Re: Help with power supply noise., Please.

On Wed, 01 Aug 2007 15:28:02 -0400, Mike <nomtrxspam@xxxxxxxxxxx>
wrote:

I am building a homebrew bench power supply and it is working very well except for too much noise on the
output. It is a single output with adjustable CV from 0 to 40V and adjustable CC from 0 to 4A. I have posted a
schematic of the regulator section and a digital photo of the noise waveform under no load on a.b.s.e in PDF
format. I posted it using the same suject line. I would greatly appreciate it if someone could give me some
ideas of what to try in order to reduce the noise. I fear it is doomed to have a noisey output since it is
point to point wired on perfboard, but I sure hope not. I tried very very hard to get the ground connections
right and have rearranged them to try minimize the noise, but those changes didn't affect it much either way.
This noise is very stable in that it doesn't change much from no load to the full 4A load, It gets fuzzy under
full load, maybe from some low level high frequency oscillation. It doesn't change with output voltage. I
increased the value of C5 to .047uf to get the noise down to where it is now. With C5 at 2200pf the noise is
about 2 1/2 times what it is now and increasing C5 further decreases the noise very little.

There is another curious thing that happens that has me stumpped. I intended to use an OPA2277 for U1. With
the 2277 installed the transition from CV to CC mode in very abrupt, I.E. with a 10ohm load resistor
connected, the voltage set to 20v, and the current set to 2.5A it draws 2A as expected. When I lower the CC
setting it has no affect on the output untill it is set to about 400ma below the current being drawn or 1.6A
in this case. The current then snaps down to 1.6A and decreases smoothly from there down. It did this no
matter what current level I used to test it. It also does it if I increase the output voltage to excede the CC
seting. With an LM6132, LF412, or a TL082 installed it transitions to CC mode quite smoothly.


The fact that the noise is largely independant of load suggests that
it does not originate in the unregulated input rail, as this source
would increase with load-induced ripple.

You may be able to spot it's earliest source - it should not be
present in the outputs of U2. The higher frequency stuff might benefit
from a little miller capacity around Q4 (picofarads) or an emitter
resistor there, (not enough to affect source rejection).

You might try re-orienting your mains transformer, with respect to
your circuit board, while monitoring the output noise. You may find
that the lower frequency line undamental and harmonics vary favourably
under some circumstances. The steel leadframes used in many
semiconductor packages (including the reference, tend to be affected
by magnetic fields that have a predictable orientation, and which
reduce rapidly in strength with increasing distance.

It was once demonstrated feasible to reduce output noise of some
models in a commercial series of linear power supplies into the
microvolt region, simply by mounting the regulator board assemblies at
right angles to convention, on the unchanged industy-defined chassis's
metalwork.

All the longer connections to pots, or larger-bodies pots themselves
are subject to pick-up. Perhaps R11 should be grounded, and not used
as a voltage pick-off point and C5 should have a small resistor in
series. C4 or a similar part might be more effective on the actual
output terminals.

A more smooth transition between current and voltage regulation might
be possible if U3 had reduced influence. It does, after all, only have
to over-ride a milliamp from the output of U2B, using it's full output
voltage compliance, to zero U1A input pin3.

RL
.

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