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UCD3138064A: UCD3138 burst mode question : control min duty

Hung Kai Woo
Prodigy 70 points
Part Number: UCD3138064A
Other Parts Discussed in Thread: UCD3138

Hi

I am using UCD3138 for LLC. I would like to know how to control a min duty of DPWM0 during burst mode, e.g. 30% duty.

Thansk

  • 0
    TI__Genius 17090 points

    Our LLC expert is out for a few days, I'll assign this question to him. 

    Since you are asking about minimum duty, I assume you are talking about doing the burst mode in PWM mode, rather than in resonant mode?

  • 0
    TI__Intellectual 1270 points
    Hi Hung,

    It depends on what DPWM mode you are operating at light load . Best would be to limit the filter output .

    Thank,
    Sanatan
  • 0 in reply to Sanatan Rajagopalan
    Prodigy 70 points

    Hi Sanatan

    I am using LLC and found that if the pulse width of DPWM0 is very small and MOSFET is running in hard switching at burst mode. I want to set a min_duty_low and min_duty_high = 30% duty e.g. PRD is 1515 and hence it is ~ 457 for min_duty_low and min_duty_high. However, it is found that the output is out of regulation from 12.2V to 13.5V. I reset the value back to orignal value. The output is ok again. I don't know why. Please advise a solution to me. Thanks

  • 0 in reply to Hung Kai Woo
    Prodigy 70 points

    Hi Sanatan

    Any updated? It is very urgent.

    Thanks

  • 0 in reply to Hung Kai Woo
    TI__Intellectual 1270 points

    HI Hung,

    You can set the minimum duty uing the following bits :

    Dpwm0Regs.DPWMCTRL0.bit.MIN_DUTY_MODE=2;

    Dpwm0Regs.DPWMMINDUTYHI.all= MIN_TON;
    Dpwm0Regs.DPWMMINDUTYLO.all= MIN_TON;

    Thanks,

    Sanatan

  • 0 in reply to Sanatan Rajagopalan
    Prodigy 70 points
    Hi Sanatan

    In our current code design, I have set min duty with following
    Dpwm0Regs.DPWMCTRL0.bit.MIN_DUTY_MODE=2;

    Dpwm0Regs.DPWMMINDUTYLO.bit.MIN_DUTY_LOW = 454;
    Dpwm0Regs.DPWMMINDUTYHI.bit.MIN_DUTY_HIGH = 454;

    It is supposed as same as your suggestion. However, output of 12V is increased from 12.2 to 13.9X and it is return to 12.2 once burst mode is existed

    Any comment on why output is shifted to 13.99 @ burst mode?
  • 0 in reply to Hung Kai Woo
    TI__Mastermind 18370 points

    In UCD3138 even when in light load mode (Burst mode), the duty cycle is still dictated by the digital filter (CLA) in a closed loop fashion.
    You can not just set the duty cycle to a random value. If you do, you basically open the feedback loop and that is why the output voltage does not regulate at 12V anymore.

    If the duty cycle is too low, you have to force(trick) the loop to send out higher output values (higher duty).
    That can be done for example by lowering the gain in the DPWM stage, or the switching stage.
    For example by modification of CLA_SCALE value in DPWMCTRL1 register.

    Hope this makes sense.

    Regards,
    Yitzhak

  • 0 in reply to Yitzhak Bolurian
    Prodigy 70 points

    Hi Yitzhak

    Is it mean that it is not recommended to use register MIN_DUTY to control duty in burst mode?

    Per your suggestion, it seems that I need to detect UCD3138 in burst mode or not and change CLA_SCALE to lower value if it is in burst mode. Is it correct?

  • 0 in reply to Hung Kai Woo
    Prodigy 70 points

    Hi Yitzhak

    I will try with below code

            if (Dpwm0Regs.DPWMFLTSTAT.bit.BURST)
            {
                Dpwm0Regs.DPWMCTRL1.bit.CLA_SCALE = 0;
            }
            else
            {
                Dpwm0Regs.DPWMCTRL1.bit.CLA_SCALE = 6;
            }

    However, the duty of Dpwm0 is still to low as same as beform.

  • 0 in reply to Hung Kai Woo
    Prodigy 70 points
    Hi Yitzhak

    I have tried to set Dpwm0Regs.DPWMCTRL1.bit.CLA_SCALE = 6 no matter it is burst mode or. The duty is still very low. I would like to share some waveform to your for reference. However, I can't find how to attach it on the mail.
  • 0 in reply to Hung Kai Woo
    TI__Mastermind 18370 points

    Hi,

    Sorry Hung Kai Woo, my bad.
    The CLA_SCALE can make the output of filter go higher, but I forgot that at the end the observed duty cycle coming out of DPWM will remain the same.

    Instead, you may raise the LLENTHRESH values for ON and OFF higher.
    This will of course make you switch to light load mode at higher load currents as well.

    This issue seem to be complex and may require more time and transfer of information about your application.
    In case this does not help,or this is not what you meant to do and you require further support, please contact your local TI sales office for further assistance.