Small transformers and LTSpice simulation

On 4/22/2011 9:12 AM, Grant wrote:
> I have a design that could use three or four winding toroid transformer.
>
> In the LTSpice notes they suggest keeping the transformer coupling at
> 1.0 or -1.0 otherwise there's lots of high frequency noise generated,
> I'm not sure if that's real world, or the simulation going silly?
>
> What I'm wondering is just how much coupling should I expect to see in
> a real world transformer of around five to ten turns on a half or
> three-quarter inch diameter toroid?  Like the ones you see on PC mother-
> boards.  Or the larger mag-amp ones from PC power supplies that come
> with two or three windings.
>
> I have lots of toroids recovered from power supplies, no idea of what
> inductance they have until I power up some circuit and try to match
> LTSpice inductance with observed waveforms ;)  I'm guessing 33uH at
> the moment for 40 or 50kHz operation.
>
> I do notice that over-voltages start at close to ideal coupling, for
> example 0.97 can give a nasty over-voltage spike on the leading edge
> in LTSpice.  I don't know how much of that to expect in a real circuit,
> any guidance here?
>
> What I plan to do is drive a transformer with a current limited latch
> circuit, +ve edge turns on a 'hc74 flip flop, current sense through an
> npn will pull 'hc74 reset line down.  Should be safe enough to watch
> the waveforms.
>
> Frequency of interest is 20 to 100kHz, current up to 2A through N-chan
> MOSFET driving the transformer, small snubber on primary as suggested
> by LTSpice, secondaries are standard flyback, pair of schottky diodes
> and caps.
>
> I might even use a 555, as it has a -ve reset line?  :)  Save me building
> a separate oscillator, add the voltage cutoff and it's done, cheap'n'nasty.
>
> Can one make a 50KHz oscillator from half an 'HC74? and an R + RC?  Is there
> a odd numbered ring of inverters hiding in there?
>
> Thanks,
> Grant.


Hi, Grant -

Take a look at the LTSpice non-linear transformer in the educational 
folder. Probably not what you're looking for, but it may be helpful in 
some way.

John