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Original question:

UCC28056: UCC28056

UCC28056: UCC28056C

Kanthavel Kumaran
Prodigy 120 points
Part Number: UCC28056

CUSTOMER Requirement:-

                                    Input voltage :115V

                                    Input Frequency : 300Hz to 800Hz

                                    Output voltage: 200V

                                    Output power: 38W

                                    Power factor :0.97

As per our Observation in the evolution board.
  1. We are able to the simulate the output voltage, based on our requirement 205V (R20 & R21 changed to 240K)
  2. We have observed Poor Power factor ( 0.44) at Higher frequencies 800Hz.
  3. Our requirement is like even in Higher frequencies also, we need to achieve greater than 0.95PF. At our maximum loads @38W

We are suspecting the Line filter choke is main factor, because the Evalution board designed for 160W @60Hz

Please suggest us the proper Choke and PFC inductor for improving the PF, based on our requirement. And also please suggest if related components for affecting power factor.

 

Thanks & Regards 

Capgemini Engineering 

Venkatesh 

9493145409

  • 0
    TI__Mastermind 31150 points

    Hello Venkatesh, 

    Thank you for your question on low PF while using the UCC28056 EVM at low power and 800Hz. 

    Actually the main cause of low PF under your test conditions is not the line filter choke, but the line filter X-caps. 

    Power factor consists of a mathematical combination of phase-shift and harmonic distortion.  PF = (cosθ) / √(1+THDi^2).
    As you mentioned, the EVM is designed for 160W max output at 60Hz.
    In "high-power", low line-frequency applications (most applications), the phase-shift part (cosθ) (where θ is phase shift between input current and input voltage) is nearly 1 and the THDi term dominates the PF calculation.  The displacement current in the X-capacitors of the EMI filter are a very small portion of the total input current.  

    If you take that same board and run it at low power, still 60Hz, the X-cap current does not change but its magnitude becomes a larger portion of the total input current (because the resistive portion has become smaller).  So θ increases and cosθ is lower, so PF is lower.  If you further change the input line frequency from 60Hz to 800Hz, then the X-cap current magnitude increases 13.33 times (because X-cap impedance decreases with higher frequency) and cosθ term gets really low.  

    That is why your PF reading is only 0.44 at 38W and 800Hz, even with the low 115Vac input.  (It would be even worse at 230Vac.)  

    To obtain high PF at your normal operating conditions, the PFC stage will need to be redesigned for those targets.  The EVM will have to be extensively modified, not simply reduced feedback network resistance.  I recommend to use the UCC28056 Excel Calculator tool for this effort so that all of the parameters are selected appropriately. 
    Link:  https://www.ti.com/tool/download/SLUC641 

    For the EMI filter, it is a little more complex.  For a 50~60Hz design, the frequency span ratio is usually 63/47 = 1.34.  For avionics, 800/300 = 2.67, which is double the ratio. 
    Therefore you must select X-caps with total low value to meet the cosθ requirement at 800Hz.  But that value may be too low for adequate noise suppression at 300Hz, so the CM and DM chokes may have to be larger to keep the EMI noise low enough at 300Hz (where the X-caps are less effective). 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 120 points

    Hi Ulrich,

    Actually we have tried ,what ever the suggestions you provided. Good to here that we are able to achieve upto 0.84 Pf @135V,800Hz.Please suggest to still more improvement upto 0.97Pf.

    And another thing we are doing simplis Simulation ,Based on the simplis model provided in the TI website.

    In this the default model is working fine for @255V 800Hz,but at the same time we are unable to achieve the output voltage using same circuitry for @135V 800Hz, we have modified based on the requirement of Vosns and ZCD pin configuration, Now we are able to achieve the output voltage of 200V .

    But we are able to change the inductor and Mosfet based on our requirement and suggestions gave by TI team, At that time the PFC boost is not triggering.

    We have done too many Modifications in the Mosfet and inductor parameters and even model also changed. Still we are facing same.

    Please suggest us or share us the feedback what things need to be done.

    Please share us the emails, for forwarding the test results.Here i was unable to upload, because of my organisation restrictions.

  • 0 in reply to Kanthavel Kumaran
    TI__Mastermind 31150 points

    Hello Kanthavel, 

    I can try to help you increase the PF higher than 0.84, but I don't think it is possible to reach 0.97.

    Besides reducing X-capacitance to a minimum (to drive (cosθ) factor toward 1), the THDi must also be minimized (to drive √(1+THDi^2) factor toward 1).
    Low THDi is difficult to obtain at 800Hz, and even more difficult when using an EVM designed for 160W as a test board for 38W.

    I have sent you an email so you can reply with your test results outside of this forum. 

    Regards,
    Ulrich