> I'm not sure I fully understand your boost circuit.
> What causes turnoff??

I puzzled over that for a while too :-)

I THINK the output transistor  goes into Beta drive limiting as inductor
current increases which reduces the rate of increase of current in the coil
which leads to regenerative action as the field begins to collapse. The 1 NF
feedback capacitor then increases the feedback voltage and you get positive
feed-back turnoff. The risk seems to be that if you stabilised it linearly
it MIGHT sit there and not switch. (As was claimed for a certain other 3
transistor circuit in the past :-) ).


        Russell McMahon


________________

> >                                        diode
> >     3-4 V o----+------------CCCCC---+---AK----+---+----o 5V @ 100 mA
> >                |            68 uH   |         |   |+
> >                R                    +------+  R  --- 100uF
> >           1000 R                    |  .001|  R  ---
> >                R      PNP           |     --- R   |
> >                |             100    C     --- |   V
> >                +--+---E C---RRRRR--B  NPN  |  |  Gnd
> >                |  |    B            E      +--+
> >         LM336  Z ---   |            |      |  |
> >          -2.5  Z ---   |            V      |  R
> >                Z  |.047|    2200   Gnd     |  R
> >                |  V    +---RRRRR-----------+  R
> >                V Gnd                          |
> >               Gnd                             V
> >                                              Gnd
>
> > Load regulation seems really solid, but I'm not happy with the
efficiency.
> > The problem is the base drive of the switching transistor,
>

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.