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Hello,
I'm struggling with this problem for a while now and I was no able to find a solution. I'm building a small motor controller based on a STM32 microcontroller and DRV8323RS integrated driver. After a while I found out that a weird noise is coming from the motor, even when all 3 PWM duties are equall (and set to 50%). The frequency that can be heard is about 10-13,5kHz changing a bit with different motors I'm using, however stays constant for a particular motor, no mater what the PWM frequency is (tested from 20- 40khz). Here is the mic fft plot:
The purple cursor is set to 20khz - the carrier frequency, however the wide peak at 13,5khz is not expected.
I decided to check the voltages and compare them to the PWM outputs from the microcontroller. I connected two probes to phase b and c STM32 PWM outputs, and their MOSFET outputs. What I saw was a bit strange for me, as the DRV is randomly injecting a delay of about 30-50ns between the PWM cycles, although the cycles from the microcontroller are perfectly aligned. Without a delay:
And after a few cycles:
I also made a subtraction between the two outputs over a wider time range:
It looks pretty random to me, and I have no idea where to look for the cause of this problem. Things I have tried already:
-Changing the circuit to 4-layer - no matter if the circuit is on a 2 or four layer the behaviour and the sound are the same.
-Changing the motor - the disturbing frequency changes a bit a few khz, but overall there is no difference
-Disconnecting the motor - it is certainly the motor that makes this sound
-Changing the dead times and drive currents in DRV - I can see it's working on the scope (extending the opening time of a mosfet) but no difference in the sound whatsoever.
-Adding capacitance to the DC bus - no improvement even at very high capacitances - the caps are mostly ceramic and their dc bias is taken under consideration.
-The capacitor on CPL CPH pin has a correct value
- Gate charge loops are as small as possible to reduce the inductance.
- I do not suspect software as it is only responsible for generating 3 PWMs.
I'm working in 3x PWM mode, with enable pin shorted with other three INLx pins.
And the sound itself - it is quite hard to catch on mic:
Click here to play this audio clip
Thanks in advance!
Dear Adam,
unfortunately the problem is still present. For now I've only tried the maximum and minimum IDRIVE settings, but no difference in sound at all. I will double check that these settings are applied using a scope, but I guess they are, since I'm reading back the registers and getting the values I previously wrote.
In that case don't you think the sound comes from these random delays, as some current must flow through the coils in such moments? I'm just guessing, but this explanation seems quite reasonable to me..
Of course there are two more layers, sorry I must have forgotten to attach them:
Best Regards,
Peter
Here you go: (highest and lowest current, measured directly on the MOSFET using the springy probe piece)
Of course (from the top) :
1. Highest gate charge current, few cycles visible on the screen,
2. Highest gate charge current, zoom on the rising edge,
3. Lowest gate charge current, few cycles visible on the screen,
4. Lowest gate charge current, zoom on the rising edge
If I may ask, do you see any symptoms of too aggressive gate charging on the waveforms ? I noticed the sound is irrelevant of the current setting, and at the same time I do not see any over/undershoots or ripples on the scope.
I also attach two more scope screens:
Falling edge zoom, lowest gate charge current:
Falling edge zoom, highest gate charge current:
Moreover I've seen a few similar projects, which were equipped with mosfets with even smaller Qgd (TPH5900CNH), so I guess this is not really the case...
