ESC design layout and noise interference question

Question

I'm laying out an ESC (sorry can't post photos) and I'm trying to squash any noise on the board. I unfortunately have limited space which requires me to run LV signal traces from my current sense amplifiers under the drive FETs. I do have a 10-layer board, but need 6 layers to carry the necessary current to from the phase outputs of the FETs to the motor wires. My plan was to make layer 7 the return path. This leads me to 2 or 3 questions:

1. My current amplifier output traces are on layer 8, below the return plane, which is below the phase outputs. Is that sufficient to prevent noise interference on the LV traces from the outputs of the FETs?

2. The ESC also reads voltage feedback from each phase. This is divided down before it is read by the ADC. Should I put the resistive dividers close to the FETs and run LV traces (same layer as the current feedback) back to the ADC, or should I run HV traces to the divider which is close to the ADC, but will bring HV traces near the processor?

3. Is one layer of return path (plane essentially) sufficient to block switching noise?

Does anyone have a good reference on laying out ESC PCBs? I could use a good guide if one exists.

Answer 1

I am no expert on PCB layout, but I can show you a typical example of a brushless 'hobby' ESC. This one was made by Castle Creations in the US.

It is a 4 layer board with the logic on one side and the FETs on the other. Both middle layers are (mostly) ground, but separate from each other except where the negative power lead is connected. So it is effectively two 2 layers boards sandwiched together (some ESCs do the same thing with two separate boards).

First the logic side. Here we have:-

• Top left: column of resistors creating the voltage dividers and virtual neutral point. Inputs through vias from phases on other side, outputs going into the MCU immediately to the right of them.

• Middle area: FET drivers.

• Right: power inputs (through vias from power input wires on other side), bulk capacitors, gnd/+5V/signal wires to receiver.

• Bottom left, top middle/right: +5 V linear regulators for MCU and receiver/servos.

Now the other side (ignore messy part where some FETs were removed, all the FETs in this ESC are burnt!):-

• Left: motor phase wires, snubbers between adjacent phases.

• Middle: FETs. Lower FETs to the left, Sources connected to Negative (gnd) on inner layer. Upper FETs to the right, Drains connected to Positive on top layer.

• Right: Power input wires, signal filtering components (from orange wire), unpopulated area for opto-coupler option.

The two inner ground layers being separate isolates the sensitive analog and logic parts from high currents flowing in the tracks on the FET side, and shields them from any EMI radiating from the tracks (which shouldn't be much if the switching times are well controlled).

As you have more layers you have more freedom to route signal wires on inner layers without compromising the ground plane. Depending on motor current, you may want to combine several layers to reduce resistance on the FET side.