I came across the LM2578, while looking really closely at a picture of eBay item 350090122136.
The LM2578 datasheet, page 25 gives an interesting flyback regulator circuit providing +/- outputs.
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I'm tempted to build one, with a supersized flyback transformer and pass transistor.
My question is this: if the load on the transformer increases, such that the pulses become very wide, then will the + and - outputs still be symmetric? In other words, is there any possibility that the |+V| output could be greater in magnitude than the |-V| output, because the square waves would no longer be symmetric?
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.
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