1. This forum section is a read-only archive which contains old newsgroup posts. If you wish to post a query, please do so in one of our main forum sections (here). This way you will get a faster, better response from the members on Motherboard Point.

flyback smps using lm2585-adj

Discussion in 'Embedded' started by johnny.dr, Jul 18, 2005.

  1. johnny.dr

    johnny.dr Guest

    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!
    johnny.dr, Jul 18, 2005
    1. Advertisements

  2. johnny.dr

    Jim Stewart Guest

    I'm not sure I could help, but I'd
    sure like to see a schematic.
    Jim Stewart, Jul 18, 2005
    1. Advertisements

  3. johnny.dr

    johnny.dr Guest

    johnny.dr, Jul 18, 2005
  4. johnny.dr

    Jim Stewart Guest

    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.
    Jim Stewart, Jul 19, 2005
  5. johnny.dr

    Bob Guest

    we've all been there...
    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.

    Bob, Jul 19, 2005
  6. Have a look an AN70 at www.linear.com The end of this app note goes
    into measuring noise on smps in great detail

    martin griffith, Jul 19, 2005
  7. johnny.dr

    CBFalconer Guest

    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.
    CBFalconer, Jul 19, 2005
  8. johnny.dr

    johnny.dr Guest

    You have to expect these deviations at the filter input.
    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:

    johnny.dr, Jul 19, 2005
  9. johnny.dr

    Guest Guest

    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.


    Guest, Jul 25, 2005
  10. johnny.dr

    johnny.dr Guest

    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.
    johnny.dr, Jul 26, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.