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TIDA-00901: filter for Vsen and Isen

Part Number: TIDA-00901
Other Parts Discussed in Thread: TINA-TI

Hi Clark,

Thank you very much for designing TIDA-00901. It is a great design.

I have a question on RC filter for sensing phase voltage and phase current. In your design, RC low-pass filter for phase current (ISES_A, ISES_B, ISES_C) is R=56 ohms and C=2200 pF. I believe it is a nice selection for the MCU. However, the filter for phase voltage and PVDD (VSEN_A, VSEN_B, VSEN_C, and VSEN_PVDD) uses C=0.1 uF. The voltage divide resistors 25.8K and 4.99K lead to the parallel resistance of 4.18K. The time constant of R=4.18K and C=0.1 uF is significant slower than that for phase current. I believe phase voltage is used to measure BEMF, which trace the position of the motor, which is required for FOC control. Do you think such slow RC time constant is enough for tracking the motor position in FOC control? Will it be better to use the same R and C in phase current and phase voltage?

Thanks and I look forward to hearing from you,

John

  • John,

                  You are correct that the corner frequencies are not the same.  Figure 20 in the design guide shows the frequency response of the motor current sense amplifier circuit as simulated in the Tina-TI circuit simulation tool. As expected, the low frequency gain is -23 dB, and the corner frequency is about 450 kHz for these values of R22 and C28.  This is high enough to pass all the significant current information for normal PWM frequencies.

                  For the voltage sensing filter, the RC time constant formed by C15 and R10 (see figure 21) give a low-pass filter with corner frequency 318 Hz. This frequency allows all expected motor drive voltage waveforms to pass (318 Hz is 19 kRPM) while averaging the PWM switching pulses and filtering out higher frequency noise.  As you mentioned, phase voltage is related to the back-electromotive force (back EMF) of the motor.  But it is more closely related to the motor speed than the motor position, and therefore dominated by the mechanical time constant of the motor, which allows the much lower bandwidth of the filter.

                  The current of a BLDC motor is closely related to the torque of the motor, and the electrical time constant of the motor winding (rather than the slower mechanical time constant).  A higher bandwidth is needed to capture all the information in the current waveform for the field-oriented control (FOC) algorithm.  The workings of the FOC algorithm are beyond the scope of this design, but there is a good overview in the application note “Sensorless Field Oriented Control of 3-Phase Permanent Magnet Synchronous Motors at this link: https://www.ti.com/lit/an/spraby9/spraby9.pdf  

  • Hi Clark,

    Thank you so much for your insightful explanation. This is very helpful.

    Best regards,

    John