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Auxilary winding config in a flyback transformer and how is it modeled in ltspice

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Oct 9, 2009
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The issue is regarding the use of the auxilary winding that can be used in flyback smps's to power the ic and increase efficiency. I'm a bit confused by how it's woven into a transformer and how it can be modeled. I do realize it's woven against the secondary winding such that it outputs a voltage (Vout + Vdiode)*(ratio between aux and secondary). Where Vout is the regulated output voltage.

It's obviously woven such that the primary voltage has no effect. Otherwise large input ranges (8-60V for example) would cause a very wide range on the auxilary.

Now i wonder how exactly this is done.

a). Is the auxilary actually a second tranformer with one winding (primary) in series with the secondary winding of the main, with inductances chosen such that it can only output a small fraction of the power of the main.

b) Same as a) but with the primary winding in parallel with the secondary winding of the main transformer.

c) The auxilary winding is woven in some way that makes it unaffected by the primary but affected by the secondary

d) ?

I can model the auxilary in spice as both a) and b). At least with regards to getting a power transfer.

If someone knows then an answer would be greatly appreciated :)

perhaps tis description can help you .
Auxiliary winding will be used , but why ? first you will need a driver for transient time . perhaps a zener diode and a high value resistor and a usual diode ! it will create a 15 volts ( linear supplying ) for a short time . then IC can work for a short time too . then a low value of power will be reflected into the auxiliary winding ( which twisted in a core that primary and secondary are twisted on it too ! ) then this winding ( auxiliary ) will have the voltage , higher than linear transient circuit hence the diode will be out of circuit ( open ) and then this winding will be able to supply the driver IC ! self supplying !

I hope i could help !
Best Wishes

A multi windings flyback transformer can be modelled by coupled inductors with respective coupling coefficient. in case of doubt, the k values should be determined by empirical measurements or estimated from text book values according to the transformer geometry.

I'm not sure what you mean with "woven against the secondary winding"? Do you refer to winding location or polarity? Usually the auxillary winding is connected for flyback operation, with the diode forward biased in flyback phase. This implies, that it's voltage is linked with the output voltage, at least in a first order.
Fvm. I have already modeled the interaction between the primary (let's call it L1) and the secondary (L2) by coupling them with a spice directive K1 L1 L2 0.995. However spice wouldn't allow me to then couple only the secondary inductor to the third (the auxilary one which we'll call L4) with the statement K2 L2 L4 0.995.

I was able to "model" the transfer of power from the secondary to the auxilary by placing a winding (L3) in parallel with the secondary (L2) with an inductance of an order of magnitude higher in order to decrease it's effect on the secondary side inductance.

I coupled this winding (L3) to the auxilary winding (L4) with the statement K2 L3 L4 0.995. Where the inductances were chosen to set a ratio of 2 between L3 and L4.

The maximum voltage on the secondary (L2) is clamped by the regulated output voltage (5V) so the third winding (L3) which is in parallel with L2 only sees 5V and because the ratio between L3 and L4 is 2 the voltage over the L4 winding will be maximum 10V which is rectified to 10V DC to feed the IC.

This works quite well. So it appears that the auxilary winding voltage is derived with a "second" transformer between the secondary side and the auxilary side where the primary is in parallel with the secondary of the main transformer.

The arrangement in spice as part of my circuit can be seen here. This is a isolated flyback smps

Goldsmith, in regards to your post. This IC has a high voltage startup regulator so the scheme you posted is not needed here and it was not what i was asking about. But how the auxilary is woven to transfer power from the secondary to the auxilary winding. But i think i know how now. Thank you for the reply though :)
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However spice wouldn't allow me to then couple only the secondary inductor to the third (the auxilary one which we'll call L4) with the statement K2 L2 L4 0.995.
In a short, the SPICE inductor model allows you to model any kind of coupling that relates to physical reality.

I don't see the point why you would need two inductors in parallel. It complicates intuitive understanding of the circuit in my opinion. It can be converted into an equivalent circuit with only three inductors.

In some cases, a model comprised of ideally coupled coils and separate leakage induction elements might be more intuitive, because it corresponds to the standard text book description of transformers. But both models are fully equivalent and can be converted into each other.
The equivelant circuit with only three inductors is what i was originally asking for. I still haven't found out how the three inductor version works in practise in regards to how the transformer is wound in reality nor how to do it in ltspice. At this point all i know i can get the same effect by using 4 inductors.

A supplement regarding multiple coupled windings. If you have three inductors and observe a coupling between two pairs each, there must be a coupling between the third pair, too. SPICE expects the coupling to be defined either for all inductors in a single statements (for equal coupling factors) e.g.
K1 L1 L2 L3 0.9
or indivually pairwise
K1 L1 L2 0.9
K2 L2 L3 0.9
K3 L1 L3 0.85
Physical plausibility restricts the possible combination of coupling factors. SPICE doesn''t strictly check for physical plausibility, but doesn't accept those cases that cause singularities for the solver.

As a first guess, if K1 and K2 are e.g. 0.95, K3 can't be smaller than 0.9.
I think i just realised how this works. What i failed to see was that all three windings are actually sharing the same core after all. The trick lies in the phase relationship between the inductors. That in addition to the diode on the auxilary winding works such that the voltage on the primary isn't rectified by the diode since it flies in the "wrong" direction. The phase relationship between the auxilary and the secondary is such though that the secondary voltage induces a voltage in the auxilary that get's rectified by the diode on the auxilary.

Thank you for your replies.

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