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Power supply poor performance.

Discussion in 'Electronic Design' started by pawihte, Jan 10, 2010.

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  1. pawihte

    pawihte Guest

    I tried my hand at making a 9V power supply with an MC34063A. I
    get the correct DC voltage output but was disappointed with the
    very dirty output. I know that, in general, simple switched-mode
    PSes have poorer performance than linear types, but what I
    observed was worse than I expected from the sample circuit given
    in the datasheet. This is the schematic, along with the
    single-sided pcb layout (in case it's due to poor layout):
    http://img683.imageshack.us/img683/4302/9vsmps.png

    My main scope is out of order and I used my backup 15MHz
    single-trace analog scope. It shows narrow spikes of unsteady
    amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
    level. Moreover, the frequency of about 15 kHz is much lower than
    I expected.

    The spike amplitudes were first observed without the second L-C
    filter. Adding that made little difference at the output of the
    first filter, and only a slight reduction at the output of the
    second filter. The load was the LED plus a 470-ohm resistor
    (total 24mA).

    I used general-purpose caps (ESR unknown) for the output filters.
    Paralleling them with non-electrolytic plastic and ceramic caps
    of 0.1uF have no discernible effect. The timing cap is a ceramic
    disc that shows 465pF on my LCR meter. I wound the inductors with
    23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

    What am I doing wrong? Is it the filter caps, poor PCB layout or
    something else?
     
  2. Yes.
     
  3. Tim Williams

    Tim Williams Guest

    Scope shot?

    More importantly, what's pin 2 look like? And the current through R1?
    Aha! Ferrite is shit for DC filtering. You're probably saturating them.
    Try a ferrite rod or powdered iron toroid.

    How many turns? What's "300uH" based on, is it measured? At what bias?
    Layout looks pretty good. D5 and C2 seem to be on the wrong sides, C2
    should be closer to the chip I guess, but with L1 where it is, that might be
    tricky. Maybe it can get just close enough to that mounting hole, or see
    what it does rotated. Anyway, that ground plane everything's connected to
    is pretty wide and this should make very little difference. I'd say it's a
    good tight layout.

    Tim
     
  4. You might want to experiment with Tantalums for C1, C3.
    And looking at your layout there is potential of coupling the input
    switching noise to the feed back pin. A small capacitance might help,
    with out affecting loop response.
    Also this type of switcher creates a lot of hash, so a input filter of
    some type (LC) is usefull.

    Cheers
     
  5. pawihte

    pawihte Guest

    Thanks for the reply. I'll try out your suggestions and see what
    happens.
     
  6. pawihte

    pawihte Guest

    OK. I'll upload the shots but it'll have to wait a bit.
    Ah. I'm not completely ignorant about the saturation thing, but I
    have limited experience with unbalanced filtering and it just
    didn't cross my mind. Thanks for the heads-up.
    12 turns. Measured without dc bias. The core is something I
    salvaged from junk.
    I could move C2 to the copper side and solder it directly to the
    IC pin but, as you said, I wouldn't expect that to make a lot of
    difference. Thanks for the reply.
     
  7. Tim Williams

    Tim Williams Guest

    The ol 'shorted' probe test?

    This is more about RFI in general than common mode specifically. When it
    switches, current loops and voltage loops throw off electromagnetic
    radiation. With the probe anywhere in the near field, you'll pick up a
    delicious burst of switching noise. The solution is a coax probe, where
    possible. For instance,
    http://webpages.charter.net/dawill/Images/RegBO1.jpg
    The red/black twisted pair running up left of center goes to a 0.47 ohm
    current sense resistor. It sees this waveform:
    http://webpages.charter.net/dawill/Images/RegBO2.jpg
    regardless of how the 10x probe is connected (notice the clip with wire
    above the heatsink, and its ground clip coming across above the
    transformer).

    If I tried watching the same current with the 10x probe, it would be
    unrecognizable from all the trash. Notice the complete absence of ringing
    and hash when the transistor switches, or when the secondary diode turns off
    (discontinuous mode).

    Tim
     
  8. Jon Kirwan

    Jon Kirwan Guest

    I'm especially interested in the inductor core situation. If
    you do change it, and it helps, let us know. There are some
    other good points made but I thought I'd focus on the core,
    because I'm still learning about these details and wanted to
    consider it a little.

