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'[PIC]: [EE]: 2-tran SMPS 5v reg now 90% efficicie'
2002\09\20@172005 by Roman Black

(Posted on [PIC] and [EE] as this is a 5v regulator design.)

I think i've found the "holy grail" of 2-transistor 5v
SMPS regulators!

After seeing the recent discussion on using a 5v switching
regulator chip I went back and spent a few hours on my
2-tran SMPS regulator with the goals of improving efficiency
and regulation to that approaching a "proper" regulator.

I tried all sorts of 3-transistor designs, with more gain
and better switching, and even tried circuit variations with
a comparator chip and a 555 timer chip. Not one of them
performed as well as this beauty;

+15v -------*-----------------*--------,
           |                 |        |  Q1
           |         470 ohm Rb       |  PNP  beta=220
           |                 |        E  BC327
     8k2   R1                *------B
           |                 |        C
           |                 |        |
           |             2k7 Rc       |   470uH (0.7 dc ohms)
           |                 |        |
           |       Q2 BC337  |        *----L1-------*------ +5v out
           |                 C        |             |
           *----*----*-----B    Q2    |             |
           |    |    |       E--------|-------------*
           |    |    |                |             |
           |    |    |                |             |
           |    |    '------C2--------*             |
    zener  |    |           2nF       |             | 47uF
      ZD1  -    |                     -             C 16v
    5.6v   ^    C1                    ^ D1  1A      |
    400mW  |    |                     | schottky    |
           |    | 6.8nF               | 1N5819      |
           |    |                     |             |
Gnd --------*----*---------------------*-------------*------ Gnd

Here are actual efficiency results with the above parts

Vin     Iin     Pin     Vout    Iout    Pout    Freq    Eff%
15v     9.1mA   0.136w  4.95v   20mA    0.099w  97kHz   72.8%
15      19.5    0.292   4.90    50      0.245   57      83.9
15      30.0    0.450   4.87    80      0.390   41      86.7
15      37.0    0.555   4.86    100mA   0.486   34      87.6
15      51.1    0.767   4.85    140     0.679   31      88.5%
15      65.6    0.984   4.84    180     0.871   31      88.5%
15      73.1    1.096   4.83    200     0.966   31      88.1
15      92.8    1.392   4.82    250     1.205   31      86.6


I was even able to get 91.2% efficiency with one set of parts
values! I imagine that using a good low-sat Q1, and a better
low-sat D1, with a well chosen inductor that even 95% might
be attainable with a good "powerband" and very cheap parts.

Waveforms, pictures and more efficiency data is here:

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2002\09\20@182824 by Roman Black

Wow! I posted too soon. I've just found another
improvement to the circuit, looks like 95% efficiency
is going to be quite possible after all. :o)
It adds the cost of a 1N4148 diode.

I'll test it out tomorrow.

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2002\09\21@022959 by Russell McMahon

> (Posted on [PIC] and [EE] as this is a 5v regulator design.)

Note that only ONE tag is effective (don't know which - probably the first).

> I think i've found the "holy grail" of 2-transistor 5v
> SMPS regulators!

That performance is utterly superb.
Haven't got time to play with it at present but I will definitely be
investigating it further. The porr regulation of the prior design made it
less suitable for many purposes than my 3 transistor version. The new
circuit's regulation make a lot of difference.

Re efficiency. Don't know what the Schottky drop is here but at 0.5v it
would add about 10% inefficiency and at 0.3V about 6% losses. This is of
course only during the "flyback" portion of the cycle. At 100 mA in inductor
losses in 0.7r are about Iout^2 x Rind = say 30 mW. This is around another
3%. [[Hard to simply calculate this loss due to two halves of cycle and
waveform shapes]].
So the result is extremely good already for what you are using. If you want
to cheat you could try it at Vout = 12 volts, where current losses will be
smaller and voltage losses will be a smaller percentage of Vout.

Adding complexity to such a fantastically simple circuit hardly seems
worthwhile but you could consider synchronous rectification with a FET
rather than the Schottky to get even less losses here. A second winding on
the inductor (complicating the coil) would provide an easy drive source or a
feed from Q2-collector is a possibility.

Don't be scared of an extra transistor if it has significant advantage -
transistors are (as you know) a very minor expense at this power level.

A vast advantage that this circuit potentially has over many simple IC
solutions, is that it has a sensibly unlimited potential maximum input
voltage capabiltity - dependant mainly on the Vceo of th transistors. At
very high voltages Rc and R1 have problems with powwer dissipation and in
due course voltage rating but this is unlikely to be a problem in most
applications this is aimed at.

The on web documentation is excellent and the full public domain release is
appreciated. .

Roman's original is at

       Russell McMahon


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