Electronic – LM2678 PCB Layout question / review

layoutpcb-designswitchingswitching-regulator

After my initial question about switching regulators I had decided on trying to use an LM2678 for an adjustable/variable voltage form 5-24V and 5A max with an input of ~30VDC.

The application will be a stepper motor tester for varying motors, hence the adjustable voltage.

In the datasheet, they mention that the PCB layout is very important for this device:

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They also give this layout pattern which I admit to not fully understand:

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I made this schematic here with the following PCB layout.

Please note:

  • The heatsink might change, but the actual Schottky diode and the LM2678 have the tab connected to ground, so I would like to share the heatsink.

  • J1/J2 pin headers are just for demo, in real design they will get replaced by beefier screw terminals of course.

  • RV1 will actually be replaced by a front panel potentiometer and not be on the board; it's for reference here only.

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And once again, with the zones filled (but harder to see silkscreen):

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My questions would be:

  1. The bold traces from the design are all very small and copper pours are used for ground and L1 to C3. Other track widths are 2mm. I tried moving the feedback wiring far from the inductor. Would this layout seem acceptable?

  2. Is there some trace/part (I'm sure!) that could be improved?

  3. They say prototype etc, but I only need to make two or so of those boards so I'd like to get this right and obviously on a breadboard this cannot be much tested layout-wise, so what other guidelines would there be to follow, or is this a bit of a tall order without tinkering on a real PCB?

Best Answer

Given the valuable inputs I received about the extra capacitors, not routing under the heatsink on the top layer, and connecting D1 and L1 on the same layer, I would like to update the schematic here for reference.

What I did was:

  1. Add the extra capacitors
  2. Moved the ground plane to top layer. This allows for better (shorter) ground connection to the LM2678 when moving the track under the heatsink to the back layer.
  3. Changed the entire track of D1 to L1 to the bottom layer
  4. Changed all critical ground pins to not have thermal relief spokes.

I think I will construct it then this way, and if I find any problems with it I will update on here with a solution.

Thanks!

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