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Bizzare behaviour from SG/UC3525

Discussion in 'Electronic Design' started by Pooh Bear, Jun 22, 2005.

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  1. Pooh Bear

    Pooh Bear Guest

    Hi,

    you may have noted my posts about problems recently with my attempts to
    make a high power smps.

    I'll give a quick overview.

    The application is a high power pro-audio amplifier.

    Traditional psus use nice toroids that we know well. You can optimise
    them for low copper losses that helps keeps weight down.

    We want to make our amps lighter though. Like some competitors that are
    also using SMPSs.

    Various mfrs have used switch-mode techonology. The fixed duty cycle
    approach appears to be universal. I expect this is since a classic
    forward converter doesn't have the bandwidth to respond to audio
    transients. Not to mention cross-regulation to other rails issues.

    Unlike most 'forward converters' - see argument about this terminology
    another day - PLS ! they run at a fixed duty cycle. I.e. max. Kinda 90%.

    Ok - so that's the rough overview.

    So - I have this pcb with my design that's 'plain vanilla' - no frills.
    The 'control circuitry' i.e. SG 3525 and some other shit is located on a
    plug-in pcb via a right-angle pcb header.

    I work-up the prototype - the waveforms look right etc etc....

    Then I get trouble as I've mentioned here before. The UVLO I can deal
    with. If the parts can't 'agree' when to stop working I'll tell them
    myself !

    So - that one's fixed. Ok - finito. ( or so you might think )

    Next problem - apparent 'jitter' on the trailing edge wavefom at the
    load ( high side ) with high input volts on the highside drive. I
    investigate this. I add a zener clamp on the highside IGBT for example
    in case of gate resonance. No good. I modify the signal path to reduce
    inductance. No good.

    I then find by accident that if I try to 'provoke' the highside jitter
    it's affected by my hand capacitance on the switch that I have included
    to 'stop switching' to the shutdown pin on the 3525. Uh ?

    I check the supervisory supply. I discover that if the supervisory
    supply ( provides drive to the 3525 ) is higher than before - changed
    transforner ratio - then the jitter occurs earlier ( I'm ramping the
    input volts on a variac ).

    I'm getting desperate.

    So - I look at the 3525 output. OutA and OutB. I checked them earlier
    and they look nice and clean.

    I have a high resistance nominal load on the half-bridge output that
    takes mere milliamps.

    I see the switching waveforms. They look sensible at the load. Apart
    from this funny 'jitter' on the high side that only occurs at high DC
    bus volts when driving my TX ( no sec load ).

    I feel I'm going mad.

    Finally I get the sense to check the inputs to my IR2110 high side
    driver ( from the 3525 ). Recall I checked these before populating the
    pcb.

    I see mismatched timing periods !

    I have OUTA used as the high side drive signal . It looked fine before
    but it's only 500ns long I discover ! OUTB is 4.5us long ( as it should
    be ) !

    I remove the IR2110 gate driver. The waveforms are perfect. Excellent
    match between OutA and OutB.

    I replace the IR2110. Mismatch occurs again. I've'blown' enough IR2110s
    to not think they're the problem. How can the logic input loading of the
    IR2110 affect a totem-pole output from the 3525 ?

    So - is the 3525 at fault ? I'm careful only to change one item at a
    time normally. I reckon maybe I should replace the TI 3525 with an STM
    part ( actually that's 2 items at a time technically - but I didn't
    expect my other TI sample to be any different ).

    I do so. Goddamit if the same bonkers behaviour isn't replicated by
    another part from a different vendor !

    I check that my pcb man has allocated every pin correctly ( he has ) . I
    read the data sheets looking for some mention of erroneous behavour. I
    find nothing.

    I noted in passing that the 3525 seemed to be sensitive to noise on the
    shutdown pin. Odd, since the IC block diagram shows that to be a 'slow'
    input. It has to discharge the soft-start cap. Soft-start is working
    fine. But my grounds should be clean - I've got a Kelvin connection to
    the low-side IGBT emitter mere cm away.

    Right now I feel like I'm going slightly mad. Maybe the pcb guy did
    something wacky I've totally missed - but he's rarely that useless.

    Does unequal duty cycle on OutA and OutB ring any bells with 3525 users
    ? It's baffling me !


    Graham
     
  2. Terry Given

    Terry Given Guest

    Gidday Graham,

    try this: lift OUTA and OUTB pins. "swap" them with little dangly wires.

