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Flyback power supplies, one last time

Discussion in 'Electronic Design' started by Joel Kolstad, May 28, 2004.

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  1. Joel Kolstad

    Joel Kolstad Guest

    While I've been muddling with software for a bit now, I went back and
    re-read one of Terry Given's comments:

    ---

    BTW, go look at what happens to your flyback converter when your first turn
    it ON. switch turns on at max. D, charging L up to Ipk (or thereabouts),
    dIprim/dt = Vprim/Lprim. During flyback, energy dumps into UNCHARGED output
    capacitor Cout through diode D. dIsec/dt = Vsec/Lsec = (Vout+Vd)/Lsec BUT
    Vout = 0 so dI/dt is MUCH less than you expected. Therefore only a small
    amount of energy gets dumped in Cout, the rest stays in Lsec (low dIsec/dt
    means it doesnt discharge very rapidly). When your switch NEXT turns on, Ip
    starts off at close to Ipeak, NOT zero. Eventually Cout charges to Vout, and
    dIp/dt = dIs/dt (or thereabouts)

    If you do not have a dedicated peak current trip, and instead have a fixed
    duty cycle, you WILL blow up your smps...

    ---

    Great, I appreciate this and can deal with it. But tell me... I'm looking
    at Abraham Pressman's book (where all the controllers are continuous time
    devices) under the section, "4.3.4. Flyback Disadvantages," and he makes no
    mention of this problem! Rather odd, no? Why is this apparently not always
    a problem with traditional designs? Seems like they'd immediately blow up
    too.

    Thanks,
    ---Joel
     

  2. Well I don't have the above referenced book to look at, but this basic
    problem isn't just limited to the flyback topology. Terry Given's comments
    are correct although I think the "WILL" (blow up your smps) shouldn't
    necessarily be capitalized.

    In any switch mode powersupply topology or even linear regulator for that
    matter start up can be quite stressful. Initially the output capacitors are
    uncharged and therefore look like a short circuit. Whether the main
    switching element (normally the weakest power element in an SMPS) or pass
    transistor fails on power on or not depends upon the relative
    voltages/impedances/output capacitor energy storage/and transistor
    ruggedness. In a low input voltage converter (IE: 12V or less) with only
    modest output capacitance values (IE: <1 joule total storage), the main
    power switch will not usually fail. Even in these types of converters it is
    wise to use some form of soft start and/or peak current limiting, but
    usually one can skimp on this and won't find any immediate failure problems
    (though in the long run reliability may still be compromised)... Staying
    within the switching element's published safe operating curves is very
    important...

    On the other hand when you start playing with mains voltages (IE: 160V) and
    beyond with output capacitances storing decent amounts of energy (IE: > ~0.5
    Joule), some form of peak current limiting is pretty much absolutely
    mandatory for any kind of remotely reliable design.

    If this isn't covered somewhere in Abraham Pressman's book I would venture
    to claim the book isn't complete.
     
  3. legg

    legg Guest

    Start-up and transient behaviours are not inherently characteristics
    of any topology - they are a side-effect of the application.

    The use of an effective slow-start or compensation for other
    large-scale or fault conditions in an application, is a decision left
    to the designer. In the case you mention, it is the potentially
    defective response of a specific control method application that might
    give problems, not the single flyback topology used as an example.

    If a fault is going to occur every time power is applied, it is
    unlikely to be missed. More of a problem are those faults that don't
    give obvious symptoms. If you examine your circuit critically for it's
    response to a full schedule of single-fault abnormal test conditions,
    you will get a pretty good idea as to it's general ruggedness.

    The flyback primary is actually extremely well-isolated from the
    output for most fault conditions. This comes at the expense of the
    loss of the reflected output current sensing, normally possible with a
    more closely-coupled topology. Simply limiting primary current in a
    flyback will not guarantee prevention of component damage due to
    excess secondary current, in the event of secondary overload.

    Sensible application of proven technology is always the responsibility
    of the designer.

    RL
     
  4. Joel Kolstad

    Joel Kolstad Guest

    One of the reason I'm playing with the flyback topology at the moment is
    that it seems relatively 'gentle' in terms of moving energy around... the
    primary gets loaded up with energy that's then dumped into the secondary,
    and even if the secondary is shorted, the primary side doesn't 'see' that
    fact directly. (Shorting the secondary seems as though it puts the flyback
    back into the startup position -- the output voltage is close to zero, so
    the secondary actually can't extract energy from the transformer quickly at
    all.)
    I'm using the 'soft start' approach. I'm glad I'm starting with a 25W, 12V
    input design... I think. :) I do know that with my design if you saturate
    the transformer it starts making scary noises but I can then switch it off
    before anything is permanently damaged.
    He's a big fan of current mode converters for anything other than low power
    designs, so I imagine he's well aware of these issues but was probably
    already 'over budget' at over 2" worth of pages!

