Roman,

>The first thing I did was re-design it to drive BACKWARDS
>so I reduced the filter cap to a small 0.68uF cap and
>then decoupled it from the final filter with a 10 ohm
>resistor. This gives a deliberate high ripple of about
>120mV on the cap, which slams the transistors hard
>into a squarewave, beautiful to see on the cro, and
>giving it good efficiency even at 55kHz which my first
>circuit ran at.

What you've done is help the circuit oscillate by increasing the loop
gain at high frequencies.  If you look at this circuit in a
small-signal sense, what you've got is a common-base amplifier stage
(Q2) whose input is at point B; this common-base stage then feeds a
common-emitter stage (Q1) which feeds back to the input (emitter of
Q2) through L1.  C1 shunts some of this signal to ground, and reducing
C1 lessens that effect giving you stronger oscillations.

>
>Here is the circuit i'm testing now;
>http://www.ezy.net.au/~fastvid/smps02.gif
>

[snip]

>...and the
>entire circuit is only 50 cents in parts or so, this
>2:1 current gain might be very handy with battery
>operated devices.

Agreed wholeheartedly, PROVIDED the user keeps in mind the limitations
inherent in this circuit: lots of high-frequency ripple (like all
switching regulators) and rather so-so regulation- a consequence of
this particular, "CheapMode(tm)" design.

>The circuit shown gives:
>* <1mA out =3D works as a linear regulator.
>* 1mA to 12mA out =3D increasingly oscillates, gets more
>  efficient. Totally reliable oscillation.
>* >12mA out, slams a nice square wave, max efficiency
>about 30mA out, good results to 100mA out, even at
>120mA out with the cheap inductor gives 1.75 current
>gain, MUCH better than any linear regulator.

The fact that oscillation ceases below about a milliamp or so is a
consequence of the basic design: there has to be enough current
flowing through Q1 and Q2 that they have enough gain to sustain
oscillation.

The varying "enthusiasm" of its oscillations between 1 mA and 12 mA is
a result of not using any hysteresis (i.e., positive feedback) in the
circuit.  If you put enough hysteresis in, like in Richard's original
circuit, it'll oscillate like a politician at all current levels
except the very lowest.

>Regulation is poor;
>4.75v @ 10mA
>4.60v @ 20mA
>4.00v @ 80mA

Yeah, well waddya want, it's CHEAP!

Good work, Roman.

Dave

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body