Hello,
I have a question about waveforms on rectifier bridge output.
In a motor controller (HV) I have an AC input stage like on schematic attached below (pic 1). I have noticed that when motor is enabled nearby placed I2C bus (usually quite sensitive to any noise) crashes so I started my investigation
All "academic" waveforms that I remember looked mostly like these on picture 2 (when motor is disabled) - my reference point is PE (because I don't have differential probe), DCBUS+ (yellow) fluctuates above PE level and hot GND (blue) fluctuates below PE. Nothing surprising so far...
Then output stage for motor is enabled. It is a triple H-bridge controlled by a motor driver IC. When it is enabled all motor outputs start switching (50% PWM). If any motor rotation is desired PWM duty is adjusted and "mismatch" between them on motor phases forces current to flow etc.
But what is a big surprise for me is the picture 3 where that switching (without any motor rotation, just motor enabled, all PWMs in phase and with the same duty cycle) affects "standard" output waveforms of rectifier bridge so much
When zoomed in you can clearily see PWM over that.
I though that DCBUS and GND levels are strongly forced by the mains network and the rectifier bridge
Of course when you subtract one waveform from another it is "ok" and you will get the DC level (on differential probe between DCBUS and GND you probably would not see any problem) but referring to PE everything looks like an EMC nightmare
Mentioned I2C bus is a first victim...
I have also checked how does it look when resistive load is connected to the output stage instead of motor windings (pic 6) and waveforms are then affected only a little bit
Do you have any idea what is going on in here? How to fix that? How to decrease the noise?