    If the OUTA pin suddenly starts working and OUTB goes funny, then its
    something to do with the external connection(s) to OUTA

    then pull the same trick with the IR2110 inputs.

    some gatedrive outputs get pissy when output inductance pulls them below
    0V (or above Vcc). a SOT23 dual schottky on each output can eliminate
    that potential problem.

    try loading them capacitively (perhaps 10-20pF) sans IR2110, see if the
    waveforms are still OK.

    set the DC bus to 0V, provide a separate high-side power supply and see
    what happens - with no actual output switching, does the problem occur.

    place a series resistor between the 3525 & 2110, perhaps 1k.

    Cheers
    Terry
     
  3. Pooh Bear

    Pooh Bear Guest

    Good day to you too Terry.
    Ummm... guess what I was thinking of doing next ! It's called desperation !

    Yuh . That figures. But a 1~2 cm track - with careful attention to routeing ?

    Interesting.

    I omitted to mention - for sake of simplicity- that I briefly decoupled the
    3525 shutdown pin with a 10 nF cap to local gnd to avoid any noise pickup.
    Frankly it shouldn't be a sensitive node but I'm really puzzled now.

    That 'helped' the odd 'jitter' on the hi-side drive. I thought I was on the
    home run - I selected the switch position that stops the 3525 outputs and
    removed power. A couple of second later the IR2110 went *bang* - well fizzz to
    be more accurate - ( no IGBTs danaged though ).

    What's weird is that it was the low side driver that went short ! About 2 ohms
    from Vs to Lout after it went bust !

    waveforms are fine sans IR2110.

    I'm driven bonkers how the pulse width on OutA and OutB is different whern
    it's in circuit ( one presumes there's a clue here but I haven't yet been able
    to make any sense of it ). One might assume a load current issue but I have
    only 33k from DCbus/2 to the switching point !

    I admit - I'm tempted to provide a specific Vboot supply anyway - using a 555
    for example.

    But this ain't the issue
    I'll try that !


    Cheers, Graham
     
  4. Terry Given

    Terry Given Guest

    Pooh Bear wrote:
    [snip]
    perhaps there is a solder short, or noise coupling into that pin. I read
    the 3525 datasheet, and its not clear from the block diagram how such an
    event might occur, but without looking at a real diagram theres no way
    to tell.
    there are some Unitrode app notes that talk about this, and IIRC bipolar
    output stages are most susceptible, FET output stages are pretty much
    immune. Some Unitrode datasheets even say "no output schottky diodes
    necessary" but alas I cant recall which ones.
    true, along with whatever stray capacitance (FETs, layout etc), which
    will cause nice fast current spikes and associated LdI/dt voltage
    spikes. the dV/dt wont change much at light load (and may even get
    worse), so capacitively coupled noise is still there.

    does the magnitude of Vdc affect the crazy pulse?
    my SOP is to provide multiple well-isolated linear power supplies for
    every gatedrive, and use those exclusively, until my power stage can
    happily drive full load all day. Or at least until I'm happy the
    gatedrive really works.

    doing this allows you to ignore the charge-pump power supply aspects,
    UVLO etc. and just concentrate on banging the gates around.

    bung a lamp in series with the VDC supply, its cheaper than a FET.


    Cheers
    Terry

    PS the PDF offer stills tands.
     
  5. Guest

    Hi, the chip has pulse limiting, a noise pulse generated by the top fet
    appearing on pin 10 will reset the pwm latch. Try a hard short from pin
    10 to 12. You must also decouple pin 13 (Vc) to ground, 0.1uf should
    do. I havent checked the data sheet but 100kHz seems a bit fast for
    IR2110.
     
  6. I have no experience of smps supplies, just posting
    to register an interest in learning along with you.

    Hand capacity effects around the Shutdown input?
    A Shutdown input that is both directly connected to the
    output OR gates, or is capable of doing an early PWM pulse
    termination via the S-R flip flop?

    Try sticking a 0.1uF from Shutdown to 0v, right at
    the chip?
     
  7. Pooh Bear

    Pooh Bear Guest

    Actually I thought 0.1uf was a bit big so I added 10nF there. It had the
    desired effect you suggest.

    It seemed to stop the 'jitter' on the trailing high side edge but then I
    went and switched the damn thing off and the IR2110 went Fizzzz as the
    rails decayed ! And it was the the *low* side driver that had gone bad !
    It measured 2 ohms form Vs to out after this incident !

    My brain is going a bit bonkers.

    I'm baffled. Doesn't happen often. Last time I was this perplexed was
    when I realised that I had a very fast voltage driver stage in an audio
    amplifier that was causing a reverse bias avalanche breakdown problem in
    the output devices. And don't even begin to ask how I worked that one
    out !

    Graham
     
  8. Pooh Bear

    Pooh Bear Guest

    OK - I'll look for that but it's the *input* signls to the IR2110 that are
    wanky - freaky even ! I keep wondering if I'm probing the correct pin !
    It's that bad !
    Loads of local decoupling.
    Nah - it's not that. Thanks for your interest and input regardless !

    Regds, Graham
     
  9. I don't seem to see complete threads these days, so
    apologies if this remark is out of date.

    Perhaps point your bifocals at the 3525 UVLO, and
    think about whether it is coming in too late, (on
    power-down), and is not making the output stages safe
    before the main supply rails drop to a damaging
    undervoltage.
     
  10. Ol' Duffer

    Ol' Duffer Guest

    Going entirely from memory...
    I thought the SG3525 was a push-pull flyback converter, but
    you use terms like "high side driver" and "half bridge", which
    make no sense in that context. I can't tell from your scattered
    verbal description what you are trying to do, but it sounds fishy.
    Got a schematic?
     
  11. "Pin 10 should not be left floating as noise pickup could
    conceivably interrupt normal operation."
    Ground pin 10 and see if it helps.
     
  12. Fred Bloggs

    Fred Bloggs Guest

    If the 500ns OUTA pulse is "clean"- no jitter, then you have something
    triggering/resetting that internal FF drive in the 3525. How does that
    get there? I haven' looked at the 3525 datasheet, but there are several
    avenues: RC timing ckt, feedback, CL shutdown, and maybe some others- it
    may take only a 5ns high frequency blip to feedthrough on the wrong hi-z
    receptor and off it goes. The high dV/dt's and the proximity suggest
    some beading, orientation, distance, and/or shielding may be required
    somewhere, but then you already have everything on a board. You can
    track this down by probing around for other events while triggering off
    the negative edge of OUTA.
     
  13. Pooh Bear

    Pooh Bear Guest

    Yes, I looked at that area. I'm unhappy it's adequate so I'm intending
    to add my own over-riding UVLO that monitors all the rails.

    In the instance I mention however I had already asserted the shutdown
    function on the 3525 before allowing the rails to decay - i.e. OutA and
    B were both inactive. Then the IR2110 expired as the volts dropped !

    Graham
     
  14. Pooh Bear

    Pooh Bear Guest

    Cheers Kristian. Did that quote come from the data sheet ?

    Ok just found it.

    Graham
     
  15. Pooh Bear

    Pooh Bear Guest

    The 3525 is simply being used to generate the timing waveforms. The 3525
    outputs drive an IR2110 half bridge arrangement.
    It's a configuration very similar to one used by one of the major
    pro-audio amplifier companies in a mildly different incarnation which
    has proven reliability. The differences are modest and I've added some
    further protective features in fact.
    Not that I can post right now sadly.

    Graham
     
  16. Pooh Bear

    Pooh Bear Guest

    Quite !
    After others' comments I'm going to concentate on pin 10 - the shutdown pin.
    Despite the fact that the block diagram of the chip's internals shows no
    obvious route for this to cause the effect, I've been pointed to a caution on
    the data sheet regarding it.

    Cheers, Graham
     
  17. Pooh Bear

    Pooh Bear Guest

    Lol ! The data sheet application schematic shows the pin hi-Z in normal use !
    Time to see the truth I guess ?

    Graham
     
  18. I'd be tempted to link the Shutdown pin to 0V and
    do external shutdown with a clamp across the slow
    start capacitor. This would avoid uncontrolled
    re-starts if the shutdown command glitched.

    Perhaps use a MOSFET with pullup to 12V so that
    any global undervoltage circuit has to generate
    an active pulldown to keep the 3524 Enabled.
    The SG3525 data sheet does not say what happens to
    the (active) logic-low levels of the OUTs when it's
    power supply goes below 8v. At some unknown low
    supply voltage the OUTs might go to an open circuit.
    Would it be prudent to have safety pull-down R's on
    the IR2110 inputs?
     

  19. It sounds like this is for your employment. Therefore my first
    recommendation is that you might consider using an off the shelf powersupply
    or working with a company that specializes in designing and manufacturing
    SMPS designs specially for people such as yourself. You could spend all day
    (weeks/months/perhaps even years) banging your head against the wall in
    frustration. In the end you might end up with a product that seems reliable
    and meets your requirements to your satisfaction only to find in your horror
    that once deployed in the field it experiences all sorts of unanticipated
    failures. It is very easy to overlook things in an SMPS even when it seems
    to be operating well. If you are a generalist doctor (or even a podiatrist)
    you probably have no business performing heart transplants when there are
    plenty of heart surgeons available.

    Nevertheless for your own edutainment it would still be very valuable to
    pursue the causes of your current SMPS ailments on your own time.

    By the sounds of it your design has multiple weaknesses/problems which may
    or may not be directly related to each other.

    In particular the IR2110 seems to have attracted your intense scrutiny,
    however I would venture to guess it is probably only an innocent victim. As
    for the OutA/OutB jitter/unequal pulse width problems (as well as the human
    hand near the design and it changes performance), this can probably be
    explained by either your implementation of or your understanding of 3525
    pins 1, 2, 8, 9, and 10 (for dual inline package). These are all extremely
    important pins which are normally driven by relatively high impedance
    signals. They are quite sensitive and are quite integral to the control
    loop and the general proper functioning of your device.

    In particular the shutdown pin is not "slow acting" like you have theorized.
    It would appear from the block diagram that even a very momentary high
    signal on this pin will instantly turn off the output NOR gates, as well as
    set the PWM latch thus keeping the outputs off until at the very least the
    next cycle of the oscillator. How fast the outputs turn back on depends on
    what happened to the soft start capacitor.

    As for your IR2110 fizzling... It seems to me that if the output IGBTs
    haven't failed, then clearly something is very wrong. I guess that is
    obvious since the IR2110 failed... Without a schematic it is very hard to
    say exactly what you are doing and how turning off the 3525 might have
    effected the IR2110. What is the nature of your powersupply source for the
    3525 and IR2110? Maybe one theory might be the IC bias powersupply is not
    properly regulated, and when the 3525 load was removed the voltage overshot
    way exceeding the IR2110 absolute maximum rating. Another theory that I
    had, but consider quite unlikely, is that perhaps when the 3525 turned off
    the output effectively went high impedance, and perhaps stray capacitance
    was coupling the output of the IR2110 signal back to the inputs, thus
    causing it to oscillate at high frequency. In this case, perhaps the IR2110
    internal power dissipation grew too large and it subsequently toasted. I
    find this theory unlikely however since the IR2110 inputs have schmitt
    triggers and internal pull down resistors, and the input/output phasing
    would have to be inverted for this to happen.

    Whatever the cause of this problem, it sounds like it has a relatively low
    chance of being directly related to your other pulse width/jitter problems.
    I would not look directly at the IR2110 for the source of your problem as
    much the circuitry surrounding and hooked up to the IR2110 for the root
    cause.

    Aside from these considerations... Current limiting? You are using it
    right? Right?? Without it you are liable to get some kind of STD or
    something. Any high power SMPS which has any hope of being remotely
    reliable must implement some kind of highly effectual current limiting. The
    soft start and shutdown pins of the 3525 may play some role in a given
    current limiting implementation, but it is up to you to decide exactly how
    best to do it in your application. I personally like implementations that
    have two levels of safety. One level instantly shuts off the output as fast
    as possible (IE: <1us, preferably <<1us) upon detection of excessive
    instantaneous current, but doesn't latch the output off for very long (IE:
    perhaps only for the remainder of the given PWM cycle). The other level of
    safety measures the time averaged current and shuts the device down for
    several cycles upon detection of excessive average current. The fast cycle
    by cycle current limit is the best, but unless it is extremely fast it may
    not be able to keep the average duty cycle sufficiently small under all
    conditions, depending upon how it was implemented. Keep in mind that any
    current limiting scheme has the potential to interfere with the voltage
    control loop whenever it gets activated.
     

  20. This is a good point, however it appears the designer of the IR2110
    envisioned this scenario and took care of it. Presumably the IR2110 inputs
    have both schmitt triggers and internal pull downs.
     
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