0
\$\begingroup\$

I'm interested in adding two H bridge circuits to a PCB also holding a cortex M4 microcontroller. 12V/1A max motors. H bridge Ln298 H bridge.

What design considerations should I make to isolate the MCU from the motor drivers?

\$\endgroup\$
4
  • \$\begingroup\$ Why do you think you need to isolate the microcontroller from the motors? You need to understand where current is flowing in your circuit and layout you pcb to ensure high current paths aren’t flowing through your microcontroller. You also need to include clamp diodes and a snubber across the motors. \$\endgroup\$
    – Kartman
    Commented Jan 18, 2022 at 2:06
  • \$\begingroup\$ I'm mainly interested in addressing EM interference on the MCU. \$\endgroup\$ Commented Jan 18, 2022 at 2:38
  • 1
    \$\begingroup\$ Just keep all the high current and high power stuff together and placed to keep all high current paths short. 1A and 12 V is no big deal. Signals from processor to H bridge driver could include a small shunt capacitor on the processor side (not loaded) and a series resistor. Keep all the low-power stuff segregated from high power, and filter everything coming back from the high power. Take a look at reference designs from ST micro and renesas and whoever else has them. \$\endgroup\$
    – user57037
    Commented Jan 18, 2022 at 2:51
  • \$\begingroup\$ Brushed motors generate a bit of ‘hash’ in operation and a spike when the current is removed. As I mentioned, a snubber is required. This is usually a capacitor of MOV (metal oxide varistor). Again, pcb layout will determine if a transient flows through your microcontroller circuit or if it takes a more preferential path. \$\endgroup\$
    – Kartman
    Commented Jan 18, 2022 at 3:24

2 Answers 2

3
\$\begingroup\$

With 12V 1A motors, You may not need to fully isolate your motor drivers. Here’s some general suggestions that are good for whether you do or not.

  1. Decouple your 12V supply well. You can use AVX Spicat or Kemet K-Sim to design the output capacitance of your power supply. The target impedance should be $$\frac{V_{out} * V_{ripple}}{0.5*I_{max}}$$ remember digital electronics can typically handle a bit of noise.

  2. Choose a power supply for your microcontroller with high PSRR.

  3. Keep the low power electronics on a different part of the board and the high power electronics closer to the 12V power source. You want to keep currents through the drivers away from the microcontroller and associated electronics.

  4. Use clamp diodes in your H-Bridge.

  5. Add snubber capacitors across the motors to handle high current transients. Make sure these capacitors are rated to at least 50V, preferably 100V, and can handle high currents for short periods.

\$\endgroup\$
2
\$\begingroup\$

Based on my experience in motor drive with full h-bridges in the transport industry, for electrical safety as well as EMI compatibility, I would isolate the MCU and related logic from the motor drive with optocouplers. I would suggest to use at least 2KV isolation. On the layout side you should have a 2 mm gap with no copper or conductive part. This gap will be bridged with the opto-couplers.

\$\endgroup\$
2
  • 1
    \$\begingroup\$ 12 volts and 1 amp. Do you think maybe your suggestions might be overkill? Anyway, if the OP opts to do this, there will need to also be a high isolation power supply from the high voltage side to run the processor. \$\endgroup\$
    – user57037
    Commented Jan 18, 2022 at 2:53
  • \$\begingroup\$ The suggestion I made is a minimum for a generic solution. What is your input source and application? May be it can be simplified. \$\endgroup\$
    – Louis
    Commented Jan 18, 2022 at 18:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.