TNY287 flyback with no opto feedback

Hi Treez,
Yes it is ok to regulate your primary side low output voltage i.e. 15 V in your schematic. You can either used Zener for regulation with 5% accuracy or TL431 with around 2% accuracy. By the way, what is your 15V primary side output current?
Then if no load power consumption is not your design concern, you can also used the internal bias on BP pin and no need to tied it up with your Primary 15V output.
In addition, to improved your Primary 15V output regulation as a recommendation in transformer design you need to enhance the coupling between the primary windings and the 15V primary output. The transformer layering must be, on the first layer is for the input primary windings, then next layer is the 15V primary windings.

Thanks.

Thankyou so much.
We actually wonder if we can get away with just using a diode as attached?....or will the diode voltage drop be too high and disable the regulation?
The power is 2W on primary side auxiliary, and 2w on sec side coil.

Hi Treez,
Yes you can remove the diode going to EN pin. Then for differential amplifier TL431 is a good choice.
Then I just saw your transformer drawing, the start or dotted pin of the primary windings I suggest you put the start or dotted connection to the Drain or D pin of the IC since this is a switching node and can help to attenuate the noise. By this configuration you will need also to revert the start winding of the low voltage output the primary 15V windings and the secondary 15V windings.

Thanks!

Thanks, but it doesnt matter which end the dot is placed on the primary, just as long as the dot is subsequently placed in the correct place on the sec and aux coils. The dot just means that those dotted (or no-dotted) ends of the coil are at the same polarity at the same time.