flyback smps using lm2585-adj

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I may be somewhat off topic but i've ran into a mountain i cannot seem
to get over regarding the hardware design of a switching mode power

I'm using an LM2585-ADJ simple switcher chip from National in a
non-isolated flyback configuration to generate 3 output voltages: 3.3,
15, and -15.  The monitored (feedback) output, 3.3V, has a lot (1.3 V
pk-pk) of transient noise at and only at every transition of the
primary switch (off to on, on to off).  Elsewhere, it is very clean.

If anyone has any experience in this area and would be so kind to offer
any help I would greatly appreciate it.  If so, I'll be happy to post
all the gory details.  Thanks!

Re: flyback smps using lm2585-adj

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I'm not sure I could help, but I'd
sure like to see a schematic.

Re: flyback smps using lm2585-adj
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no problem.

Re: flyback smps using lm2585-adj
Hi Johnny,

What kind of capacitor is C8? It's sometimes better to use 2 or 3 capacitors
to get lower ESR (and spreading heat at higher power levels.). Further more,
normally I'm using snubbers across the output shottkies.



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Re: flyback smps using lm2585-adj
C8 is a 25 V, 3300 uF capacitor, part# UPW1E332MHH
( )

The datasheet specs it to have an ESR of 0.018 at 20 degrees C (my
nominal operating temp).  I agree with you, I probably need to spread
this out across a couple of capacitors but I don't think that it is the
source of my problems.

While we're on the capacitor subject, does anyone have any idea how to
measure the ripple current in a capacitor to make sure it falls within
the capacitors operating ranges?

I have, since posting the schematic, placed a snubber (series RC in
parallel w/ the diode) around my feedback output shottkie.  Values of
0.1 uF and 20 ohms really cleaned up the ringing reflecting back to the
switch.  I failed to mention the hardware update on my july19 post with
the waveform snapshots.

Bob mentioned earlier that you can't probe a smps using the alligator
clip ground lead.  I followed his suggestions and he was right, the
noise was not of the magnitude that was first expected (0.5 Vp-p spikes
instead of 2V).  This makes sense; however, I do not understand why I
can pull any off-the-shelf smps and probe it using the alligator clip
ground lead with successful readings.

I apologize for not updating the thread but I figured no one was
looking at it further.  Thanks for any input and don't hesitate if you
have questions, this thing is killing me.

StackTools wrote:
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Re: flyback smps using lm2585-adj

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Try moving C8 over to the far side of the
inductor, along with the feedback source.

My gut tells me the inductor should be
closer to 470uh and the output cap more like
330ufd.  Without knowing your current requirements,
it's just a stab in the dark though.

Re: flyback smps using lm2585-adj
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we've all been there...

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1: is the noise really there? you can't probe a SMPS using the alligator
ground lead and the normal snout of you scope probe. Pull the snout off the
probe. Now you can see the tip and the shield of the probe. If your probe
comes with a low inductance ground clip (springy wire ~2cm long) then slide
that on to the sheild. Otherwise, use a jewler's screwdriver to make the
ground connection by holding the probe and the scewdriver in a little
triangle on top of the output cap. It's tricky to make all 3 connections
simultaneously - having someone else there to handle the scope helps a lot.
The output cap is the only place to look for the noise - anything else is
common-mode and can be managed at the system interconnect level (careful
return path design).

2: If the noise is still there when properly probed, then the problem is
poor circuit layout. The LM2585 data sheet should have a section on layout -
follow the advice on keeping the output current loop small.


Re: flyback smps using lm2585-adj
On Mon, 18 Jul 2005 16:26:39 -0700, in comp.arch.embedded "Bob"

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Have a look an AN70 at The end of this app note goes
into measuring noise on smps in great detail


Re: flyback smps using lm2585-adj
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A quick glimpse at the schematic shows some possible anomalies.
First, you should do your measuring, and sampling, after the HF
filter.  You have to expect these deviations at the filter input.
It is possible that the regulator has its own hidden filter in the
feedback loop.

Second, you should be taking great pains to avoid any ground
loops.  The surge current in the 3.3 regulated rectifier circuit
should not have any impedence in common with anything else.  The
way the schematic is drawn indicates a casual attitude.  Draw that
ground area in as a big fat buss, with explicit connections.

"If you want to post a followup via, don't use
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Re: flyback smps using lm2585-adj
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I would expect some deviation but my gut instinct says it should be
less magnitude (see screenshots below).  Although I couldn't honestly
say as I have no prior experience in this area.

Agreed, layout could use some work.  I tried to avoid ground loops
during the first pass layout but have learned a great deal since then
and I now see areas for improvement.

I will try all the above suggestions this afternoon, thanks for the
responses.  I'll send another PCB with improved routing soon if the
problems persist throughout the troubleshooting process.

I thought I'd post some waveform snapshots as well:

CBFalconer wrote:
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