I would suggest you ditch this circuit, you level of understanding worries me especially where a circuit like this is being used.... These circuits are used, but are not safe and are usually found in totally potted LED mains light bulb replacements where NO parts especially conductive can be touched. This is not the realm of hobby/playing circuitry!
If you cannot find out the role of the AP1684* then leave this sort of stuff alone you will be a danger to your self and others.
* It took me less than 15 seconds to find its function.....
Thank you yassin, i had readed the datasheet, but it does not explain how it work (it is about performance and components) i had some question about the circuitery:
what is the role of the two diods D1 D4
in the schematic the schottky diode output is directly connected to the V+ pinout ( so we will have 200v in v+)