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UCC28056: Question about selecting a boost inductance

Jongyoon Chae
Prodigy 30 points
Part Number: UCC28056

Hi

I'm Jongyoon.

I'm Currently designing a PFC with UCC28056 and have some questions regarding it. 

First, my specifications for I/O is as follows : 

Vin (AC) : 198 Vrms ~ 242 Vrms

Pout : 200 W / Vout : 400 V 

Here is my questions 

1. Selected GFF based on input voltage 

: I Understood that GFF is automatically changes when input voltage varies. And I found out that the selected GFF based on the input voltage can be calculated through 

Kzc (ZCD resistor divider ratio) and VFF,fall. I want to know that if i am correct about it .

For example, assuming Kzc = 401 (value on the datasheet), since VFF0Fall = 0.331 V 

Kzc * VFF0Fall / sqrt(2) = 93.85 Vrms. Therefore I believe when the input voltage goes lower than 93.85 Vrms, GFF0 will be selected for operation. 

Am i correct ? 

2. Boost inductor design 

: So if my understanding in question 1 is correct, is it okay for me to design my boost inductor by following ? 

my input voltage range is 198 Vrms ~ 242 Vrms, so i believe that only GFF5 ~ GFF7 values are utilized during operation 

LB = (VinRMSMIn ^2 /(110% * Poutmax) )* TONMAX5 *0.5 

LB = ((KZC*VFF5FALL)^2 /(110% * Poutmax*2))*TONMAX5 * GFF6 *0.5 

and i should use the smaller value 

I want to know if i'm correct about it. 

I will look forward to your reply 

Thanks. 

  • 0
    TI__Expert 6745 points

    Hello,

    We are on holiday today for Good Friday and an expert will respond next week.

    Regards,

    Jonathan Wong

  • 0 in reply to Jonathan Wong
    TI__Guru* 80446 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 80446 points

    Hello,

    Even though you are not designing for 85 V to 265 V RMS input.  With the UCC28056 you need to set up the ZCD/CS sensing as if you were then you don't have to worry about selecting the correct GFF.  Also the systems engineer that developed this device had made these recommendations to customers in the past as well.

    I did not know if you were aware of this but there is an excel design calculator that will help you setup the design.  You can find it at the following link. 

    https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-jJuGvp2EMI/01.00.00.0B/sluc641b.zip

    Regards,

  • 0 in reply to Mike O'
    Prodigy 30 points

    Hi thanks for your answer. 

    I understand that the set up for ZCD/CS sensing should be maintained to a value which is mentioned on a datasheet (Kzc = 401) 

    I am wondering that since my input specification is 207 ~ 233 (range changed from my initial question)

    My converter works only at GFF6 (since I used the same ZCD/CS sensing pin as recommended) 

    Since boost inductor should be designed based on the maximum  ON TIME (TONMAX) which varies according to GFF 

    Then I think my boost inductor should be designed considering TONMAX6, not TONMAX0 

    I already looked at the design excel you attached, but in the design file the boost inductor is designed by TONMAX0 

    So should my boost inductor be designed based on TONMAX0 or TONMAX6 

    Bit confusing... 

    Thanks & Best regards 

    Jongyoon

  • 0 in reply to Jongyoon Chae
    TI__Guru* 80446 points

    Hello,

    I believe because of the way the internal multiplier is setup that you have to design this as suggested.

    That has been the recommendations from the systems engineer and IC designer of this device.

    Regards,

    Mike