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UCC28251: UCC28251 schematic

Wei Cheng Liu
Prodigy 250 points
Part Number: UCC28251

Hi 

i did the schematic for uCC28251 /140Vdc input with 60Vdc output half bridge converter.
there are several things I can't figure out,
I don't really sure what's going out about the circuit. the Vcomp level isn't going well to control the duty.
Perhaps the secondary side control circuit I mad is way going wrong.
do you have time to help me with this?

Best Regards

KevinLiu

ucc28251 schematic.pdf

7774.ucc28251 schematic.pdf

  • 0
    TI__Mastermind 27135 points
    Hi Wei Cheng Liu,

    Thanks for your interest in UCC28251. Does your output voltage appear to be unstable? Are you able to measure the closed loop response of the converter? Is you duty cycle inconsistent for a given load current?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 250 points

    Hi Ben

    My output voltage appear not correctly and unstable, the duty cycle is inconsistent for light load, and can't output full load.

    Best Regards.

    KevinLiu

  • 0 in reply to Wei Cheng Liu
    TI__Mastermind 27135 points
    Hi Wei Cheng Liu,

    Do you have the ramp resistor (R803) populated? Could you show me waveforms for OUTA and OUTB?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 250 points

    Hi Ben

    I did really missed R803, after pull up to Vref by a resistor, the switching behavior is better.

    This resistor is to decide duty maximum, right? How the way of duty modulation is going? higher resistance vs larger duty?

    If there is fixed ramp generation, so the UCC28251 is becoming voltage mode, am I right?

    ILIM pin voltage is only for protection. am I right?

    Best Regrad.

    Kevin 

  • 0 in reply to Wei Cheng Liu
    TI__Mastermind 27135 points
    Hi Kevin,

    The resistor is used to make the ramp signal. The ramp signal is compared to the output of the error amplifier to set the duty cycle. Take a look at section 8.3.7 of the datasheet for reference.

    Yes, voltage mode control uses fixed ramp generation (or with voltage feedforward from the input). In current mode control the ramp signal is generated from current information such as through a current sense resistor. This app note covers the major differences between voltage mode control and current mode control: www.ti.com/.../slua119.pdf

    Yes, ILIM is for overcurrent protection.

    Are you seeing the correct output voltage now?

    Best Regards,
    Ben Lough