Modify AC/DC supply from 6.5Vout to 6Vout

[gkmaia]

I am trying to understand the following circuit but really stuck on how the NCP1203 controller works here.

so that is an existing power adapter I have at home and I traced the PCB to understand better the circuit.

Seems the NCP1203 works as an oscillator so current can flow through the transformer.

But I cannot see anywhere a voltage regulator here.

Also what seems to be the feedback coil in the transformer has pin1 not connected to anything.

Is there a way to use NCP1203 to regulate the output voltage or it's purely based on whatever comes out of the transformer?

Also, will the addition of a pF cap in parallel to EC3 will make the output more stable?

[wraper]

All you need to do is use zener with lower breakdown voltage. Basically it acts as voltage reference. Once output voltage increases current through it and optocoupler emitter increases. On the other side of optocoupler, controller IC reduces duty cycle in response to that.  To reduce output voltage by 0.5V, you need zener with 0.5V lower breakdown voltage.

[wraper]

BTW on input voltage side circuit has very serious errors. But I won't comment on that further as I'm going to sleep.

[gkmaia]

BTW on input voltage side circuit has very serious errors. But I won't comment on that further as I'm going to sleep.

That is just copy and paste from the existing power adapter's pcb. I did not come up with this design i am just reverse engineering it. but thanks for the feedback!

[Zero999]

I think you made a few errors tracing the PCB.The GND pin isn't connected and VCC doesn't look right. The circuit is probably similar to the one on the data sheet, which can be found using a search engine:
https://www.onsemi.com/pub/Collateral/NCP1203-D.PDF

Take another look at the PCB and compare it to the data sheet schematic. There will be a few differences but it should be similar.

wraper is right, the zener diode is used for feedback and the voltage can be changed by replacing it with one with a different breakdown voltage. What are you powering with this PSU? It's likely 6.5V vs ^V won't make any difference.