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

B=3D=D8/Ae

Thus:

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 ofa) 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?

Reggie,