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UCC28070: Active power factor correction

Part Number: UCC28070

Hello,

I am using UCC28070 interleaved PFC controller for 1.2kW battery charger design. I have few queries regarding this IC. These are mentioned below.

1. Does duty cycle in boost PFC varies according to input sine voltage? 

2. What are the advantages and disadvantages if we keep duty cycle constant?

3.  What are the advantages and disadvantages if we keep duty cycle variable?

4. Why loop bandwidth of PFC is always kept low such as at 20Hz?

Regards,

Dhananjay

  • Hello Dhananjay,
    I will answer your questions in the order you wrote them:
    1. The UCC28070 is a CCM type pfc controller and the duty cycle varies with the instantaneous value of the input voltage.
    2. The UCC28070 cannot be used with a fixed duty cycle. You can use the UCC28063 transition mode controller in this way but 1.2kW is beyond the power handling range of this controller. The peak currents are simply too high and the power losses great.
    3. The advantage of variable duty cycle is that the current can be forced to accurately track the voltage and achieve good power factor.
    I can think of no disadvantage
    4. The loop bandwidth is is usually set at about 0ne tenth of the minimum rectified input frequency. This is so that the loop does not attempt to track each cycle of the input voltage but rather the average value.

    Regards,
    John
  • In reply to John Griffin1:

    Hello,

    Thanks for the quick response.
    If the loop bandwidth is one tenth of rectified input frequency, then how does the duty cycle vary at input frequency?
    Thanks
  • In reply to Dhananjay Yadav:

    Hello John,

    Can you please answer to above query?
    Regards,
    Dhananjay
  • In reply to Dhananjay Yadav:

    Hello Dhananjay

    John is travelling at present and his responses may be delayed.

    In the meanwhile - I'd suggest you look at the material on PFC at http://www.ti.com/ww/en/power-training/login.shtml?DCMP=psdslibrary&HQS=tlead-power-psdslibrary-apec2015-pwrhouse-20150312-lp-en      The article at https://www.ti.com/seclit/ml/slup203/slup203.pdf is a good introduction to PFC in general.

    Overall, the slow loop response (typ 10 Hz or so) allows the controller to set an AVERAGE current level in the PFC inductor which is then modulated (or multiplied) by the instantaneous input voltage (a sinusoid). You can see the MULT block in the UCC28070 data sheet block diagram. This modulation is what modified the duty cycle as the line voltage goes through it's cycle. This way you get a sinusoidal input current whose AVERAGE level is correct for the load conditions at the output.  there are some subtlties of course but that's the basic idea.

    Regards

    Colin

  • In reply to John Griffin1:

    Hello,

    What is the minimum duty cycle that UCC28070 can offer?

    Regards,
    Dhananjay Yadav
  • In reply to Dhananjay Yadav:

    Hello Dhananjay

    The minimum duty cycle is zero.

    Regards

    John

  • In reply to John Griffin1:

    Hello John,

    Thanks for reply!
    Please let me know at maximum instantaneous input AC voltage duty cycle will be minimum or maximum? 

    And at zero instantaneous AC input voltage what will be duty cycle value?

    Regards,
    Dhananjay Yadav

  • In reply to Dhananjay Yadav:

    Hello Dhananjay
    Equation 42 in the datasheet shows how D varies with the input voltage.
    You can see that the peak value of Vin corresponds with the min value of D and the min value of Vin corresponds with the max value D

    Regards
    John
  • In reply to John Griffin1:

    Hello John,
    Thanks for reply!
    But I am not able to realize this concept. This is for the 1.2 KW battery charger PFC stage.
    Can you please tell me is duty cycle is in phase with input AC wave or it is out of phase?
    Please find below image for reference. Is it like the same as shown?

    Dhananjay Yadav

  • In reply to Dhananjay Yadav:

    Yes the duty cycle waveform looks like it is behaving properly.

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