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UCC28051 MAX HARMONICS/ THD ISSUES

Tony Nicotra
Prodigy 10 points
Other Parts Discussed in Thread: UCC28051, UCC3817A, UCC28019

WE ARE HAVING  DIFFCULTY  MEETING THE  3%  HARMONIC &  5% MAX THD LEVELS  ON THE 3RD  & 5TH  HARMONICS   USING THE UCC28051 PFC CONTROLLER ON A  35W CV SWITCHING SUPPLY

WE MUST MEET THIS SPEC DOWN TO A 10W LOAD LEVEL FOR  A  MILITARY PROJECT.   OUR  3RD HARMONIC MEASURES ABOUT 4.67% AT A  20W LEVEL.  THE MAIN PWM IS RUNNING AT 65KHZ.   WILL VARYING THE SWITCHING FREQ LOWER THE HARMONICS? 

I UNDERSTAND THAT THE EFFICENCY OF ACTIVE PFC CONTROLLERS  DROPS  WITH  OUTPUT LOAD.  WHAT SUGGESTIONS CAN YOU ADVISE SO WE CAN LOWER  THE HARMONICS.   

WILL THE UCC28051 BE ABLE TO CONTROL THE HARMONICS  DOWN TO THAT LEVEL OR SHOULD WE LOOK FOR ANOTHER CHIP USING A DIFFERNT PFC SCHEME

 

TONY

APX TECHNOLOGIES

 

 

  • 0
    TI__Guru 70735 points

    The UCC28051 was designed to meet EN61000-3-2 input harmonic requirements.  Their is a User's guide that can be found for 100W PFC pre-regulator that can be found at the following link to show the expected PF and THD performance.  http://www.ti.com/lit/ug/sluu152a/sluu152a.pdf

    Your specifications seem slightly different and for ENC61000-3-2  you only require PFC pre-regulator above 75W.  Most PFC controllers will only give you less than 5% current THD at full load and it will decay as the load decreases.  The reason for this is the flat spots in the input current waveforms caused by amplifier offset or internal offsets of the controller.  One way to improve this is to use the highest current sense signal possible.  However it's performance is limited.

    Transition mode PFC does have higher THD compared to average current mode control.  You might consider using a UCC2853, UCC28019 or a UCC3817A. However I am still not sure you will be able to achieve current THD of less than 5% at 30 and 60% loads.

  • 0
    Expert 1920 points

    What is your capcitance just after your diode bridge rectifier?