Help understanding transformer AUX winding

Question

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?

Answer 1

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.

Answer 2

There is an isolation (insulation) barrier between the primary (or high voltage) side and the secondary (or low voltage) side of a power supply transformer. However, just because a winding is on the primary side doesn't stop it from being used as a secondary -- it just means it isn't isolated from the primary. Hence, the auxiliary winding is wound on the primary side of the isolation barrier to power primary-side control circuits that need low voltage DC to run.

Answer 3

The primary winding of a transformer is the input. All other windings are secondaries (outputs), regardless of where they are drawn on the schematic symbol, or where they are physically placed on the transformer core or coil former.

You can use what the maker considered as a secondary winding as the primary, if you wish.

Answer 4

Most SMPS use mosfets which have considerable input capacitance .This means that the drive current needs get higher with increasing frequency .The lower on resistance devices are proportionally worse in this respect. Mosfet gates and the chips that drive them generally want 20V or less.If you were to drop this voltage from say rectified mains the losses would be far too great .Maybe 10W for a 100W SMPS .If you went back to a BJT your drive needs would not be so frequency dependent but your frequency would have be lower to maintain low switching losses.The low gains of most HV BJTs mean that the drive needs could be worse than the mosfet anyway .In fact Aux winding technology was alive and well on old SMPS that used BJT. The AUX winding efficiently provides this low voltage and is cheaper than a seperate supply.Because the SMPS is now powering itself its less likely to have stability problems ay no load .If the output is shorted the Aux winding voltage falls greatly shutting down the drive.Some time later the boot cap recharges and the SMPS retries .This hiccup mode can be a good thing in some applications.