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

No Leg,

It is a forward with feedforward and a fan.

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

Cheers Terry

.....to determine peak flux density.

RL

I thaught so, I need to think...

Reggie.

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

The questions I need answering are:

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?

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