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
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.
What is your capcitance just after your diode bridge rectifier?
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