    Some years back I had one of those (cheap) 12V->120V inverters rated at 100W
    that I smoked while asking it for no more than some 50-70W (it was powering
    a small laptop computer). I wasn't impressed with that design, but I figure
    that for the $40 or whatever it was I got what I paid for.

    ---Joel
     
  5. Terry Given

    Terry Given Guest

    As usual, great points from Legg. A trivial example proves his last point:
    consider a dual-output flyback - 10W at 5V, 1W at 12V. now short-circuit the
    12V supply. watch all the transformer energy now flow into the puny 12V
    rectifier (1N4148), not the monster 5V rectifier (which must handle about 8A
    peak, every cycle).

    Inductor saturation is most likely to cause Joel catastrophic problems - his
    duty cycle is fixed, based on the assumption that it operates in DCM. When
    it doesnt, the primary current increases by roughly Ipk_nom every on time,
    and hardly decreases at all during the off time. If the switch can provide
    enough current (say a FET) then the inductor will eventually saturate, at
    which point the primary current rises rapidly (if you have a gap in the
    inductor, the new dIp/dt will be le/lg where le is effective path length of
    core, lg = gap spacing - in other words, the inductor now behaves as if air
    cored). usually the new dI/dt is so high that the current ends up limited by
    the series R (or beta limiting with a bipolar, or 10xIrated with an IGBT)
    and the switch goes BANG.

    peak primary power limiting can stop this (if you do it well) but as Legg
    pointed out, that wont necessarily save your circuit from other problems. I
    have dealt with this in the past using digital soft-starting (pld/micro
    ramps up D from 0 to setpoint over some time) in conjunction with feeble
    transistor drive (and a big enough junction to soak up the watts). Making
    sure the transformer doesnt saturate too soon is a good start, too. You know
    its a good design when it can run continuously from Vinmax at Tmax into a
    dead short on any output, without failure, and run correctly once the short
    is removed. some smps chips have 2x current comparators - the usual PCMC
    comparator, and one set about 20% higher that is the "oh shit" comparator,
    and usually triggers hiccup-mode shutdown/auto-restart operation.

    Joel, in theory you could measure Vout, and knowing Vd, Ls calculate dIs/dt
    and reflect that back to work out your duty cycle - thereby always choosing
    the duty cycle that enables all energy stored in the core during Ton to be
    dumped into the core during Toff. This would automatically deal with an
    output short-circuit. In practice whether or not it works depends entirely
    on your sampling time, not the PWM frequency - because you have 0-1 sample
    times before anything can be observed (worst case events always happen just
    after you sample the ADCs :). add in the time to do the calc and output the
    new duty cycle (often a whole sample time, but can be quite quick) and you
    will see that these numbers are what determines xfmr saturation. IIRC your
    sample rate aint too fast.

    Why didnt Pressman talk about it? I can think of lots of reasons. Firstly if
    you use PCMC (and virtually everyone does), it will automatically limit the
    peak primary current (and usually therefore power). This reason alone is
    often enough justification to use PCMC - the primary of a converter usually
    has more energy available to destroy things in the event of a fault (esp.
    with off-line smps), unlike the secondary. so preventing the primary from
    failing is usually a good way to prevent catastrophic damage when things do
    go wrong. Its also a big book, with lots of goodies. room or time
    constraints? quite possible.

    real psu's usually have soft starting, too. then again, cheap taiwanese crap
    smps (like the NZ $25 "200W" smps I used to put into videogames) typically
    dont. And they also periodically blow up when you turn them on (actually one
    of the more common failures with el-cheapos is the rectifier diodes blow up
    trying to charge the DC bus cap, as their Ifsm isnt big enough). The cheaper
    the smps, the greater the likelihood it will go bang when you short its
    output (PC repair guys are a great source of cheap heatsinks etc. as pc smps
    crap out all the time - they usually will give you boxloads of dead ones :)

    And of course it is precisely these "unusual" conditions that differentiate
    the bad "designs" from the good ones. After I designed my first flyback with
    built-in self destruct and found this issue (my smps actually blew up
    because of a sneaky snubbing trick I pulled that required DCM), I got into
    the habit of building smps-testers that power-cycle the unit at full load -
    using a timer and relay to turn mains on and off (and discharge Cbus, Cout
    when off. yay for multi-pole relays). Do that about once a minute, and leave
    it running until it fails (or you leave the company a few years later) -
    thats about 1440 starts/day, which is (usually) at least 100x greater than
    the unit will see in practice, so its easy to do a "lifetime" test (10 yrs =
    3650 days/100 = 3.65 days of accelerated testing). This is how mains gear is
    tested (often with a plastic "finger" that pushes the switch. a million
    times.

