This is in a way a follow up to my previous post Advice on ground plane in my first PCB. I wanted to continue my questions there, and I updated my post after receiving feedback from others, but couldn't tell if my post re-appeared as unanswered. So here is a fresh post. The PCB below (prepared using ExpressPCB) is for a flyback converter implementation using the LT3748 boundary mode flyback controller IC. The circuit is taken from the LT3748 datasheet itself (see very last page). This is for a class project.
Two constraints I am working with are: 1. Transformer is designed by us, but is mounted outside PCB and connected thru the 4-pin connector (since there's no time to order bobbin - plus they won't sell a few) 2. 2-layer design only (lab course instructor wanted a simple design)
My questions are:
Datasheet advises to isolate or physically separate the high current ground from small-signal ground. The secondary ground plane is completely separate, so that's done with. As for primary, the only high-current grounds are those of Vin and R8 (the sense resistor). So, is it a good idea to connect the negative terminals of Vin and R8 with a track and then connect that track at one point to a ground plane that will have all the other small-signal grounds. The only penalty then would be that the R8 to Vin track would be quite long (since I'm using 2-layer only and wouldn't like to break my ground plane underneath, unless it is less worse than running this long track).
As you see from the schematic, I'm running two tracks underneath the IC (i.e. on the same top layer). Do you foresee any problems there? My IC shouldn't get hot, as it works with small current only.
Is it bad practice to have the ground plane run under a switch. I don't have any grounds around my MOSFET so I can cut out from the ground plane there, but I thought this would make it less effective (in that some return paths become longer) .
We're going to order the board from ExpressPCB, but to get a good price we need to stick to a standard board size, which is almost 3 times the populated area (so two thirds is empty space). So the question is do I fill ground in that space, or leave it empty? What about the top layer - also ground?
I realize the shape of my ground planes are somewhat funny. I just tried to fill only as much ground as needed to make straight return paths. What other considerations should one make in this regard (e.g. fill as much as one can? but then do I extend both ground planes, or one of them? how to choose?)