FSFR1700XSL circuit network?

[Inari]

Hello.
My very first post here.

I please some help to figure out one component in FSFR1700 psu.
Part of this circuit which I draw is in below.

I need to figure out zener diode value (marked as D404 in red circles) - how can I choose correct value or which parameters it depends for? In FSFR1700 datasheet there is no this kind of circuit application example. How I can calculate this value? And what happens when this value is not correct or lower/higher than desirable?

Sorry my bad language skill.
Thanks for all!

[f4eru]

Hello

Interesting component.
You shcematic seems a little strange.
Where does this zener come from ? seems not necessary according to the DS.

Please try to draw schematics with ground lines on the bottom of components, this makes it much easier to read.

[Inari]

This is bad drawing, I agree completely 

I drew it in Paint because I have never drawn a circuit diagram in for example PCB CADs (only layout..)

If I am completely honest - this psu is in commercial product part. And there is no any service manual (countless hours of searching in English, Chinese, Russian etc - no exact match).


Problem is that I dont know exact value of this zener and this zener is broken - dead short in both way. Then I dont know what is right part to replace it. It is SMD, glass MELF zener - no code printed in case
All other parts is measured separately- I was hoping that maybe some engineer (I read that in this forum there are lot of them and hope...that maybe someone take this little moment...) explains how I can be sure to choose right value - at now, I am confused why this zener is used here? It is reliable or maybe I can make it little better?
I think that output short circuit may be a reason why this zener is shorted and maybe also FSFR1700XSL itself but I dont know exactly. Anyway, I have new FSFR1700 and 17V, 18V, 24V zeners to replace this and I hope to be sure that before I solder new IC in place then I know what I can to do minimize change that maybe new IC burns out.

I cant to test old IC (then I am not sure is this old IC work or not) - I have some previous experiences Fairchild SMPS controllers which burns out "soft" - no spark, no smoke, no fuse blow - just quietly they dont work anymore. I think that maybe this time too and I have only one new FSFR1700 - I dont want "surprises".

I hope little that maybe someone explain. I think that maybe 18V zener value (like other good zeners) is not very bad but I am not sure

 :(Maybe seems that this topic is also in wrong place but this is power solution, this is because it is in here.

[f4eru]

it seems to be only a protection, so you can try without.

One option to repair your screen backlight is to replace the LED psu by a separate led driver...

[f4eru]

if you want to be more careful, take a small voltage zener, and try, then increase the voltage gradually.