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Core power handling Capability

Discussion in 'Electronic Design' started by Hammy, Apr 25, 2010.

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  1. Hammy

    Hammy Guest

    I have some Flyback transformers (from CoilCraft) these were
    originally intended for use in a 70W 60kHz application using the
    NCP1200. If I increase the switching frequency to 100kHz is it
    reasonable to expect 90-100W using the same core?

    I've been running simulations and the rms and peak current for 100kHz
    equalizes at 90W ,with the 70W 60kHz flyback. There is a higher DC
    component of course (about 10-15%) due to a reduced switching cycle,
    the converter is deeper in CCM.

    I had a sheet giving approximation on core power throughput vs
    frequency and it shows that for 70W 70kHz increasing the Frequency to
    100kHz the same E-Core could handle a little over 100W. I know I could
    expect higher DC winding losses; in a pinch I could add another
    parallel winding there is already two it would be tight though.

    Any magnetic experts out there?
     
  2. Hammy

    Hammy Guest

    It is from this application note from On Semi.

    http://www.onsemi.com/pub_link/Collateral/AND8076-D.PDF

    This is pretty well all they have for the transformer.

    "Transformer

    Below are the key parameters you will pass to your
    transformer manufacturer to help him select the right
    winding size and tailor the internal gap:
    Maximum peak primary current, including 160 ns
    propagation delay: 1 / 0.33 + 374 × 160 n / 700 m = 3.2 A
    Maximum primary RMS current at low line: 1.6 A
    Maximum secondary RMS current: 6.9 A
    Primary inductance: 700 mH
    Turn-ratio, power section: Np:Ns = 1:0.166
    Turn-ratio, auxiliary section: Np:Naux = 1:0.15"

    I'm not exceeding any of those ratings upto 90W with a safety margin.
    If I had to guess and I do it may be either a ETD34 its about 1/2 a cm
    longer then an ETD29 core that I have or an EE32.
    Yes your probably right. I will just hook it up and monitor for
    saturation and temp rise.
    Components have improved since they did the AN I'm using a FET with
    1/5 the rdson with smaller gate charge then the one they used as well
    as sync-rectification.That alone reduces the losses compared to
    original design by about 4W.
     
  3. legg

    legg Guest

    The spread-sheet assumes that you are reconfiguring turns, gap and
    wire guage for the different operating frequency.

    Simply increasing the frequency may reduce flux peaks, but core loss
    will not reduce proportionally if the frequency is increased at the
    same time. The net result is increased total loss even without an
    increased current through-put.

    Copper losses increase with the square of the current density.

    What actually happens depends on how the original transformer was
    designed. If you don't know the core shape, material, turns, wire
    gauge, voltage ratios and frequency, there's little point in
    speculating.

    It'd be faster just to fire up the circuit and measure deltaT as the
    frequency-setting component is altered - if the circuit is
    pre-existing and you nkow no more about it.

    If the circuit is not pre-existing, you'll get better results working
    with fewer unknowns. Transformer loss is not the only consideration as
    a flyback circuit's throughput power increases.

    RL
     
  4. The loss in a ferrite core should be mostly hysteresis loss, which is
    roughly proportional to frequency and square of magnetic field intensity.
    Power throughput, at least in an oversimplified case, is proportional to
    the squares of frequency and magnetic field intensity.

    Ideally, ratio of throughput to core loss is proportional to frequency.

    Meanwhile, there are other issues:

    1. Resistance of the copper will be higher at the higher frequency,
    approaching proportional to the square root of frequency once the "skin
    depth" gets much smaller than the wire radius.

    2. The transformer may have enough inter-layer capacitance to cause
    a significant lowpass filter effect. Do you know that it will work at the
    higher frequency?

    Or are you planning to rewind it?

    3. If you rewind it and use fewer turns of thicker wire, keep in mind
    that the wire's resistance at the frequency in question may be closer to
    inverse proportional to the wire's circumference than to its cross section
    area due to the skin effect.

