Using neighboring traces for PCB return current

Question

I am developing a 2-layer board for a 3-channel BLDC motor driver. I am using an IC for the driver (DRV8313). I will PWM it at 20kHz with around 10% or less duty cycle. Current is less than 1A.

It seems ideally, the return current for the BLDC power use neighboring (or nearly neighboring) traces, as that is the actual loop. But those are on the same layer. The PCB will be a standard 1.6mm thick; the current is low enough that I can make the traces even closer together on the same plane. Will this keep EMI as low as having an actual ground plane reference, or is the ground plane still preferred? Is the fact that traces are wide and flat a reason for this?

I want to do this so I can route +12V and GND on neighboring traces on the bottom of the board (which means I need the return current flows of the +12V trace and GND trace/plane to cancel each other out too). I'm trying to avoid going to a 4-layer board, and avoid running wire jumpers on the top just to feed the IC.

Answer 1

With the 20kHz being the max PWM, the frequency is low enough to not be a concern. FCC limits don't apply until 1.7Mhz and the harmonics will likely stay below that value.

If your worried about mutual inductance between trace loops on your own board, then make sure you keep the loop area small, as to create smaller inductors.

At 1Amp a 200mil long conductor will need to be at least 13mil wide to keep the temperature delta below 20C

Take care to not 'chain' traces (this is just an example and not the full h-bridge of a BLDC controller):

^{simulate this circuit – Schematic created using CircuitLab}

Each trace has resistance, this can be calculated, make sure you take this into account as small resistances can add up.