piclist 2002\09\23\042935a >
Thread: 2-tran SMPS 5v reg now 90% efficicient!
www.piclist.com/techref/index.htm?key=tran+smps+reg+efficicient
picon face BY : Morgan Olsson email (remove spam text)



While i think this idea using C2 is really neat, i think it can be substituted for a wire... wound as a feedback winding on L1 and placed in series with Q2 base instead.

+ Cheaper parts (one less component)
- More work (winding)
+ Higher efficiency (no current pumping through C2 and dumped in Zener)


Copying from Romans web page and adjusting:

+    -------*-----------------*--------,
           |                 |        |  Q1
           |                 Rb       |  PNP
           |                 |        E
           R1                *------B
           |                 |        C
           |                 |        |
           |                 Rc       |
           |                 |        |
           |                 |        *--[+L1a-]----*------ +v out
           |                 C  Q2    |             |
           *----*--[-L1b+]-B          |             |
           |    |            E--------|-------------*
           |    |                     |             |
           |    |                     |             |
           |    |                     |             |
    zener  |    |                     |             |
     ZD1   -    |                     -             C 16v
           ^    C1                    ^ D1          |
           |    |                     |             |
           |    |                     |             |
           |    |                     |             |
Gnd --------*----*---------------------*-------------*------ Gnd

Values:

L1b:
Turns: design for about 3v amplitude at highest input voltage.
Maybe 1V is enough (less to wind) but never more than 5V (getting near Q2 Ube reverse limit)

As we are winding this on a inductor with unknown number of turns it is trial, or if you have a signal generator to drive L1a and measure output from i.e ten external turns L1b, then it can be measured.

R1:
No current pumping through C2 and dumped in Zener, so R1 can be increased (doubled?) without decreasing Q2 base drive, = lower loss.

/Morgan

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