Thank you for all your answers. I will take into account all your comments (missing connection to transformer on primary side, inappropriate sizing of bridge rectifier, wrong connection of optocoupler, inappropriate sizing of the capacitor C5). Maybe a few words about me and the circuit: I am reading a few books on SMPS, including one by Sanjaya Maniktala, and I am missing the connection between the theory and practice. I do not plan to produce "my" SMPS tomorrow, turn it on and die. I just thought I'd start making something and then ask any questions I stumble across. So I can better understand the theory of SMPS.
I will focus on the following questions now, because I think I'm having trouble understanding "what is a ground and where is it used?" Since my school days I have seen the GNDs from time to time in the circuits, but one never addressed when and how exactly to use them in the real circuits (well, I am not an electrical engineer either).
What makes you think you need "a ground"?
I think, so that the current flow is possible, there should always be a "plus" and a "minus", whereby "minus" is often defined as GND. Otherwise the current cannot flow.
What do you mean by "a ground"?
Something that has a potential that is almost zero compared to "Plus".
What physical thing would this schematic symbol turn into when the PCB is built?
I have four layers board. On the top and bottom are my components. The layers in between are both GNDs, they are both connected to the "minus" of the input. This would allow me to go directly to a GND layer in the middle through via's without drawing a wire to GND on the "minus" of circuit. In other words, I just wanted fewer wires on the PCB.
Tim (T3sl4co1l) has already explained well what a GND is in reality. I would now let the thoughts go through my head and maybe ask something in this thread a little later regarding my circuit. Maybe after I have worked through all the other comments.
And thanks again to all. The topic SMPS fascinates me, I deal with it for some time, but unfortunately all alone, have no friends who are electrical engineers which I could ask. A few days ago I thought to myself, it can not go on like this and have created an account here on the forum. Rather I ask stupid questions and get smart answers than if I would sit alone again with all my stupid questions.