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max flux density of 250mT for TDK PC40?

Discussion in 'Electronic Design' started by reggie, Feb 21, 2008.

  1. reggie

    reggie Guest

    A rule of thumb I have heard about and want to check its validity is
    that:

    For a fan cooled forward converter transformer, employing feed forward
    should have a flux density lower than 250mT as not to saturate for TDK
    PC40.

    1) Is this a reasonable estimate of max flux density at say 100 Deg C
    100KHz.

    I know that in PSU terms (not described accurately mathematically)

    V=NdØ /dt

    B=Ø/Ae

    Thus:

    V=N*Ae (dB/dt)

    therefore following integration etc...

    V *dt = N *Ae * dB

    And thus

    dB = (V*dt)/N*Ae---(1)

    and by similar methods

    dB = (L*dI)/N*Ae----(2)

    (d indicated change or delta)

    2) For a continuous forward converter, would this 250mT = (dB) limit
    be as a result of

    a) the ripple current peak to peak in the coil (from low value to high
    value of the trapezoid) during the fet on time = (dT) or

    b) the max current (from zero to the top of the trapezoid) during the
    fet on time = (dT)

    I think a)

    Have I the wrong idea?

    Reggie,
     
  2. Fred Bloggs

    Fred Bloggs Guest

    The main limitation is power dissipation density (W/cm^3) for a maximum
    temperature rise (usually 40oC) as a function of frequency, and 100KHz
    is pushing it, almost always requiring a derating of Bmax,pkpk. You're
    working with DC specs.
     
  3. reggie

    reggie Guest

    Thanks Fred,

    I take your point about power dissipation and temperature rise.
    Are you saying that at 250mT Peak running at 100KHz the temperature
    rise would be to great, ie above your 40 Deg C ?

    I am not sure what you mean by DC specs, could you please explain.

    In essence I am after what dI causes which dB in relation to the
    current waveform and the resulting BH curve.

    Reggie.
     
  4. Fred Bloggs

    Fred Bloggs Guest

    Those TDK datasheets should have a power density graph of dissipation
    density versus frequency with flux density peak variation as a
    parameter, you are interested in the ac-component of flux density. This
    dissipation results from the energy required to force the material
    through its hysteresis loop and is nonlinear with frequency or the rate
    at which you flip the polarity of the magnetization. The 2500 Gauss
    sounds more like a DC saturation flux density for the material, at
    100KHz, you may have to back this off to 1000 Gauss or less. The DC flux
    bias also sets a limitation on your ac-component of flux but does not
    directly contribute to power loss and heating. And you're right, you
    have to use the equations you mention relating flux to voltage,
    frequency, and effective cross-sectional area, this is your starting
    point for estimating the magnitudes you need to size the core.
     
  5. reggie

    reggie Guest

    I have looked at the data below:

    http://www.mhw-intl.com/assets/pdf/...0_PC47_PC90_PC95_Material_Characteristics.pdf

    From page 7 onwards I get blank pages because of erros in installing a
    new character set. But I see what you mean on other materials, Can I
    get this data from anywhere else. I use to have it in a booklet from
    TDK. Could I request this from someone? Anyway I see your point

    Now, In a forward converter the flux swing = the working component =
    ac component as you put it (I need a diagram!) on a B-H curve, moves
    the working flux density up towards saturation. If there is too much
    DC component the top end of the working flux density will start to
    saturate and one gets that classic non linear spiky primary current
    waveform. If you catch it before the fet blows up!

    The 2500 Gauss I think is a rule of thumb not to exceed flux density,
    when using feed forward and fan cooled, as the saturation flux density
    of TDKs PC40 is 390 mT sorry 3900 Gauss at 100 deg C.

    2500 Gauss must be the peak flux density, because if it were the
    working (ac component) for PC 40, the circuit would have to be
    discontinuous, ie the flux density would have to reset to zero.

    I am assuming that the reset winding in a continuous forward converter
    takes the flux density from the peak to the dc level.

    Does this make sense?
    I understand, It's the "loop area" that contributes to heating.

    Thanks again,

    Reggie.
     
  6. legg

    legg Guest

    I think Reggie is working on a flyback with incomplete energy
    transfer.

    The 'peak' flux density could greatly exceed deltaB.

    RL
     
  7. reggie

    reggie Guest

    No Leg,

    It is a forward with feedforward and a fan.
     
  8. Eeyore

    Eeyore Guest

  9. Eeyore

    Eeyore Guest

    You are CORRECT to use mT. Gauss is a deprecated cgs unit.

    Can you use PC44 or PC47 ? These will have lower losses and the core calculations will hardly differ much.

    Graham
     
  10. Terry Given

    Terry Given Guest

    in which case the magnetising current is what you are interested in, not
    the reflected load current.

    Cheers
    Terry
     
  11. legg

    legg Guest

    ......to determine peak flux density.

    RL
     
  12. reggie

    reggie Guest

    I thaught so, I need to think...

    Reggie.
     
  13. reggie

    reggie Guest


    I have been thinking, not alot (it hurts!)...

    The questions I need answering are:

    1) is 250mT peak, forward converter, with feed forward and a fan an ok
    target?
    2) how would I calculate the B peak, to make sure the transformer does
    not saturate
    3) is saturation B peak for TDK @100 deg C 390mT (or is it peak or
    peak-peak)
    4) can someone explain this with relation to ripple current and what
    this looks like on a BH curve.

    I want to be able to plot the BH curve for my forward converter using
    instantaneous values of B, so to check that it isn't saturating.

    I know that one uses different "B" formulas for bidirectional and
    unidirectional driven chokes., since this is a forward converter in
    need unidirectional, so need to take into account residual flux
    density remaining in the core prior to the next cycle.

    I think this is what I need..

    Any ideas?
     
  14. legg

    legg Guest

    It won't saturate, but how hot will the transformer get? If you add a
    fan, you give yourself some leeway, but you will need thermal
    protection, because the safety isolation transformer is one of the
    few items for which there are published acceptable maximum temperature
    limits under single-fault ( ie jammed fan) conditions.
    Bpk = v * t / ( N * A )

    Best calculated at normal minimum operating voltage ~ dropout.

    Bpk = peak flux in Teslas
    V = applied primary voltage in Volts
    t = maximum on time in seconds
    N = primary turns count
    A = core xsectional area im meters squared
    Saturation is a flux level that can be achieved on purpose or
    accidentally. In a forward converter, with flux effectively reset to
    ~0 during the off time, it would have to be achieved in one swiching
    cycle. This is not impossible, but in a static/stable regulated
    forward converter system it would not be expected.
    In a coupled transformer, flux levels are independant of load current,
    due to transformer action. Secondary current flux cancels primary
    current flux. Only magnetizing current is uncoupled and controlled by
    Lmag.

    Ripple current in the output choke has no effect on the transformer
    magnetization.
    The Bpk formula is valid for any specific point in the swiching cycle.
    If the voltage reverses in sign, the direction of deltaB reverses.


    RL
     
  15. Eeyore

    Eeyore Guest

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