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Original question:

UCC2626: Setting the PWM-I, C-TACH, and PI network

UCC2626: PWM Duty Cycle maxes out at 50%. Increase PWM_I and PWM Duty Cycle

Jack Barone
Prodigy 10 points
Part Number: UCC2626


Hello, 

I am working on a bldc motor project. The issue I am having is that the duty cycle will max out at ~50% and PWM_I will max out at 5V(does not go any higher). The duty cycle maxes at 50% because the PWM_NI is a triangular wave between 2V and 7.5V, and when it is compared with the 5V PWM_I signal(at the PWM comparator), a duty cycle of 50% is produced. The result of this is that at high loads, the speed drops below the setpoint because the motor doesn't have enough power.

My question is how can I increase the voltage of PWM_I? If the voltage of PWM_I could reach up to 7.5V, the duty cycle could reach 100%. My board is nearly identical to the example showed in figure 17 of the data sheet(two-quadrant, analog speed control, etc). 

Thanks, 

  • 0
    TI__Mastermind 27676 points

    Hi Jack,

    Thank you for your question! I will need some time to look into your request and provide a response, hopefully by the end of next week (next week is a shorter week due to Christmas).

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 10 points

    Thanks. Do you think increasing the gain of op-amp u 5a would allow pwmi to reach a high voltage? 

  • 0 in reply to Jack Barone
    TI__Mastermind 27676 points

    Hi Jack,

    What is the voltage that is used to power the U5A op amp? The output of the op amp will not be able to go above the voltage that is used to power the op amp. If you are using 5V to power the U5A op amp, then I am not surprised that the PWM_I input is not going above 5V. 

    Regards,

    Anthony Lodi

  • 0 in reply to Anthony Lodi
    Prodigy 10 points

    Anthony, 

    The supply voltages are 0V and 12V. I did more testing today and PWM_I does go above 5V, up to 7V eventually at high loads. The issue is that at medium loads, the speed drops below the set speed. I want the speed to remain constant across various loads. 

    This makes me think that the gain should be increased. How would I go about playing with the op-amp gain? 

  • 0 in reply to Jack Barone
    TI__Mastermind 27676 points

    Hi Jack,

    If my analysis is correct, to increase the gain it is necessary to either decrease R29 or increase the total effective impedance of the C7||(R10+C8) network. According to my calculations,

    VPWM_I = VSPEED_SET + (Zeff/R29)*(VSPEED_SET - VPWM_TACH)

    where

    Zeff = the effective impedance of the C7||(R10+C8) network,

    VSPEED_SET = the reference voltage for the commanded speed

    VPWM_TACH = the output of the buffer that senses the filtered TACH_OUT output

    As can be seen from the equation, the difference between the reference voltage for the commanded speed and the filtered PWM tach output voltage is amplified by Zeff/R29. when the Zeff/R29 ratio is larger, then the differential VSPEED_SET - VPWM_TACH voltage is amplified more.

    Let me know if adjusting the gain helps resolve your issue. 

    Regards,

    Anthony Lodi