Small transformers and LTSpice simulation

Started by Grant April 22, 2011
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