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Encouraged by Roman's input and results with his version of Richards two
transistor buck converter (not to mention Dave and Olin's input  (-
encouragement takes many forms :-)) ) I decided to try a purpose built 12 to
5v design and look at addressing some of the concerns raised.

Results were "pleasing". This is by no means a fully optimised design but
indicates what is able to be easily achieved.
The circuit is slightly more complex than Roman's. Whether the extra
complexity or the circuit itself are justifiable is up to potential users to
decide.

In the following discussion and data figures are quoted to RESOLUTIONS in
excess of their probable PRECISIONS . eg Iout = 10.78 mA. The use of
extended precision is to allow similar results to be compared and trends to
be seen as other  variables are changed. The number of significant figures
used are consistent with the observed resolutions in each case.

Efficiencies are measured including losses in R10 and also in a 10r
decoupling resistor in the Vin lead.
At 6 mA in and 10 mA out these add a few percent to the overall loss of
efficiency.

TARGET SPECIFICATION:
Target Vin / Vout was 12 v / 5v as per recent discussion.
Target nominal Iout was 10 mA.

I tried Vin from 7.9 to 30 volts and Iout at nominal 10 mA and 20 mA.

Best efficiency at 10 mA was 76% at 9v in.
At 12v in efficiency was about 72% .

"Current multiplier ratios" over that achievable with a linear supply
increased with voltage in giving 1.7 times at 12 volts, 2.4 times at 20
volts, 3.1 times at 30 volts. This figure is identical to the ratio of
efficiency of this circuit to efficiency of a linear power supply under the
same conditions.

This circuit was designed with the aim of operating at Vin = 12v and only
then was tested at other Vins.
Operation in the range 5 < Vin < 9 v could be improved to provide a
converter intended to optimise power output from a "PP3" 9v transistor radio
battery


Core component values were as shown on the diagram.

CATCH DIODE                Catch diode was BYV26 for most measurements with
results for a 1N4148 being compared for a few data points. The BYV26 was
superior but not by as much as I had expected. Say 1N4148 is ~~ 97% as
efficient.

INDUCTOR:                Inductor was either 120 turns or 30 turns on a
Micrometal 20 x 13 x 8 (OD ID height) core.
The 120 turns produced measurably better results (eg Iin = 6.27 mA versus
Iin = 7.83 mA at Vin = 12 V)
This was probably due to the very short switching period with the smaller
inductor.
This core is MUCH larger than that used by Roman. Results with miniature off
the shelf prewound will follow "in due course".



C1 was usually a 1 uF mylar but a 0.1 uF monolithic ceramic was tried for a
few data points.
The 0.1uF was about 95% as efficient - probably mainly due to the increase
in switching frequency which resulted.

CAPACITOR ESR        Dave raised the issue of output capacitor ESR which he
said was crucial to frequency of operation. I produced a pessimistic numeric
expression for affect of ESR on frequency of operation.
A 1 ohm resistor was placed in series with C1 to simulate a capacitor with
significant ESR. At 30v frequency of operation shifted from 20 kHz to about
38 kHz when  the series resistor was added. This shift is expected and is a
natural consequence of the method of feedback and is entirely acceptable in
envisaged applications. An ESR of this magnitude would be "unusual".

ZENER CURRENT        Dave and Olin have noted the low zener current in my
original design which would lead to somewhat reduced voltage regulation and
potentially extra noise due to operation on the flatter part of the zeners'
V/I knee. I increased zener current by adding R1 to ground from Zener anode.
The R1/R2 junction must now be driven to about 0.6 volts plus by the zener
before Q1 is driven on. At this stage zener current will be about 600/R1 mA.
This current can be made arbitrarily large by selecting appropriate R1. In
this case with R1 = 600r Izener is about 0.6 mA. In practice the regulation
achieved suggests the zener is being operated well enough up its knee for
typical applications. This "wasted" zener current represents an efficiency
loss of about 5% at 10 mA out!

A 5v1 zener was used. Output voltage was 5V +/- 0.05 volt over most Vin.
The fact that Vout < Vzener + 0.6 v indicates the zener is still operating
further down the knee than per spec sheet. Given Iz is about 0.6 mA this
is expected.


STATIC FEEDBACK / HYSTERESIS        I considered adding explicit static
hysteresis as suggested by Dave & Olin. While this would meet the
theoretical concerns raised as to the possibility of the circuit failing to
oscillate in some cases, in practice the circuit is so utterly convinced
that it wants to oscillate that I did not pursue this at this stage. I will
examine this in due course. Roman's latest design removed the static
feedback which had been present in Richard's initial design and relies
entirely on dynamic hysteresis as in this design. He also reports
enthusiastic oscillation in practice in all cases to date. Adding static
hysteresis should help switching waveforms and I will examine this aspect in
due course.

TURN ON/OFF TIMES                                 Comments were made about
the potentially long switching times of the main switching transistor. Drive
values were semi-optimised by tuning to minimise input current at a fixed
Vin
and Pout. The very small value of R5 (1k) was chosen for best efficiency.
With R4 = 10 k as shown, this has the effect of limiting circuit operation
to Vin > about 7 volts as R4/R5 form a divide and Q3 cannot be turned on
when Vin * R5/(R4+R5) is less than about 0.6 volts. For operation at lower
Vins R4 & R5 would need to be reoptimised. Use of a high side driver
transistor (current amplifier) for Q3 as per my previously posted FET based
design would allow good tune off drive without  increasing dissipation in R4
excessively. Turn on is generally less of a problem than turn off.

CIRCUIT OPERATION:

All transistors off.
R6 turns Q2 on.
Q2 on turns Q3 on via R4.
Q3 on supplies current via Q3, L, R10 to Rload and Cout.
V at  L1/R10 junction rises until Z1conducts and pulls R1/R2 junction to 0.6
volts.
Now Q1 on so Q2 off so Q3 off.
Q3 off causes L to 'ring" so left hand end of L goes to ground (actually
diode drop BELOW ground) due to D1.
V on C1 CONTINUES TO RISE die to stored energy in L.This is a primary part
of the feedback mechanism.
When energy drops Vc1 drops again and cycle repeats.

RESULTS

See attached Excel V2 spreadsheet.

Output voltage regulation at Io = 10mA nominal was 4.96v <= Vout <= 5.13v
for 8 <= Vin <= 30 v or better than +/- 2%.
Regulation was about +/-2.2% at 20 mA load.

Efficiency was slightly better at 20 mA out at all values of Vin.

FREQUENCY / PERIOD OF OSCILLATION

At 10 mA the period of 1 cycle for Vin = 10, 20, 25, 30 was 25 uS, 35uS, 42
uS, and 50 uS respectively.
(40 kHz to 20 kHz).

At Io = 20 mA frequency was about 33 kHZ at 12v in and 24 kHz at Vin = 30
volt.


**** ANYONE INTERESTED ADVISE ME OFFLIST AND I WILL SEND EXCEL RESULTS FILE
(10 KB)









_____________________________________




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