(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
values:

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

:o)

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:
http://www.romanblack.com/smps2.htm
-Roman

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics