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Buck Converter Output Capacitor

Discussion in 'Electronic Design' started by QQ, May 8, 2004.

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  1. QQ

    QQ Guest

    Hi all,

    I would be grateful if you can help me clear this basic doubt
    regarding buck converters:

    During startup of a buck converter I notice there is an "inrush"
    current that flows into the output capacitor. I think that this inrush
    current can be reduced by increasing the soft start capacitance. But
    increasing the soft start cap reduces the rate at which the output
    voltage comes up, which is something I don't want to do. So I select
    FET's which can handle the inrush spike and and inductor which doesnt
    saturate at the level of current. Also I inrease the R_over_current
    sense resistance value. But what parameter of the output capacitor
    should I be looking at to determine whether the output cap can handle
    the inrush current?

    The datasheet for the Cap seems to specify Rated Voltage, Rated Cap,
    EST, Rated ripple current, Tangent of loss angle, Leakage current.

    Thanks,
    QQ
     
  2. R.Legg

    R.Legg Guest

    Current ratings in a capacitor are thermally derived. The only parts
    I'm aware of that are sensitive to surge currents are some solid
    tantalums.

    With fet's and inductors as series limiters, there is generally no
    concern for the capacitor, as fets are more likely to be damaged
    first. Even when the charge transfer is from capacitor to capacitor,
    or source impedance is extremely low, damage is more likely to occur
    first in the switch - even if (or perhaps especially if) it's a
    mechanical one.

    In a buck regulator, if the current is controlled in any way, it's the
    controlling switch that will suffer first, if only from agravated
    ground bounce affecting drive integrity. You should use the soft start
    to indirectly limit currents, with the aim to keep components within
    their intended operating ranges and to avoid disturbing the input
    supply, if the current is not controlled directly. This control need
    not limit the rise severely, but disabling it entirely is not a wise
    step.

    How large a dV/dT are you attempting on the output and for what
    reason? How large a filter capacitor are you using and how was it
    selected?

    RL
     
  3. R.Legg

    R.Legg Guest

    I forgot to mention that the capacitor parameter controlling peak
    current is most commonly found as a dV/dT limit. Ipk = C x dV/dt. This
    is usually applied to film capacitors and reflects the ruggedness of
    internal metalization and it's contact integrity to the schoopage.

    RL
     
  4. James Meyer

    James Meyer Guest

    Good question. One way to reduce the inrush current is to reduce the
    value of the output capacitor to a value as small as the converter can tolerate
    consistent with the ripple requirements.

    Jim
     
  5. Joerg

    Joerg Guest

    Hi James,

    Not necessarily. A switcher inductor attempts to instantly dump whatever its peak
    current is into that output cap. Smaller caps might not live long if they were
    rated way under that current level.

    Regards, Joerg
     
  6. QQ

    QQ Guest

    Thank you all for your informative answers. I have another question
    which I would like to know the answer to:

    On one of the (voltage mode) buck converters I have designed the
    switching frequency is 700KHz. The output voltage which has a ripple
    of about 5mV is modulated by a low frequency sinusoid (25KHz
    frequency) of ripple about 15mV. I have used appropriate probing
    techniques. I am quite new at this and I dont know why this is
    happening? Instability is my guess but I had designed for a phase
    margin of about 45deg and gain margin of about -70db. Any ideas as to
    what the problem could be?

    Thanks
    QQ
     
  7. R.Legg

    R.Legg Guest

    Check your measurement set-up.

    If a computer is involved, disconnect it to see if there's any
    difference. Common culprit is ground noise through a serial port or
    similar hookup.

    I note that you haven't answered any of the questions your previous
    post raised, nor have you enlightened us on any cure that may have
    been found to the originally posted problem, in the meanwhile. That's
    pretty shabby use of a two-way communication channel.

    RL
     
  8. Roger Gt

    Roger Gt Guest

    : Thank you all for your informative answers. I have another
    question
    : which I would like to know the answer to:
    :
    : On one of the (voltage mode) buck converters I have designed the
    : switching frequency is 700KHz. The output voltage which has a
    ripple
    : of about 5mV is modulated by a low frequency sinusoid (25KHz
    : frequency) of ripple about 15mV. I have used appropriate probing
    : techniques. I am quite new at this and I dont know why this is
    : happening? Instability is my guess but I had designed for a
    phase
    : margin of about 45deg and gain margin of about -70db. Any ideas
    as to
    : what the problem could be? Thanks QQ

    There may be some random resonance in the circuit, not at all
    uncommon in power circuits. Also hard to find if your not used to
    this level of trouble shooting.
     
  9. QQ

    QQ Guest

    Sorry Legg, I didnt answer the previous question. I want dv/dt to be
    fairly large becasue this DC/DC is on a plug in card. Some of the
    power supplies that the chip uses already are present when this card
    is plugged in. So this particular rail cannot come up too slowly or
    the chip begins to draw current from this rail. Anyway I think I have
    found a compromise value of Rocset but thanks for your answers. It was
    quite informative.

    Measurement setup seems to be fine since I cannot see a similar effect
    on other DC/DC rails. No computer is connected to the device. One
    other points I should probably mention. The ouput cap is 100uF
    ceramic. Also I have "heard" of a condition called subharmonic
    oscillation due to instability. Do you think this is an instability
    issue? I tested the power supplies with load tranisents and it does
    seem to recover from load transients fast enough which makes me
    suspect that that is not the problem. Also the low frequency ripple is
    in the range of 15mV peak to peak while the "actual" ripple voltage is
    5mV peak to peak only. I think this low frequency ripple is of a very
    low level which again makes me think, not an instability issue.

    What are your comments on this?

    Thanks,
    QQ
     
  10. legg

    legg Guest

    Is this condition occuring when plugged into the system, or when it is
    stand-alone?

    It is not subharmonic instability, as 25KHz is too far off to be a
    half, third or quarter tone of 700KHz.

    If the supply is running in a system with other converters operating
    at a near-frequency, this could be a beat-frequency. This occurs when
    another unit is operating 25KHz away from the DUT. It is not
    guaranteed to be inaudible as the frequency can be anywhere within
    your circuit's tolerances. Multiple units in the same system only
    avoid this through synchronization.

    You may find that it is an effect already common in the system, using
    previous supply types in parallel, unless they were somehow configured
    to avoid it.

    If there is any sharing circuitry, you might check them out for
    contributing funny business as well.

    By the way, at what frequency is your '45degree margin' obtained?

    Your inrush problem sounds like a shortcoming in the control
    circuitry, or mating configuration. Paralleled and hot-swappable
    circuits have to be designed for the function - they will not occur
    accidentally.

    RL
     
  11. QQ

    QQ Guest

    Hi Legg,

    Thanks for your time. I am getting more and more convinced after
    showing the waveform to others that it is probe noise. They say that
    the scope probes can pick up random small magnitude noise components
    which could show up when the voltage scale is reduced to the 5mV per
    div range. Also load transient testing gives good recovery which is
    comforting. Its a digital tektronix scope by the way. And the probe is
    'fashioned' from a 50 ohm BNC cable to avoid noise pickup
    (ironically!).

    The gain crossover frequency is designed to be about 20 KHz and that
    is the point where the phase margn is lowest ie. 45 deg. I havent
    verified this using a frequency response analyzer though.

    Ive heard that stability can be tested using load transients. How
    exactly does one set that up? Of course if the supply recovers on load
    transients then it is stable. But is it possible to say, just looking
    at load transient response, what the gain margin and phase margin is
    etc.?

    Thanks,
    QQ
     
  12. legg

    legg Guest

    And no other plotters, printers or other devices are connected to the
    Tek?

    Using 'fashioned' test harnessing can mean you end up having to ship
    one of them with each unit. A calibrated scope probe, plugged into a
    hard-soldered socket, is probably a better solution, even if you have
    to add an amplifier to get measurable amplitudes.
    I thought you said you'd done load transient testing on the DUT.
    Fiddle with your compensation and note the differences. Don't forget
    to vary the supply voltage over it's range. Apply the heat gun and
    freeze spray, if you don't have a thermal chamber. Look at the input
    current, too, while hitting the output with the load change.

