max flux density of 250mT for TDK PC40?

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=3DNd=D8 /dt



V=3DN*Ae (dB/dt)

therefore following integration etc...

V *dt =3D N *Ae * dB

And thus

dB =3D (V*dt)/N*Ae---(1)

and by similar methods

dB =3D (L*dI)/N*Ae----(2)

(d indicated change or delta)

2) For a continuous forward converter, would this 250mT =3D (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 =3D (dT) or

b) the max current (from zero to the top of the trapezoid) during the fet on time =3D (dT)

I think a)

Have I the wrong idea?


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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.

Reply to
Fred Bloggs

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.


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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.
Reply to
Fred Bloggs

d text -

I have looked at the data below:

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=46rom 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 =3D the working component =3D 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,


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