I needed some advice on my first ever PCB. I am trying to build the circuit below (schematic drawn using OrCAD Capture), which is straight out of the datasheet of the flyback controller LT3748.
I did some reading (including some of the posts on this forum), and decided to adopt this plan:
- 2-layer PCB: All components/tracks go on top, bottom layer has ground plan only
- All components are surface mount only.
Ok so my first attempt on the PCB is below (using the ExpressPCB software). This is a class project and we must build our own transformer. We couldn't get the bobbin we wanted and so decided to just lay down the transformer somewhere next to the board and connected it through a 4-pin connector.
We couldn't really mimic the layout suggested in the datasheet, as we're using only a 2-layer design for various reasons. We do realize that a 4-layer design along with a small commercial SMT transformer would have been ideal, but we're working with what we've got.
So based on some of the feedback below (and thanks to everyone who commented), I've made slight improvements to the PCB (as shown below).
I also have some questions:
The datasheet advises to isolate or physically separate the high current ground from small-signal ground. The secondary ground 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.