    The main kinks in load transient testing are in obtaining meaningful
    response around zero load or near the device's limit, and also in
    getting good definition and control of the load current wavefront
    di/dt. You will find that most shunts give erroneously high reports,
    as their bandwidth is abysmal.

    RL
     
  13. Well, is the signal still there with the circuit unpowered?

    Try unplugging everything else in the vicinity (computers, power
    adaptors, everything you can except your circuit and the scope!).
     
  14. Dario

    Dario Guest

    NTC resistors are widely used to ward off inrush current attack !
     
  15. legg

    legg Guest

    Your problem was control of rising output voltage in a hotswap
    circuit. Methods of charge equalization, prior to shorting the output
    capacitors to the functioning bus, are what I thought you should be
    looking at, if the controlling circuitry was not inherently
    bamboozled.

    The problem is normally considered to be one of firstly avoiding dips
    and spikes in the functioning bus due to the back-surge or turn-on
    overshoot; secondly avoiding connector pin damage - an issue in larger
    or higher current circuits; then finally, of course, you want the
    newly plugged-in circuit to run normally.

    NTC's have an energy rating. The problem of operation, hot, is not
    that the energy increases, as this energy is usually determined by the
    capacitors and application voltage. The problem is that they no longer
    provide the intended inrush limit.

    If they are present when someone tries to energize a circuit that has
    already failed, then obviously the energy that they experience is much
    higher than normal. An exploding NTC is a sign of an over-sized fuse
    or overstressed NTC.

    RL
     
  16. Terry Given

    Terry Given Guest

    yes they are. And if you look carefully, you will find (as I did) that
    virtually NO NTC can handle the energy required to charge a typical bus cap
    when the NTC is hot - they tend to explode, instead. which they do.
    Unfortunately, NTC's are designed to start cold then run hot - so power
    cycling is a great way to make them explode. Legg, whats your take on NTC's?

    Terry
     
  17. Terry Given

    Terry Given Guest


    I did some work on a number of smps that had NTC inrush limiting. As usual
    Legg is spot-on wrt energy ratings, and that .5CV^2 governs E, not the NTC
    resistance. What the NTC does govern, however, is the peak power
    dissipation, V^2/Rntc (ie the rate of change of energy). If the NTC is hot,
    E doesnt change but Ppeak does - for 100:1 cold:hot resistance, Ppeak is
    100x higher when hot. It is this I found most NTC's couldnt handle. The eqpt
    in question was 90-265Vac rated, Rhot = 1R, so (265*1.41)^2/1 = 115kW peak
    pulse power; (115*1.41)^2/1 = 26.3kW. I tested a range of NTCs, and couldnt
    find one that would survive this (and fit in the space allowed). Some (the
    smaller ones) exploded, most failed short-circuit.

    When I discovered this, I went and looked at some real units in the field
    (115Vac), and found about half had shorted NTCs. Note that this argument
    alos applies to resistors, too - most people consider average power
    dissipation, but ignore peak power (I have re-worked several smps designs
    where large FET gates were driven from +12V supplies thru 4R7 0805
    resistors, which see about 30W peak, and eventually fail)

    I once had to do a forensic analysis on an ac motor controller that caught
    fire. soft-charge was resistive, bypassed by a relay. The actual problem was
    that the customer, in a fit of inspired stupidity, had shorted the DC bus
    terminals together. These of course were after the soft-charge circuit, the
    relay of which was controlled by the main smps - no DC bus, no relay, so the
    customer had dropped the national grid across the soft-start resistors and
    left it there. It turns out that RTV burns nicely at 900 degrees C, and of
    course the input current was nowhere neare enough to blow the fuses. We
    ended up selecting Vitrohm for the soft-charge resistors, as they failed
    open-circuit under these conditions. The customer also had to foot the bill.

    Terry
     
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