First of all, before answering your questions, let me make some suggestions about the layout. I have seen your previous post and this has been a good attempt, but:
1.- The "Switching Loog" is too large. (Switching loop is Vin, Trafo, Q1 and R8, you call it,"high current loop", maybe it is no quite precise) And moreover, this loop include the control area. This is really EMI problem!! I suggest something like this.
(Think in current loops always!)
Make smaller the switching loop
- Place R8 near from C1. Really near.
- Rotate Q1 90ยบ clockwise
- Place R6 and C6 as close as possible (acap) from Trafo pins.
After that,
- Think about the "Gate net", "Sense net" and "pin 15 from controller net", It must be route separatly! The "gate net" is a radiant net and the others ones are sensitive nets. Route them.
- Route Vin and Enable controller nets.
- Complete the route of primary
(The controller has not a Decoupling Capacitor ? Are you sure of this?)
2.- A connector for trasformer sounds a problems maker. Could you solder your Custom Transforme in the PCB? It would be better. If you cann't, I you sugggest a 2 row 2 columns conectors. In this way you can gain more space between primary and secondary, and more space between trafo pins.
OK and now. Your questions.
1.- Yes, try to keep separate switching ground from control ground, but connect to the same ground, in your case Bottom layer. For this, try to keep the "switching loop" components together according the placement I suggest.
Connecting R8 to Vin through one track and then one via to GND is not a good idea in your current layout.
2.- Ummm... I would try another layout before consider this question.
3.- I cann't undestand this question. What do you mean?
4.- As a rule, It better If you fill the empty area with hatched copper. It improve the etching process in PCB manufactureing.
5.- As a starting point, fill as much as you can. Do you need consider any electrical isolation between primary and secondary.
Good luck!
Wow, Well we could spend hours discussing ground.
I'm not that much of an expert, but I have made lots of mistakes.
First I would say there are two "ways" to think about ground.
At low frequencies (DC to audio) the "star" ground concept is useful.
Here I'll draw a picture, with big heavy lines for paths with a lot of
current. And then bring all those separate paths to one point.
(the point where power enters the circuit.)
The idea is to keep the high current's out of the more sensitive parts of the circuit.
At higher frequencies ~10 MHz and above, one starts thinking about a ground plane
that is everywhere. With lots of ground connections between the case and the
ground plane. At the higher frequencies on is not so much worried about the DC drop in the ground lines, but the inductance of the ground. You want the fast signal wires to have a (continuous) ground plane underneath them that carries the return current.
I'm sorry I haven't answered your question. Whether you have a loop or not depends on how you lay it out. And a small ground loop on a pcb may not be that bad.
The best thing is to build it and try it, and remember that you may have to "fix" the ground.
Best Answer
A ground plane:
provides a low-impedance ground connection,
This is important since "ground" is how different components relate to each other. When ground lifts because of impedance noise margin is reduced and errors can result.
acts as a EMI shield,
If your product needs to undergo any sort of official certification then you can know that half of it is already shielded and you only need worry about the other half.
and can act as a heat sink.
This is not that important for small-signal components, but any sort of power electronics, or even just a voltage regulator for the rest of the board, can benefit from this.