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I'm designing a three-phase inverter and have been looking up information online with respect to grounding, which led me to a somewhat misapprehension.

In my system I firstly planned on separating the ground of microcontrollers (PWM, digital signals etc..) and sensors (analog signals such as current/voltage measurements) from the power ground, which is the one in the 400 V DC LINK of the three-phase legs. I intend to do that with optocouplers and hall sensors.

However, I'm still confused as to why there's a need to isolate these grounds. Can I not simply be careful with pcb routing not to create dangerous return paths and I'm good to go? Is there really a specific need to isolate grounds that PCB routing cannot solve?

For instance, this design from ST uses a shared ground for both MCU and power transistors : https://www.st.com/resource/en/data_brief/steval-esc001v1.pdf

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  • \$\begingroup\$ "Is there really a specific need to isolate grounds that PCB routing cannot solve?" Ask the guy who gets sapped on the comms-interface of your device with 400DC from 1000uF in case of a fault. I would be hell of mad at you, if i were him. \$\endgroup\$ Nov 30, 2023 at 18:55
  • \$\begingroup\$ @ElectronicsStudent I see that, I'd be mad at myself too. The point is, can pcb routing solve that or will it always be hazardous? \$\endgroup\$ Nov 30, 2023 at 19:01
  • \$\begingroup\$ This depends on your application. Nobody can tell without a block diagramm or a more detailed system description. My sarcastic (sorry for that) comment above trys to say: If there is any other interface on the device than AC-In and DC-Out: Yes, please use isolation. If you post more information (Block-Diagramm, More description, Functional Decomposition, whatever) i will formulate an answer based on that. \$\endgroup\$ Nov 30, 2023 at 19:08
  • \$\begingroup\$ @ElectronicsStudent Sure, i'll add a schematic based on a similar design to illustrate what I mean \$\endgroup\$ Nov 30, 2023 at 19:11
  • \$\begingroup\$ Hmmmh a schematic of a three-phase inverter wont be helpfull! We can google that. What we need to know: Is it AC-In/DC-Out only? Or does it have a RS485/SMBus/Whatever interface? Do you want to measure DC Voltage/current only, or also AC-Phase Voltage/Current? Does it have a "programming port" for in field programming via e.g. USB? Or does the MCU just sit there? Stuff like that \$\endgroup\$ Nov 30, 2023 at 19:14

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Based on the information you provided, i'd recommend the following system layout. This only isolates the CAN-Interface for external use.

schematic

simulate this circuit – Schematic created using CircuitLab

(1) Your CAN interface is isolated from your main MCU with a digital Isolator. I'm not aware (From the top of my head), that i used isolators intended for CAN specifically. We usually use a "Communications Co-Processor" connected to the main MCU via SPI.

(2) You main MCU is on the same ground as your entire power circuit. The gate-drives, the ADC to sample phase voltage and current, and what-not.

(3) Your main circuit needs a "helper power-supply" to bootstrap the circuit from the AC-Input. The gate-drivers can require a significant ammount of power, so do your calculations.

(4) Programming/Updating the device in the Field is done via a e.g. USB interface. The data is routed through the Co-Processor and updates the main MCU. Make sure, the power circuit is in a safe state per design while the main MCU is in reset.

(5) The Co-Processors flash image is stored in the main controllers flash image. It is "updated" every-time the device boots. This way, the Co-Processor can't suffer from firmware inconsistency issues after the main processor was updated.

(6) Power is supplied to your communcation side via the external interfaces (USB for updating, CAN for data exchange). This is extremly common. Take care of EMI/TVS protection on the CAN interface if intended for industrial use.

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  • \$\begingroup\$ Thanks a lot. That's pretty much what I wanted to know. The MCU may stay in the same ground as the power circuit along with sensors. Communication on the other hand is isolated. With respect to grounds not exceed the 1kv rating, is it ok if these are connected using a capacitor? \$\endgroup\$ Nov 30, 2023 at 21:17
  • \$\begingroup\$ With regard to GND connection between the isolation i usually use 1MEG in parallel to 10p (Make sure to get parts actually rated for 1kv!) \$\endgroup\$ Nov 30, 2023 at 21:22
  • \$\begingroup\$ Great to know that, thanks a lot ! \$\endgroup\$ Nov 30, 2023 at 21:48

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