# Help understanding transformer AUX winding

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?

• Please post links to SMPS schematics that you are referring to. The purpose of an aux winding varies from design to design. Dec 31, 2015 at 18:50

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.

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.

• Ooh ok. So for an example there could be a primary winding with power applied to that winding and right next to it another winding that uses the flux created by it to output. I was merely picturing one side of the core used for primary and the opposite side as the secondary. Didn't cross my mind that the same side of the core can be used with another output winding on the primary side Dec 31, 2015 at 18:50
• @ohmmy -- that's the idea, although in SMPS transformers the windings are layered vertically instead of being "one side primary/other side secondary" as the schematic symbol implies. Dec 31, 2015 at 18:56
• Hmm could you provide a drawing or pic of such internals to help me see it visually? Dec 31, 2015 at 19:50
• Sure thing, see IR AN-1024 fig 5 Dec 31, 2015 at 22:26
• Great thanks. What do those little dots denote on the transformer schematic? Do the positions mean anything? (Top/bottom) Dec 31, 2015 at 22:37

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.

• So when an output of the transformer on the secondary is being used to drive an ic on the hot/primary side of the circuit the secondary windings are just routed to that side? I'm trying to visualize the design of the transformer itself Dec 31, 2015 at 18:13
• In most transformers, there isn't really a physical "primary side" and "secondary side". Often the primary winding is nearest the core, and the secondary (or secondaries) is wound over the primary. The primary and secondary terminals may be placed wherever convenient for the transformer manufacturer or for the end user. The transformer symbol you see on a schematic diagram will usually show the windings placed as convenient for the schematic, without regard for the actual physical arrangement of the windings and terminals on the transformer. Dec 31, 2015 at 18:46

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.