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TMS320F28069M: Current Ripple for one phase during open-loop control in Proj_01b

Part Number: TMS320F28069M
Other Parts Discussed in Thread: MOTORWARE, DRV8303, DRV8301,

Good afternoon,

I'm revisiting an old BLDC motor controller project and trying to reduce the current consumption when running at full load. I've ported my old hal files (in v14) to the latest v18 of motorware and have the lab01b project running with my motor. Interestingly, I'm seeing an unusual current ripple in one of the three current measurements (see below). I wanted to reach out to see if anyone had ideas as to why I'm seeing this affect, and if there could be anything to change either in hardware, or software settings that can mitigate this affect. The PWM outputs from the SVGEN match the example in the lab tutorials, but the current looks quite different.

I should mention that the current in phase A is amplified by an external amp, while the current in phases B and C are amplified in the DRV8303 chip. I've attached a screenshot of the schematic for phase A. Please let me know if you have any ideas.

Best regards,

Chris

  • What pwm and current control frequency are you using? And did you have any current waveform of oscilloscope? You may increase pwm frequency if the inductance of motor is very small. The current waveform of datalog/pwmdac should be similar to the waveform of oscilloscope, if no, you have to check the current sampling circuit since one phase current is not current, PCB layout design is very critical for current sampling and motor control.
  • Good afternoon Yanming,

    Thank you for your quick reply. I did actually try a few different PWM frequency and control frequencies to see if that had any impact. It is a very low inductance motor, so control has always been challenging. I've tried PWM frequency of 45kHz and 60kHz and control frequencies of 10kHz and 15kHz. Is there anything in the op-amp design schematic that may be causing this issue?

    Also the current measurements on the other two channels, B and C, are a little noisy. Is there any way to enable internal filtering to smooth out this signal. How can I adjust PWM and control/sampling frequency to smooth the response and reduce noise?

    Thank you for your help again.

    Best regards,

    Chris

  • 45kHz pwm and 15kHz current control frequency should be good for most low inductance motor. You have to go back PCB layout design review to make sure the current/voltage sampling signal is enough clear. General signal deal rules must be followed, like using separated ground for analog, digital and power signals, current signals use the differential input to amplifier...
  • What do you recommend for a filter for the B and C channels connected to the DRV8303? It looks like there is some high frequency noise on the analog signal. Shall I increase my 1 nF capacitor on the current sense lines to a 0.01 uF capacitor or higher?

    Also I found that the DRV8301 datasheet recommends a ~1MHz filter between DRV8301 and TMS320F28069M (56 Ohm resistor and 2.2nF capacitor), however, the DRV8301 dev-kit design does not include one. Do you recommend that I add a low-pass filter between these two components per the datasheet or should I follow the reference design in the dev-kit?

    Best regards,

    Chris

  • 1. 10nF is too high, 2.2nF or 3.3nF is better.
    2. You'd better to add a RC filter between the output of DRV830x PGA and adc input of C2000, and place this filter near the ADC input pin. The value of 56(OHM) and 2.2nF is ok for this filter.
  • Thank you, we'll give that a try. Is the project lab01b supposed to produce smooth open-loop control at any frequency? I've found that it works well ~1kRPM for my motor, but if I increase to ~1.5kRPM then it becomes unstable. Is the control only as smooth as V/Hz approximation or could there be another issue with my setup?

    Best regards,

    Chris

  • It's not a ideal v/f control for full speed range since the v/f profile maybe not match the motor specification, just only use it as h/w verification for next labs.
  • Good afternoon,

    The oscillation in the phase A current channel was stemming from an oscillation in the 1.65V op-amp circuit. We solved the issue by adding a 1 kOhm resistor to the output of the op-amp and this eliminated the ringing in the current sense channel. Thank you Antonio, Randhir, and TI's internal engineers in helping identify and provide a solution!

    Best regards,

    Chris