    You mentioned 300uH measured and 24mA load and 12 turns on
    the inductor. With ferrite, let's use a Bsat = 0.1T to be
    safer. We assume not to know mu_r, for now. That still
    tells a lot.

    L = mu_0 * mu_r * N^2 * A_e / l_m

    but also,

    Bsat = mu_0 * mu_r * N * I / l_m

    This second equation can be solved to place the unknowns on
    one side and the knowns on the other:

    (mu_r / l_m) = Bsat / (mu_0 * N * I)

    That can be stuffed into L, as:

    L = N * A_e * Bsat / I

    But we know L, so solve for the unknown A_e as:

    A_e = L * I / (N * Bsat)

    Stuffing in L=300e-6, I=24e-3, N=12, and Bsat=0.1 yields a
    value for A_e of 6e-6 m^2. Assuming a circular profile, this
    is about a radius of 1.4mm or a diameter of 2.8mm. However,
    I don't imagine that the average load current is the peak
    inductor current. So this is actually too small to avoid
    saturation -- the diameter will need to be greater, I think.

    L goes proportional to N^2 while B goes proportional to N, so
    if you reduce the mu_r of your core you will only have to
    wind more turns by the ratio of change in mu_r and reduces B
    allowing more Ipeak. So you might consider finding a
    significantly lower mu_r core and winding a few more turns to
    get back to your L value. Here, I'm talking about L1, I
    think. L2 looks like part of a filter to me so I have little
    comment about that, now.

    Jon
     
  9. You did attach a load to the output? Compare the input to the regulator to
    the output at no load and then try adding a load of maybe 10% of max and see
    if it does any better. I don't see why you need a second L-C stage. The led
    may not be a large enough load.

    Also, your circuit seems a bit different than the buck in the datasheet.
    Some of your resistor values are different(not sure if thats intentional or
    not). Other than the the circuit looks correct.

    On page 7 they give the characteristics of that circuit so you should be
    seeing approximately the same(again, some of your component values are
    different(R2 and R3).

    Try a larger load of at least 100mA or even shorted and see what you get. As
    has been mentioned, in an smps design, the output depends on the duty cycle.
    For small loads it is non-linear and the duty is significant. For large
    loads the regulation is almost independent of the duty. Hence the first
    thing is try a larger load and see if that improves anything.
     
  10. Hammy

    Hammy Guest

    I have a small 0.5W NCP3063 pcb inverting. Heres a shot of the input
    and output ripple below.

    Is this what your output looks like?

    http://i50.tinypic.com/2l9l0zl.png

    That's just using a bead and a cheap 100uf AL cap for my post LC
    filter. Yellow is input ripple; blue is output ripple 38mVpp full
    load. Are you talking about those skinny spikes riding the ripple on
    the blue trace? I'm measuring this with a short ground pin on my probe
    and right over a 0.1uf X7R 0603 cap right at the output.

    You're always going to have weird ripple on Hysteric controllers but
    yours does sound really high. The inductance on your post LC filter
    seems unnecessarily large as well.

    The advice mook gave is good particularly when measuring with your
    scope.Your probe acts like an antenna and picks up switching noise.

    Try holding your probe in the air and keep moving it closer and you
    will see waveforms on your scope without even touching the circuit.
     
  11. Tim Williams

    Tim Williams Guest

    That implies A_L = 2.08 uH/T^2 (unbiased), which is pretty high, typical for
    an ungapped high-mu ferrite toroid. They're typically around 1-10 amp-turns
    (AT) saturation, pretty easy to saturate. If the peaks are four times
    higher than average current, that's easily 0.024A * 12T * 4 = 1.15AT, which
    might be enough to saturate it. If it's highly discontinuous, the peaks
    could be much higher, bringing it into saturation. And if it is
    discontinuous, that could explain the unusually low frequency.
    Bsat ~ 0.4T is more typical, though you might want to drop it to 0.2 or 0.1
    for better linearity, or for high frequency use (transformer duty only, DC
    choke is different). I usually go with ~0.2T, which is a practical
    factor-of-2 headroom, just so I can say I've made the allowance.

    Notice that assuming peak B is equivalent to a certain amount of applied
    voltseconds. This is simply because EMF = -dB/dt * A_e, or Vs = -B * A_e.
    In terms of easy-to-measure parameters, you can get AT(sat) and A_L from the
    inductor's V-I curve, which gets you V*s = AT(sat) * A_L / N.

