I was wondering in an SMPS (switched mode power supply) can we supply an opamp or comparator with the auxiliary winding? For example in an application using UC3844 can we do that for over voltage/current protection?

I am building a power supply rated 20V 4A, and I need a DC OK signal.

Could they also be powered from the supply output?

  • \$\begingroup\$ It depends on what kind of power supply it is and what currents the op-amp would measure. Is it a mains powered switch mode power supply with isolated output? Would you be meuasuring the current on mains input side or isolated output side? Is the auxiliary winding used for powering the UC3844 too? \$\endgroup\$
    – Justme
    Feb 1, 2023 at 6:45
  • \$\begingroup\$ Yes, auxilary winding also supplying power to UC3844. Actually ı am building a power supply 20 Volts 4 Amps and ı need a DC OK signal. And ı am plannig to do that with using a comparator or opamp from the secondary side. And yes it is an isolated power supply. I have another question; from output of the power suppy can ı supply directly OPAMP or Comparator? \$\endgroup\$
    – John Reese
    Feb 1, 2023 at 7:26
  • \$\begingroup\$ @John Reese - You said it is an isolated supply. But what is the input? Is it 115 VAC, or something else? And a lot depends on the application. \$\endgroup\$
    – SteveSh
    Feb 1, 2023 at 20:53

3 Answers 3


"I am building a power supply rated 20V 4A, and I need a DC OK signal.

Could they also be powered from the supply output?"

Maybe, with a proper circuit design. You would have to design it such that the "DC OK" signal is in a NOT OK state for any of your fault conditions (supply not working, over current or over voltage trip, output out of range). We usually use an opto coupler for this output, configured so that the opto is off under any fault condition.

Note on A Design Option:

Many of our systems have multiple power supplies providing a half dozen or more regulated voltages. We have one of the power supplies, usually the first one that's turned on, generate a secondary referenced voltage of 12 V to 15 V. This voltage is then distributed to the other supplies for them to use to power their secondary referenced monitoring circuits such as over/under voltage detection, over current detection etc.

This eliminates a chicken and the egg scenario which could occur if the local secondary voltages were used to power their fault detection circuits, in that is guarantees proper operation of the fault detection circuits regardless of what's happening with the local voltages.


The answer is strictly no.

The AUX winding is already powering the mains/primary side components so the AUX supply is already mains referenced.

It must definitely not be allowed to power anything on the isolated side.

In addition, the transformers are usually designed so that there is less isolation between primary and AUX coils as they are both for the mains side. The secondary coils will usually have better isolation from mains side.

  • \$\begingroup\$ I think the OP's question should be rephrased as "Can I add (a possibly additional) auxiliary winding to a SMPS transformer to power additional circuitry. He's apparently not asking if the can use "the" auxiliary winding, which is typically supplying primary side circuitry. He also doesn't specify which side of the isolation barrier the additional circuitry is on, so if it's on the primary side he may be able to get away with stealing some current from the existing aux winding. \$\endgroup\$
    – John D
    Feb 1, 2023 at 16:30
  • \$\begingroup\$ Why do you assume that "the AUX winding is already powering the mains/primary side components"? There is nothing magical about this winding. It's just another secondary, in the general sense. \$\endgroup\$
    – SteveSh
    Feb 1, 2023 at 20:55
  • \$\begingroup\$ @SteveSh I am not assuming it. The OP confirmed that the same AUX winding is already powering the SMPS chip on the primary side. And that it needs to be isolated. \$\endgroup\$
    – Justme
    Feb 1, 2023 at 21:42
  • \$\begingroup\$ Ahh, I see that now, in one of his comments. \$\endgroup\$
    – SteveSh
    Feb 1, 2023 at 22:52

There are three topics I'd like to discuss: rating; isolation; and other sources.

1 - Rating

In a typical application, such as UC3842 flyback with primary-side auxiliary winding to supply the controller:

The winding is rated for whatever current it is designed for, and the only limitation is cross-regulation, or voltage range, or whatever else is set by design.

Cross-regulation refers to the voltage change in one output, due to the change in load current on another output. Typically a flyback supply is limited by the leakage inductance between windings. The output voltages track when all outputs are loaded proportionally, and differ when loaded differently.

Typical designs are not rated for much on the aux winding, limited mostly by wire size I suspect. For example, many off-the-shelf inverter transformers provide a "15V 50mA" winding, or something like that. (The voltage rating of course is merely a suggestion, where the transformer will operate best; within reason, as the aux/sec ratios are fixed of course.)

As example, I once built an LED lamp with a small fan powered by the primary aux winding. This required a start-up circuit, as the fan is too much load to permit startup otherwise (it would brown out the controller's meager startup bias; further exacerbated by a widely adjustable output range). I just used a time delay driven by the gate drive output, thus the fan stops if gate drive stops, and the fan starts after some delay after the gate drive starts.

2 - Isolation

Specifically for your application (DC OK output signal), the primary winding is unusable. It is connected to mains voltage for one, and even if not used to power the controller, will generally have inferior isolation rating -- as it only needs to provide functional insulation, not reinforced.

Which means you might not be able to use such a winding on an off-the-shelf part, even if you didn't need it for the primary side.

This is easily solved with a custom transformer design, or by not needing it at all.

A 20V output is well within the range of many comparators and op-amps. Or regulators to power them. For example, a 78L05 (max input 30V) into a MAX6897, with its sense divider connected to the 20V supply.

3 - Other sources

Another question that may be of interest is, can current be drawn from the VREF pin? The answer is yes: the current rating is in the datasheet. See also TL494, etc.; many controllers provide such. This can be used not only as an accurate voltage reference, but to power logic, comparators, amps, etc., up to a total load less than the minimum limit. (One could even power a small microcontroller this way, though I would recommend avoiding pulsating loads like this, as they may feed noise into the reference supply, affecting stability or regulation.)

  • 1
    \$\begingroup\$ Please note that the aux coil already powers the mains side components, so obviously you can't use that for powering the isolated side components any more, as it will remove isolation and be hazardous. \$\endgroup\$
    – Justme
    Feb 1, 2023 at 11:22
  • \$\begingroup\$ Yes you are right but we can use another auxilary winding at the other side. And ı am wondering it can supply or not an opamp \$\endgroup\$
    – John Reese
    Feb 1, 2023 at 12:48
  • 1
    \$\begingroup\$ Wth another secondary winding, then yes, it can. Many SMPS's are designed/built with just that sort of architecture. As Tim Williams mentioned, it all comes down the the cross regulation performance. For example, one recent SMPS I was involved with provided 3 different voltages from a common power train. A 4th voltage was created using a linear regulator off of one of the other voltages. \$\endgroup\$
    – SteveSh
    Feb 1, 2023 at 23:02

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