Re: Flyback regulator / transformer problems.

Don't forget the vital formula relating minimum number of Turns N, Core
cross sectional area A (m^2) RMS Voltage V, and Saturation flux density B
(use 0.15 to 0.2 Tesla for most ferrites) :

V = 4.44.F.N.A.B

copied from Andy G4JNT

interstingly enough path length and gap length afect current but not
saturation.

another cuase of saturation is lack of de magnetization

Colin =^.^=

"Paul Mathews" <optoeng@xxxxxxxxxxxxxx> wrote in message
news:1117644315.089089.69280@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> to prevent saturation: shorter pulse duration, more turns, longer path
> length, large cross section, ...
> Paul Mathews
>
>
> seegoon99@xxxxxxxxx wrote:
> > Robert Baer wrote:
> > > seegoon99@xxxxxxxxx wrote:
> > >
> > > > Hi to all.
> > > > Thanks the help.
> > > > The phasing is correct as far as I know. I changed the aux winding
> > > > around , but things got worse , so I assume I had it correct.The 5V
> > > > output seems good , so I assume that it is correct.Tried changing it
> > > > around and the supply weny haywire!!
> > > > As to the core saturating. Surely if the core saturates the output
> > > > voltage would fall.
> > > > The output seems nice and steady under load.
> > > > I'll try to go through all the tests you mentioned , sounds like
fun!!
> > > > I don't have a current probe , but I'll use a resistor to measure
the
> > > > current.100 ohm to 1k seems like a large value though. I'd have
thaught
> > > > a few ohms would be more appropriate , or am I miss-understanding
you?
> > > > Once I have the amp/turns figure ,
> > > > I'm not sure where to find the "effective length" from the data
***.
> > > > As you may have guessed ,I'm fairly inexperienced with this sort of
> > > > stuff , so please excuse my ignorance!!
> > > >
> > > >
> > > > I'm not sure what you mean when you say
> > > > "
> > > > In any event, with a Ferroxcube P14/8 core of 3C18 material (or
> > > > similar for that matter), i can easily get 1000V for a 56 turn
1000uH
> > > > inductor; supply from about 5V to 200V (adjusted pulse width for
> > > > constant peak current of 240mA).
> > > >
> > > > "
> > > >
> > > > cheers
> > > > Rob
> > > >
> > > One ohm from FET source to ground; for current sensing.
> > > SMT for low inductance; if cannot do, use two 1/4 watt 2 ohm carbon
> > > comps, at 30-45 degree angle ontop of the ground plane - shortest
leads
> > > possible.
> > > Then a 1K from the source to a test point; short lead at the soruce
end.
> > > This isolates capacitive loading and other effects of probe loading
> > > on the circuit.
> > > **
> > > Every core has a magnetic path with an effective length associated
> > > with its size and shape.
> > > Perhaps you can access a PDF of the core i mentioned via:
> > > http://www.ferroxcube.com/prod/assets/p148.pdf
> > > Look at the second page, top left for effective length (for that
core).
> > >
> > > **
> > > I gave an example of what i had done that was chosen to be close to
> > > what you had mentioned.
> > > That way, you have a rough idea as what to expect.
> > > Sorry cannot give examples of power inverters, but i thought
> > > something was better than nothing.
> >
> > Hi there.
> > Thanks for the help.I see you are refering to magnetic path length , so
> > I should be able to make some progress from here, I hope.
> > I did a measurement of the current on my board as it's running , and
> > the core may very well be saturating. The FET source is connected to
> > GND via a 1ohm
> > sence resistor which is connected to the uc3842 via a low pass filter
> > consisting of 1K to the current monitor pin and 470pF to gnd.
> > The voltage across the 1ohm resistor rises linearly(about) to 400mV ,
> > and then sharply to about 3V. This indicates to me that the current is
> > rising as it should to about 400mA and then the core is saturating and
> > the current ramps up to 3A os so. The only thing stopping the smoke
> > comming out is the uc3842s current sensing switching the output off.If
> > the core is indeed
> > saturating , how can I prevent this.I have used 2 app notes from
> > different manufacturers(unitrode and infineon) and come out with
> > similar parameters(turns , inductances etc)for the transformers.
> > If the transformer is saturating , I must be doing somethig
> > fundementally wrong:0(
> > Any ideas.
> > Cheers
> > Rob
>


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