Re: rectifier waveform

On Mon, 29 May 2006 05:27:52 +0100, Pooh Bear
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:


John Larkin wrote:

On Mon, 29 May 2006 03:16:13 +0100, Pooh Bear
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:

John Larkin wrote:

Following a discussion that John Fields and I have going in a.b.s.e...

Consider a conventional power transformer running off the AC line.
Assume a 12 volt RMS untapped secondary that drives a bridge
rectifier, filter cap, and a modest grounded load, producing roughly
+15 out.

What is the waveform at either end of the transformer secondary?

A slightly 'clipped' sinewave.

Rarely!

Uh ? Always IME. So what do you think it is ?


It could vary over a wild range of waveforms. Helmut did some really
nice sims... I hope he can post them to a.b.s.e. John F has posted
some actual scope waveforms which include interesting features.

I'm just now appreciating how peverse this circuit really is. How
about this one:

Call the ends of the transformer X and Y (Speff has already used up A
and B.) Suppose we're at the top of the cycle, and that X is the
positive side. Two diodes are now conducting, call them Dx and Dy,
where Dx is charging the cap + side and Dy is clamping Y to near
ground. Everything is nice and symmetric.

Now, after the sinewave peaks, voltage X:Y is dropping below the cap
voltage, so we are about to turn the diodes off.

First assume diodes that don't store significant minority carriers,
like schottkies or fast-recovery pn diodes. They still have a severely
nonlinear capacitance vs backbias curve. As the winding voltage
droops, "reverse" current flows through the diode capacitance. Since
the winding stray capacitance to ground is of the order of the diode
capacitance, the effects on secondary voltage can be nontrivial.

The higher the reverse voltage on any diode, the lower its
capacitance, so the faster the reverse voltage increases. That's a
positive feedback mechanism, and if one diode has slightly less
capacitance, it will lose the tug-of-war, and the corresponding
transformer terminal voltage will "jump off the rail" first. This
jumping off the rail is visible in JF's schottky waveform, although it
is pretty symmetric in his case, and there are complex reasons for
that, too.

Things get even more interesting if the diodes store lots of charge,
like a slow pn rectifier. Helmut simulates some astounding asymmetric
spikes, and I've seen stuff in real life that was almost as bad, but
with lots more ringing.

And serious primary-secondary capacitance can make the secondary
waveform even more complex, as can asymmetric diode reverse leakage,
which can be significant with schottkies.

This really *is* interesting.

John


.

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