    4. The switching transistor's switching loss, as a percentage of power
    throughput, is proportional to frequency. The transistor's rise and fall
    times will probably be in the tens of nanoseconds, possibly more.

    Consider the energy dissipated assuming rise time times half the current
    initially conducted by the transistor (possibly zero) times the input
    voltage,
    plus the fall time times half the current being conducted by the
    transistor shortly before switch-off times the voltage that the transistor
    experiences during switch-off (always more than the input voltage, usually
    by a factor of more than 2, in flyback circuits).

    Keep in mind that rise and fall times in transistor datasheets are at
    junction temperature of 25 degrees C and with ideal or very good driving
    of the transistor. In actual applications, these times are usually
    slower (longer periods of time).

    Multiply the switching losses by frequency, add conduction losses
    (increases with temperature if the tyransistor is a power MOSFET), and
    determine if that is going to be too much heat for the transistor to
    dissipate. If the heatsinking is currently minimal, then it is *probably*
    easy to hack additional or greater heatsinking onto the switching
    transistor.

    - Don Klipstein ()
     
  5. legg

    legg Guest

    Quite frankly, I've never seen a power transformer described in this
    manner, nor is the list actually complete without it's surrounding
    article, in which topology, operating frequency, input voltage range,
    and output voltage are actually mentioned. It has to be assumed that
    the part must meet the requirements of some coordinated safety
    standard ~ 60950.

    The article itself seems a little quirky - focussing more on control
    chip pecadiloes than power train. As the part is being made available
    for this application, there is little emphasis on practical
    transformer design issues.

    One example; the choice of primary inductance is made arbitrarily on
    the basis of full load transition from complete to incomplete energy
    transfer mode (an irrelevant feature) at an arbitrary input voltage,
    somewhere (the author hopes) the psu will never actually have to run.
    Then this careful calculation is (equally arbitrarily) approximately
    doubled.

    Another interesting issue is the fact that, in the end, the author
    somehow achieves the design spec output power only at a higher output
    voltage than intended. Whether this indicates that the final iteration
    was incapable of the design spec current and voltage, or that the
    author was just to lazy to complete an accurate set of drawings for
    his article - is a mystery to me.

    You'd be better off haunting the old Unitrode seminar app notes, if
    you're interested in flyback transformer design iteration.

    http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014

    RL
     
  6. Hammy

    Hammy Guest

    Yes it is from Coilcraft; here;

    http://www.coilcraft.com/y8844.cfm

    Its the Z9260-AL

    I've noticed that some game consoles have the exact same specs PSU
    wise. 16.5V, 4.2A. I never took one apart but I'm guessing they are
    the PSU from the application note.
    Well it is written by C.Basso for Onsemi so I view like I view most
    application notes like commercials.

    http://pagesperso-orange.fr/cbasso/Spice.htm

    This I understand why they would do it. I don't think you would want a
    Flyback much above 30W operating in DCM at low line peak current could
    start to get high.

    The reason they give for adjusting the inductance is to reduce the
    peak current; the controller enters a burst/ skip mode at one third
    full load if the peak current is high the supply would generate
    excessive noise. The burst mode is in the audible frequency range.

    They probably tested and determined that 700uH is when the supply is
    tolerable noise wise.

    They could have just as easily decreased the level at which the
    controller enters burst mode.
    You noticed that too:) I was curious about why they did that as well.
    They mentioned earlier in the AN about having difficulty in
    maintaining sufficient voltage level on the AUX winding during burst
    mode this might have something to do with it.

    They are trying to highlight the amazing low standby power the chip
    can achieve. This can only be obtained with the DSS inactive and the
    controller being supplied from the aux winding.

    I'm hoping to have the time to get a board done up some time through
    the week.

    I'm using the NCP1217 100kHz version. I found an appnote using that
    controller and the same CoilCraft transformer. The power output is 85W
    but again it's higher output voltage at 3.5A.

    http://www.onsemi.com/pub_link/Collateral/DN06038-D.PDF
     
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