switching regulator questions

The data sheet has 17 pages, where is page 25?

Whoops, wrong chip - try this one

This is a flyback configuration meaning that the secondary delivers power when the primary is off. Energy is stored in the inductor/transformer during the on part of the cycle then transferred to the secondary side during the off time.

Regulation is determined by feedback from the + side of the output, back to the chip. The negative side goes for a ride without direct feedback.

Regulation will depend on how closely coupled the + and - windings are. It would be best to bifilar wind them, both wires together to reduce leakage inductance. Leakage inductance will store energy that is not necessarily coupled to the secondary or may couple unevenly. The minus probably will NOT track the plus very well especially if they have differing currents.

Depending on the application, it would be best to follow the +and - outputs with linear regulators like LM7815 and LM7915 to insure reasonably matched and quiet + and -15 volts. Of course the outputs would have to be a few volts higher.

You should avoid very wide on time voltages because the secondaries have to ring off and supply power with the same (volt )(time)/(turns) product as the primary. Usually the on time should not exceed the off time, depending on turns ratio.

You can make it better if you do this:

Imagine two isolated outputs that both make +15V. The diode is in the positive side. of both windings.

Now hook those two supplies in series.

The plus end of the two windings still have the same AC waveform on them but are at different DC points.

Now put a capacitor between the two positive ends of the windings.

They are now forced to have nearly exactly the same waveform even with the leakage inductance ringing.

The result is much better regulation on the non-servoed output

Ah. Thanks. Requires two coils, sets of diodes, ... but more stable...

Thanks to all who replied

Michael

Say... the key word here is isolated.

Am I correct in assuming that using two circuits with simple inductors, connected in series, would be a bad idea?

So, I should use two 1:1 isolation flyback transformers in series, correct?

Michael

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There are not more coils or diodes just one more capacitor.

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I think you have it completely wrong. There is:

One core.

On this core there are 3 windings

One winding is the primary and is driven by the MOSFET

The two others are identical windings that are the secondaries.

Gotcha!

I'm missing the forest for the trees... thanks.

Michael

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Yes, the same as that only different.

The OP was talking of a flyback so there are more windings in the question.