Dave Dilatush wrote:
>
> Roman,
>
> You're getting some very encouraging results!

Thanks for the input Dave. Being spice deficient
i'm doing it the old fashioned protobaord and cro
method, which has done me for 20 years so far. :o)

> >...I'm going to try
> >a lower H and lower R inductor, and play with it to
> >find the upper speed limits of the circuit. Since
> >these BC3?7 transistors are good for 100MHz I think
> >we could get good efficiencies right up in the 100's
> >of kHz range.
>
> If you do try to raise the operating frequency, don't be too
> disappointed if it doesn't work as well as expected; even using a
> transistor with a high Ft you could end up losing significant power
> due to transistor turn-off time.

Actually I should explain, I reduced Richard's circuit
to the basics, which was mainly simplifying all the
base drive circuits to a simple resistor. Then I decided
the weak point was that he relied on large enough ripple
at the output to make it oscillate, but not too much
ripple to make it worthless...

The first thing I did was re-design it to drive BACKWARDS
so I reduced the filter cap to a small 0.68uF cap and
then decoupled it from the final filter with a 10 ohm
resistor. This gives a deliberate high ripple of about
120mV on the cap, which slams the transistors hard
into a squarewave, beautiful to see on the cro, and
giving it good efficiency even at 55kHz which my first
circuit ran at.

Here is the circuit i'm testing now;
http://www.ezy.net.au/~fastvid/smps02.gif

Point A slams a very nice square wave, giving good
efficiency from about 20mA out to 80mA out. Point B
is a beautiful smooth sinewave, about 120mV P/P.
Freq is about 40kHz to 45kHz through the whole range,
and output voltage is totally ripple free.

Regulation suffers a bit from the 10 ohm resistor
in the output filter, but even as shown this circuit
is VERY usable for driving a 5v PIC from a 12v style
battery, giving 2:1 current gain with output currents
from 15mA to 100mA. Considering this is a cheap RF
"choke" inductor with 10 ohms dc resistance, and the
entire circuit is only 50 cents in parts or so, this
2:1 current gain might be very handy with battery
operated devices.

I'm charting stuff now, and will put it up on my web
page soon. Compared to a 5v regulator IC, even a pricy
low-dropout one, this circuit is at least 2x more
efficient, and maybe 3x more efficient than a 7805
regulator. And we're not even started yet!

Please comment, Dave and Russell, and I wish Richard
would add input as well. In my version of the circuit
i've sacrificed a bit of regulation to give a very
cheap, very squarewave, SMPS regulator with excellent
oscillation. The tiny RF choke is good to 100mA!!!
:o)
-Roman


> The speedup capacitor (C3) in the "Modified Prosser" circuit I posted
> improves the turn-ON time of Q2, but it has no role in helping turn Q2
> off any faster; it can't, because there's no pull-up drive from Q1.

With my mod, the transistors turn on/off very
well, even at 80kHz I tested. The trick was
ENCOURAGING a higher ripple at Point B, giving
much harder biasing of the transistors. The loss
is now shifted to regulation, which is a non-problem
if the device uses current in a set range. It still
compares ok to a resistor/zener regulator for
regulation, at about 3x more efficient. :o)


> If you intend trying to operate this circuit up around a hundred
> kilohertz, however, such a pullup resistor might help.  I'd suggest,
> as a starting point, using the same resistance value as R5.

I've eliminated the transistor biasing issues,
from what I can see on the cro, the next issue is
testing a toroid inductor that has 0 ohms dc
resistance, as the cheap RF choke i'm using
costs a lot in efficiency. Then the 10 ohm
output resistor can be reduced, and C1 "tuned"
to the match the inductor depending on what
current range is required.

The circuit shown gives:
* <1mA out = works as a linear regulator.
* 1mA to 12mA out = increasingly oscillates, gets more
  efficient. Totally reliable oscillation.
* >12mA out, slams a nice square wave, max efficiency
about 30mA out, good results to 100mA out, even at
120mA out with the cheap inductor gives 1.75 current
gain, MUCH better than any linear regulator.

Regulation is poor;
4.75v @ 10mA
4.60v @ 20mA
4.00v @ 80mA
These figures are easily adjustable based on the
zener voltage and the desired current range.
But it's usable! :o)
-Roman

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