# Re: max flux density of 250mT for TDK PC40?

• posted

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.

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

The 'peak' flux density could greatly exceed deltaB.

RL

• posted

No Leg,

It is a forward with feedforward and a fan.

• posted

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

Cheers Terry

• posted

.....to determine peak flux density.

RL

• posted

I thaught so, I need to think...

Reggie.

• posted

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?

• posted

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

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