Isolating the motor control signals from the microcontroller from the high voltage/current lines

All QuestionsCategory: Electrical EngineeringIsolating the motor control signals from the microcontroller from the high voltage/current lines
Isolating the motor control signals from the microcontroller from the high voltage/current linesBrittany Williams asked 4 months ago
We need to design an integrated DC motor driver/controller, which will be under the control of the microprocessor. Due to space requirements, the digital and analog circuits will be on a single PCB (think of the size of a PC104), or they will be stacked on top of each other (again, similar to how pc104 boards are stacked). The load requirement is about 10 amps at 30 volts. The stall current can reach 25A.
In order to protect the control unit from the motor, we want to isolate the control signals. In fact, even the power source and the grounding of the digital and analog aspects are different. (Note that this means that the ground of the analog side can be with a completely different voltage than the ground of the digital side)
My questions are: What is a good way to isolate these two domains (optocouplers, perhaps?). The signal rate between the two domains should be around 1MHz.
Also, I’m a little concerned about noise from the motor affecting digital circuits, even with proper isolation of the control signal lines. Just being physically close to the engine sometimes causes problems, not to mention physical contact. I’d love to hear about your experiences building motor/drive controllers so we don’t make the same mistakes.

1 Answers
Isolating the motor control signals from the microcontroller from the high voltage/current linesStanley Stanley Delapena answered 4 months ago

Optocouplers can be used in many applications but be aware that conversion speeds are limited. Digital insulators (analog devices, et al) that use magnetic or capacitive coupling are much faster, but are a bit more expensive. We’ve had good luck with all of these methods. In general, optocouplers (regular with drivers or external drivers such as the Avego HCPL-3120) work because switching speeds are rarely above 100 kHz on actuators. Use gate motors that have outputs that are fast and powerful enough to keep switching losses under control. For analog feedback, consider Isolation Amplifiers (TI, Analog Devices) LEM or Optoamps (Avego) We will often heat up the control circuit and just input and exit control information. Regarding noise, avoid running power through ground to any control or measuring circuit. Use a ground control PCB with a single ground-to-ground connection if possible. I’ve had success using a small 2-layer SMT control board (grounded on one side) on a power system rated at 83A peaking at 385V and 62.5kHz and I haven’t had a small issue with noise so far. SMT is installed directly on short positioned power devices and obtains its control signals through an 8-pin header.

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