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