Russell McMahon wrote...

[snip]

>This design is simple to build, forgiving, well behaved, easily scaled =
for
>power and voltage and reasonably efficient.
>It SHOULD have use in many amateur applications.

Since you've built this circuit and are using it, I'd be interested in
seeing some performance data.

What efficiency do you actually get, at whatever operating point
you're using (Vin =3D _____, Vout =3D _____, Iin =3D _____, Iout =3D =
_____)?
What frequency does it oscillate at?  What are the rise and fall times
of the voltage waveforms on BUKFET's gate and drain?

>This circuit is special - it was provided by God (no kidding).

Watch out, there!  One always needs to be aware that God has a sense
of humor, and one of His more subtle pranks was to create Zener diodes
with weak knees.

With the resistor values shown, your circuit operates Zener diode
ZBUK1 with about 60 microamperes of reverse current; while this may be
enough for some diodes when operated at room temperature, it is
definitely NOT enough for ALL diodes over any appreciable temperature
range.

Zener diodes frequently have a rather gradual transition between the
non-conducting state (at voltages well below Vz) and reverse
conduction beginning at voltages approaching Vz.  They generally
should not be operated in this "knee" region because they can be very
noisy, have wretched temperature coefficients, and appear to have
breakdown voltages that are WAY out of spec.

A good rule of thumb is that a Zener diode usually needs at least one
milliampere of reverse current in order to reliably "do its thing". =20

With only 60 microamps of Ir, you could run into troubles with this
circuit- poor regulation, large changes in output with temperature,
noisy operation, etc.- that show up in production, particularly when a
new batch of diodes arrives.

I tried simulating this circuit in SPICE; it works, after a fashion,
but it appears to operate more like a linear regulator that's decided
it would be fun to oscillate a little, than a true switching
regulator.  Others have pointed out the problem caused by the weak
turn on/turn off drive to BUKFET, and it's a real killer: in
simulation, at least, I get an efficiency that's not appreciably
greater than that of a linear regulator, mostly because of BUKFET's
long, long, LONG turnoff time.  You need sharp on/off transitions to
avoid wasting power, and that means solid gate drive.

Part of the problem is BUKFET itself; the IRF6215 you've chosen for
BUKFET is a real monster of a FET, with a huge gate charge
requirement.  If this design is primarily for low-current use (a few
tens of milliamps output), you could use something like a ZVP3310 from
Zetex which would switch a lot faster.  Total gate charge for this
device is about 0.8 nC, compared with 60+ nC for the IRF6215.

Got any measurement results to share?  I'd be interested in seeing
them, particularly if they summarize results for a number of units.

Dave

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