    Cheers
    Terry
     
  6. Joel Kolstad

    Joel Kolstad Guest

    Thanks for all the feedback, guys. I see now that the idea of a 'pure
    software' feedback loop is really quite limiting when it comes to preventing
    catastrophic failures. I'll try Terry's test of cycling a dead short and
    see what happens... I'm guessing that either my fuse will blow (hopefully)
    or the main switch will (oops -- followed shortly thereafter by the fuse if
    it fails short-circuit). I'm also a little concerned about what happens
    when I go from full load to zero load -- will the feedback loop react
    quickly enough to prevent my ~300V rail from shooting up over the voltage
    ratings of the various secondary side devices before something croaks?
    (It's a 400V capacitor and 600V rectifier.)
    I'm thinking of doing something a little cruder whereby a lookup table is
    used that limits the maximum duty cycle regardless of what the feedback loop
    would like to do. This will inherently generate soft-start as well, I
    believe.
    I've been planning to increase it... up to about 10kHz; above that I'd have
    serious concerns about whether or not I'd have enough time to process each
    sample. That's still 5 switching cycles per ADC sample, though.

    ---Joel
     
  7. analog

    analog Guest

    One low parts count way to prevent short circuit runaway is to make the
    flyback's operating frequency inversely dependent on output voltage.
    With a UC384X based design, the oscillator capacitor is normally charged
    via a resistor to Vref (5V). Usually there is a small slave flyback
    winding to provide nominal 12 volt power to the PWM IC. A shorted output
    is reflected to this voltage. If a large enough percentage of the PWM
    IC's oscillator charging current is feed from an additional resistor to the
    slave 12 volts, then a shorted output can result in enough off time for
    flyback current to ramp to zero. A practical implementation usually takes
    a few more parts than I've indicated here, but the idea is the same.

    analog
     
  8. legg

    legg Guest

    SW control brings up the hairy question of housekeeping power. This is
    the Abbott and Costello side of power conversion.

    "Who's 'on', first?"

    In circuits where logic or control power is obtained from a low-power
    auxiliary on the main transformer, the shutdown can be enforced by
    simple collapse of the supply voltage. The output voltage collapse in
    a flyback load can be crudely represented by the loss of reflected
    flyback voltage in all coupled windings.

    Your control circuit should be prepared to re-initialize safe
    operation, following the passing of each 'undervoltage' event. This
    could simply mean getting a PIC with a good UVreset, low power
    consumption and a reliably starting oscillator.

    Obviously, while this is happening, any 'protection' functions in the
    hardware has to be the natural result of analog control sections.

    Control circuitry that won't run off the same power as everything else
    in the box, without a very good reason, is an offense in the eyes of
    your creator.

    RL
     
  9. Joerg

    Joerg Guest

    Hi Joel,

    Here I side with Terry and even all my low power SMPS designs have been
    current mode controlled. No exceptions.

    The point he brought up regarding unintended transfer from DCM to CCM
    was once illustrated with sparks and kaboom: I redesigned a client's
    system from Thevenin termination (a.k.a. space heaters) to AC
    termination. The not so intended consequence was that the power
    consumption became so low that the fixed duty cycle SMPS obviously
    didn't want to stay in DCM. A squeal, then it started screaming, smoke
    billowed and then it produced an impressive pyrotechnic showdown. All
    within very few seconds, not enough to run to the back of the unit and
    pull the breaker.

    Regards, Joerg
     
  10. Terry Given

    Terry Given Guest

    good thought. when in doubt, sample faster.
    doing 32-bit fixed point DSP with 24-bit mantissa, I used small LUTs to
    calculate sin(x), atan(x), 1/sqrt(x) etc in 8 cycles, accurate to +/- 1/2
    LSB. so much for crude.....

    almost do-able. pretty easy to stop L from saturating. a decent size
    junction on your switch to suckup the transient power, and it may well work.
    The real trick is to figure out the necessary tests to PROVE it does go. Get
    a faster micro.....

    cheers
    Terry
     
  11. Terry Given

    Terry Given Guest

    Nice. Just filter the piss out of it (break R into 2 parts, with cap
    inbetween), lest problems arise - dunno if they will, but poor decoupling on
    5Vref is usually a good way to cause problems, so feeding ac @ Fswitch back
    into osc might not be a good idea

    Cheers
    Terry
     
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