FEDEVEL
Platform forum

Mehrdad , 04-26-2020, 01:18 PM

Hi all,

I am designing a board that contains control and power circuit. It needs to drive four 24-volt stepper motors as well as heater cartridges. The power circuit can draw 15 Amp if everything works together. Unfortunately, the PCB size is limited by the mechanic and its case and I cannot use optical isolation or the isolated DC-DC convertors to isolate and separate the control and power circuit grounds. Because the components placed on the PCB are very tight, I am going to use a 6-layer stack-up. Now my question is in order to minimize the ground bounce and effect of the return current of the power circuit on the control circuit, how would you assign the PCB layers? I am thinking of assigning one ground plane to a digital (control) circuit and another ground plane for the high current drive circuit and connecting them in one point to have a kind of star ground distribution. What about the power plane? Should I assign one layer for the digital power plane (+3.3V) and another one for the +24 volt?
Thanks in advance for your comments.

robertferanec , 04-28-2020, 03:07 AM

I am not expert for power electronics, but personally I would still maybe try to isolate the power electronics somehow from the rest of the circuit (islands, location, etc). If you just use separate ground layers on different positions in stackup, I believe, the noise from the grounds still could in some cases "jump" to the other planes or signals routed close to that planes and the result could be maybe even worse comparing to using all GND planes in stackup for both (power and also control circuits). But as I said, I am not expert for this, I may be wrong.

Please, anyone with experience in designing power electronics? I would be also interested to hear some thoughts on this topic.Thank you

mir0mik , 04-28-2020, 04:31 PM

Hi Mehrdad,

Can you give away more info? How many layers do you have? What's the placement? Each board is different, there is not one rule fits all.

omid.fotouhi , 04-28-2020, 09:22 PM

Hi
Usually when my board has high voltage or high current part and digital part , away from that how many layer I used , I route all high current in top or bottom and another digital signals and power (3V3) in inner layers.This is very common in automotive control board . This form of design makes 2 effects : First If your board is small you can clean power(with high current) traces solder mask and in this case you can cover this trace with solder and this increases the height of the trace and more current flows through the wire. Second is distance between high current and digital power is PCB's core, And this reduces the destructive effects on digital parts and power .

If you want to use inner layer , you have to separate the digital part and the high current part according to the high current and high voltage standards for example in home made boards you must separate digital from power with 5mm gap (5KV experiment).

In case of separation of analog from digital, your solution is good but in high voltage or high current cases never do this.

I hope you found it useful.

omid.fotouhi , 04-28-2020, 09:28 PM

https://www.powerelectronicsnews.com...l-section-3-3/
Search about :Motor deriver PCB design guide

mir0mik , 04-29-2020, 02:02 AM

Hi Mehrdad,

Thanks, I meant it in the opposite way to be able to give you better help. I know I'm new on this forum, but I'm in SMPS (IND, SSN, RAILWAY, eMOBILITY) business since 2007...

Mehrdad , 05-02-2020, 10:16 AM

Hi Omid,

Your comment is appreciated. I agree with your reasons regarding the stack up. However, removing the solder mask and thickening the power traces by soldering and tinning is still a controversial issue that some people believe it cannot increase the current capacity that much while others believe it considerably decreases the trace resistance. I personally use this method for the relay traces that carries high current. Tow opposite opinions can be seen in the below links:




BTW, do you know any application note or reference design that explains the board layout for the Automotive industry?

Mehrdad , 05-02-2020, 10:35 AM

Hi,

I am sorry that I cannot share the current version of the board because it is not my personal board. However, the placement is not complicated and it is like below if I show it in a block diagram.
The board uses 6 layers.

mir0mik , 05-02-2020, 12:39 PM

Hi Mehrdad,

Looking at the placement - is it possible to move the MCU block away from the high current path? I suppose the (SMPS & Linear) are for the Stepper motor drivers too... this way all the current will flow trough the MCU section. Regarding to grounds - it's hard to help without proper schematic. But personally I would analyse the schematic and try to do the placement in such way that the power lines avoids the logic in all layers (if possible) and did a good thinking where the grounds will be connected together...