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TMS320F28379D: Fast Current Loop Question

Adam
Expert 1225 points
Part Number: TMS320F28379D


In this video:

https://www.ti.com/video/series/c2000-designdrive--solution-for-industrial-drives-and-motor-cont.html

it is said you do not need to increase switching frequency (called in video carrier frequency) when you want have faster response time.

it is said that this can be done with sub-cycle pwm update technic.

what i understand sub-cycle pwm means you can update pwm register with new value right after adc conversion is finished. But even that you have your value earlier available the regulation speed will be still limited by the switching frequency, am i not roght? To get faster regulamin you need to increase switching frequency in my view. I don’t understand the point in the video. Can somebody explain this a little bit more?

  • +1
    TI__Expert 4620 points

    Adam,

    Yes - There is no need to increase switching frequency for faster response, that is because C2000 MCU's ePWM peripheral can update duty value in the same cycle with "Direct Load Mode". It means that it will change the duty within the same cycle. But with this configuration, care need to be taken while making this update in the algorithm by adding limit within which you can make this update so that the algorithm doesn't miss the compare event in PWM module. There can be multiple update in duty in one switching cycle as long as you are in that limit to make inner most current loop faster to make subsequent speed/position loop faster to match the reference speed value. This mode is crucial especially in servo application. This is where other C2000 peripheral like faster ADC for sensing and floating point unit (FPU)/trigonometric measurement unit (TMU) for computation comes handy so that feedback and calculation can happen faster. Currently Ti's FCL algorithm (fast current loop) showcases < 1microsecond response for current loop, which enables updating duty within 1% in case of switching cycle of 10KHz frequency. So to answer you question, regulation speed is not necessarily limited by switching frequency direct loading is used.

    For next cycle update, C2000 MCU's ePWM peripheral have "Shadow Load Mode" which is more useful in appliance motor where updating speed is less critical than industrial servo application. 

    Regards,

    Sumit

  • 0 in reply to Sumit Patil1
    Expert 1225 points

    Thank you for the replay.

    For a typical next-cycle uptate the rule of thumb is that switching frequency and sampling frequency should be at least 10x the electrical frequency of the motror.

    So when my motor has 5kHz electrical frequency, I would go for 50kHz sampling and 50khz PWM.

    What is the rule of thumb using sub-cycle update? Total delay is per TI presentation is reduced from 1.5Tsamp to 0.5Tsamp, so I can aim for at least 20kHz sampling and switching or I can have even 10kHz sampling and switching with sub-cycle PWM (assuming FCL calculations takes 1us)?