I'm new here and I'm looking for someone to help clarify the AUX winding of a transformer, more specifically in a switched psu.
I see schematics where it's being used either as feedback or voltage input to an ic on the primary taken from a winding on the primary (at least that's how it seems to me). Here lies my confusion.
My understanding is a magnetic field created in primary creates a flux in secondary that outputs different voltages dependent on winding/s. So how can there be a second winding on the primary side that induces voltage in the hot side used as an AUX? Am I missing something simple like are the windings really on the secondary but the pins are connected to primary/hot side?
Best Answer
Consider this circuit of a smps circuit:
You can clearly see three windings. Right one is secondary which is producing the output. Top left is primary and bottom left is auxiliary.
Auxiliary is nothing but an additional secondary. Usually in a switching circuit, we need some low voltage power source for the working circuit. You can't power them using 220V or 110V line directly. So you are left with two options -
1) Somehow use the low voltage output which is being generated which will be a very bad idea because the sole purpose of using such a schematic is to provide galvanic isolation to the secondary side and by connecting output side to switching IC, you have defeated the purpose.
2) Make a separate low power winding just to provide enough power for the chip to run which is what we do by making an auxiliary winding. When you cut open a transformer, you will find that secondary winding will be usually made using thicker wire because it has to handle a larger current whereas auxiliary will be made using thin wire because it has to carry just small amount of current to keep the chip functioning.
Both secondary and auxiliary windings get their energy from primary winding. You can add more windings to the transformer and call them aux2, aux3, sec2, sec3 etc in case you need such an arrangement.