as a voltage regulator, using a grounded emitter
resistor driven by a zener to output to clamp the base of Q1, allows a full
regulator using half a ULN2003 / 2004 / .... This means two such ccts could
be implemented in a  single IC. BC337 etc has a cheaper component cost but
more parts.

The design is perhaps less suited for the single cell to LED application in
its present state as its output trails off rather badly as Vin drops.
Probably another transistor to add some dynamic drive variation with Vin
would help. A multi-cell LED version may be OK. The droop of current
capability with voltage out would allow a LED to find its own operating
point rather better than a design with good hard regulation which would then
need current control.

5v to Vpp: Discrete version:

   1 IC (dual SOTxxx  transistor), 2 R, 2 C, 1 D, 1 L. 1 Z.

ULN2003 version (theoretical at this stage)

   1 IC (1/2 actually), 1 R, 2 C, 1 L, 1 Z

  Assumes R1 is internal & R2 added.
  Assumes internal catch diode OK (Add 1 D if no go)

A ULN200x version may allow a low voltage optimised version which starts on
0.9 or less volts to "pump up" / bootstrap  the Vin_local for a second
higher power optimised version which then takes over. This is precisely what
is done in some commercial designs which are intended to operate down to
very low Vin. The inductor ALWAYS draws power from the true Vin but the main
controller runs from a higher voltage bootstrapped Vin thereby allowing far
more efficient operation. Once the main system is running the startup supply
can be disabled. Component count could still be modest and startup to below
commercially realisable voltages may be possible. For the standard dry cell
(Le Clanche / Carbon Zinc / Alkaline etc) the usual endpoint is considered
to be 0.9v but useable energy is still there down to about 0.8V. A converter
that ran WELL at say 1V would be much better for eg NiCd applications where
the bulk of energy is available at lower voltages for std primary dry
batteries. Good low voltage operation allows a minimum number of solar cells
to be boosted to useful voltage levels.



       Russell McMahon

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