I am designing a offline power supply which will 24V as output with 2.5A. I found UC3843 as better option. I used TI we bench tool to design my power supply. This is the schematic I got from it. The given transformer design, I given to one manufacturer and I got a transformer. I rigged up the circuit as it mentioned. But in this I am getting three Issues
1) Audible noise across the transformer.
2) Sometimes(randomly) restarts while I am connecting 1A load.
3) Small ripples present in output when there is no load(I made AC coupling so we can see the signal clearly and we can enlarge).

 

Have a look at the power supply to the UC3843. You may find that it is going in and out of UVLO.
What is the DC output voltage?

 

Have a look at the power supply to the UC3843. You may find that it is going in and out of UVLO.
What is the DC output voltage?

Hi,
I am getting 24.7V output voltage (Here I have used AC coupling to show the ripple, with that I can change volt/div, so it is enlarged).
UVLO happening at 7.8V around. And the ripple problem happens during the no load operation

 

400mV ripple isn't bad for a 24V supply on no-load, but if there are not problems elsewhere, it does indicate some instability in the feedback loop.
Have a look at the voltage on the emitter of the optoisolator, the COMP pin of the 3843 and the output pin of the TL431 to see if there is a similar waveform. Did you use the specified optoisolator?
Have you read SLUP340? The section on Isolated feedback with optocoupler is relevant

 

I don't see the point of Q1 and it's surrounding items, just put the led from the optocoupler to the supply, .

 

I don't see the point of Q1 and it's surrounding items, just put the led from the optocoupler to the supply, .

I tried with only Rled,Rbias,O1,VR1,Rfbt and Rfbb in secondary feedback, even same problems I am getting.

 

Just take the correct and well checked circuit from Danyk.cz
For example https://danyk.cz/svar_en.html
And change only the voltage and current limits.

 

I don't see the point of Q1 and it's surrounding items, just put the led from the optocoupler to the supply, .

I think that the computer chose a TLV431 instead of the normal TL431, hence it needs a reduced supply.

 

I think that the computer chose a TLV431 instead of the normal TL431, hence it needs a reduced supply.

Yes value of Dz is 20V so it is providing 20V for the tl431 and optocoupler.