>        >Could you have a saturating core?
>
>        are you saying that the humming is a symptom of a
>        saturating core?


Could be, I'd want to slap a current probe on the primary winding, and
see that the current ramp is linear. If not, you're probably
saturating.

>        yes. actually, i calculated #primary turns based on maximum
>        flux. according to mfg. data sheet, for my desired power
>dissipation and
>        operating frequency, i can be running at over 300mT in a
bridge.
>        since i am using a forward converter, my flux is in only one
>direction,
>        and i should be safely able to double that flux density. but
i
>didn't.
>        so i should be safe.


300mT is a lot of flux!  I think you may be in trouble here.
Especially about doubling the flux.
The core limit is regardless of polarity, so X gauss in either
direction will saturate you. With AC, you can drive from -X to +X, but
with DC you can only drive to +X or -X, not 2X.
There is a trick that will let you get to 2X, but it's evil, and hard
to use in a pot-core or toroid.



>        >a CS3844 or some such would
>        >be a lot more predictable, plus operating nicely at 300Khz.
It also
>        >monitors the core current, and will not let you saturate the
core, as
>        >long as your sense resistor is picked properly.
>
>        now that could make life easier. who makes this device? do
you have
>        any preferred devices?

Cherry, Ericsson, and Unitrode all make these, 3842,43,44,45 The
differences are what startup voltage, and wether or not they have a
max duty cycle limit.  They also have a nice beefy output drive for
turning on a FET, IIRC it's about an amp in both directions. Plan on
adding a resistive divider in the gate drive, this will let you
control the gate ramp. In my applications, it turned the FET on and
off so fast that I was getting a fair bit of EMI, (from the rapid
dV/dT) Slowing it down slightly made the fet a little warmer, but much
quieter

I prefer the cherry parts, the internal zener is a bit more robust,
but they all work well.