PCB ground planes with isolated voltage
I have designed a PCB that I need to work in which a microcontroller drives a 24 V brushed motor. The input power to the board is 24 V, this is then lowered to 5 V by a step down converter, and this 5 V is then made "isolated" by an NTE0505. The isolated 5 V is used to power the microcontroller, RS485 communication and motor current sensing, while the 24 V and 5 V (not isolated) are used for the motor and its driver.
The PCB has 6 layers, all separated by dielectric, which are:
- Top layer
- Ground Plane
- Signal layer 1
- Signal layer 2
- Power Plane
- Bottom layer
On the fifth layer (power plane) the non-isolated and isolated 5 V are routed, suitably separated from each other by planes that do not touch.
The second layer (ground plane) is a single layer on which only the ground vias of the isolated 5 V are routed.
The 24 V ground (defined as POWER GROUND) is positioned on the top layer and bottom layer, carried on the two layers with vias that cross all the layers (as well as the 24 V). The ground of the 5 V (not isolated) is the same as the 24 V and is also routed to the two outer layers.
My doubt is mainly related to the fact that there are two grounds, that of the 24 V and 5 V non-isolated (POWER GROUND) and that of the 5 V non-isolated: does it make sense not to have at least a portion of the ground plane for the POWER GROUND? What are the best techniques for the correct use of the ground plane in the case of having two supplies isolated from each other?
The vias used cross all 6 layers of the PCB from top to bottom layer, and only the designated vias "touch" the relative planes they are connected to.
Unfortunately for copyright reasons I cannot share schematics or PCBs.
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Your description is confusing due to lots of hand waving, so it is hard to be specific. However, an isolated section should be separated from the non-isolated parts laterally. It depends on the exact spec or certification you are going after, but usually 5 mm is good. It can be more than that, like for Intrinsic Safety. Check your spec.
With the sections isolated laterally, there is no issue about which planes go to which isolated sections. Everything in a section (all layers) are either isolated or not.
Do you really need a power plane? Maybe the motor section needs it for high current, but likely the rest of the board doesn't. My first knee jerk reaction is that you could probably do this with 4 layers instead of 6.
Having a ground plane is good. In this case, you'd have two ground planes both in layer 2, with the 5 mm or whatever gap between them. When you have a good ground, then a power plane is rarely necessary. Unless the problem is DC current capacity, good decoupling of power to the ground plane at each point of use gets you the low impedance at high frequencies.
Low impedance at low frequencies is about brute force DC current capacity. A plane can help, but there are often simpler and cheaper methods that don't require an entire layer.
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You have a 6 layer design for controlling 24V brushed DC motor, galvanically isolated voltage rails(24V and the rest) and your question is how to separate them properly.
There is no further reasoning why 6 layer design, why did you choose to isolate the PSU, is there something extremely noise sensitive that you would like to protect in the circuit(right now there is only MCU, RS485 and motor current sense).
Based on the amount of information given, I don’t think you need 6 layer design, unless you are extremely tight with space or motor draws that much current that you would dedicate the whole plane to it. Even then, you can increase the copper thickness further(4oz is something most manufacturers easily support nowadays).
Rather use a standard 4 layer design and dedicate one layer to ground, usually the one closest to the layer where components would be mounted.
I see no valid reason to galvanically isolate the 24V rail, at least you didn’t give one so I would challenge it. Use a star ground system, where your motor ground and PCB ground would be connected at exactly one place. This would largely prevent motor currents flowing through your circuit. As a bonus, your current sense becomes simpler and cheaper too.
RS485 could be non isolated as well, but without further information I will let you judge if it makes sense or not.
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This isn't a direct answer to your questions, but you should be aware of that to maintaining isolation means careful design off the PCB as well.
You would do well to remember that if you are planning to have two isolated grounds you have to arrange everything so that the isolated grounds (or their other signals) do not touch in unexpected places due to shielding, mounting hardware or conductive actuator or sensor housings as you would then loose your isolation or worse yet cause ground loops or loss of noise or safety protection.
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