    I like to work with Vs because it's more useful to circuit analysis. How
    many turns do I need? Integrate voltage over a quarter wave period to get
    V.s (it's usually a square wave, so that's just Vpk * 4/f), and I've already
    measured the core's A_L and AT(sat), so just divide and you get turns.

    In this case, if AT(sat) = 1AT and A_L = 2uH/T^2, then Vs = 2*1 * 12 =
    24uVs. At 9V, that's only 2.7us on-time. Pawihte, are you seeing ~3us wide
    pulses?
    Notice that mu_r disappears -- that means it doesn't matter how much
    magnetization you apply, it's the volts and time that gets it up to
    saturation. (You could do the same to a powdered iron core (let's say
    0.1uH/T^2 and 300AT(sat)), although magnetization current gets absurdly
    large for transformer duty!)
    L2 has to carry the same current, so it'll need the same characteristics,
    although it has less delta B, which basically means it can be lossier =
    cheaper (one of those ugly yellow/white toroids?).

    Powdered iron cores that size are in the 80nH/T^2 range, so you need about
    sqrt(300 / 0.08) = 61 turns to get there. Saturation is in the >200AT
    range, so you can safely dump over 3A though it -- more likely you won't
    even be able to get enough turns inside it to see it saturate before I^2*R
    losses overwhelm it.

    Tim
     
  12. Tim Williams

    Tim Williams Guest

    Oh, I remembered something else about probes and grounding:

    If you put the probe itself through a ferrite bead, you can observe its
    effect, if any. Ideally, this won't change anything. If it changes, then
    you have current going somewhere it shouldn't, either up through the mains,
    or between probes (if you're using more than one).

    Last time I did this and saw the effect, I got:
    - No bead: fast ringing, medium amplitude, medium decay
    - With bead: slow ringing, same amplitude, medium decay
    - Damped bead: slow ringing, low amplitude, fast decay

    Bead setup: three turns through large ferrite bead. Last test: 10 ohm
    resistor soldered in place through the ferrite bead (so it's acting like a
    leaky shorted turn).

    If you are getting probe current, adding a ferrite bead doesn't really help
    your measurement, but it does change it, so you can at least guess what
    hides behind the "probe cable resonance" jigglies.

    BTW, I happen to have some 3" high-mu ferrite toroids, which are *beautiful*
    for putting a couple turns around a big fat probe. (They're supposedly
    12uH/T^2, so 3T will get over 100uH easily!)

    Tim
     
  13. pawihte

    pawihte Guest

    You're right. I touched the probe tip to the ground-clip and the
    spikes remained almost the same. In fact, I should have thought
    of this myself. Although it must be obvious that I have large
    gaps in my knowledge, I've certainly been aware of such induced
    pickups for a long time - decades actually. My only excuse is
    that it was already the small hours of the morning when I ran the
    trial. Thanks for pointing it out.

    As a quick test, I placed the whole thing inside a tincan. I
    wrapped the whole thing, including the mains transformer, in
    bubble plastic wrapping without even grounding the circuit to the
    can. Only the 9V output lines (7" of flex) and the mains wire
    were outside. The spikes dropped from about +/- 2V to +/- 20mV !

    I'll check out the suggestions made by others and report back.
     
  14. pawihte

    pawihte Guest

    Correction: I was too quick to draw a conclusion. I took out the
    whole thing from the tincan again, but the spikes are still only
    +/-20mV. (This time I left the scope probe and ground clip
    attached to the ends of the output wires). I further observed
    that even slightly moving the setup (millimeters) caused the
    spikes to shoot up again, and they remain high until I turn it
    off and then on again. Even just touching the wires have an
    effect. It seems something's unstable.
     
  15. pawihte

    pawihte Guest

    I increased the load to ~0.1A, then to 0.6A. The spikes remained
    fairly stable at approx +/-10-20mV until I momentarily touch the
    output +, at which point the spikes shoot up again until I turn
    it off and on. Normal ripple, which was effectively nil at light
    load, was about 5mVp-p at 0.6A.

    Frequency is now about 45kHz. The ~15kHz I reported earlier was
    probably an observation error on my part. There was a lot of
    jitter. I think it's time to work on the inductors.
     
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