Filtering power to stepper motor drivers for EMI purposes.

Thread Starter


Joined Jan 18, 2017
I have been using Allegro A5977 stepper drivers for a while now (a few years) and I had not too bad of a result meeting the EMI compliance targets even for CLASS B.

However I went by the seat of my pants when filtering the VBB (the Motor power voltage). I use a filter consisting of an electrolytic cap to the GND, 68uF SMT can right at the chip's VBB, then I put a BLM18KG221SN1D ferrite bead in series (220 Ohm at 100MHz, 50mOhm DC resistance 2A rated) and finish it off with another cap, this time a X7R 22uF chip capacitor to the GND.

This filter is at every driver. Then it runs to my 12V input where it meets the board's input filter which could vary depending on a board and POL DCDCs I use. But no matter what, there will be a 100uF electrolytic and 1uF film cap there.

I have my doubts as to the ferrite bead efficiency as the currents that could go into 1A peak will render the bead not very... resistive to the noise which is quite low in frequency in the first place and is certainly not in multi-MHz region.

How can a I design a board with stepper drivers and ensure that CLASS B FCC is met?

PS. I do not have motors outside the box: all motors are inside a metal enclosure (which of course does have some openings for ventilation and for moving parts).