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UCC28722: How to design a bias, supplied with the 390 VDC of a pfc converter?

Ras Sharif50
Genius 3455 points
Part Number: UCC28722
Other Parts Discussed in Thread: UCC28019A,

Hi Everybody,

I am designing a bias supply by UCC28722 which is supplied with a DC voltage ( of a pfc converter using UCC28019A controller ).

The supply voltage is 390 VDC typically which is from 380 to 400 VDC and has a 23.4 V peak to peak ripple. ( ie the minimum voltage equals 380-23.4/2 V ~ 368VDC )

I use the provided excel calculator SLUC529.

In the row 35, the primary peak current  of the transformer is calculated by:

Ippk=Pout*2/(Eff*(Vinmin*SQRT(2)*0.6)*Dmax)

I modified the above equation to the bellow one ( to adapt the minimum input voltage ):

=Pout*2/(Eff*(Vinmin*SQRT(2)-Vrip/2)*Dmax)         --->               Pout=5W, Eff=0.75, Vinmin= 269VAC ( 380 VDC ), Vrip=23.4, Dmax=0.495

Which results in 73.1 mA peak to peak.

And then the Lpm=31.2 mH.

Also the Nps~77.

Now please consider the TIDA-00355 reference design  which is very similar to my application.

The transformer used in the the design is a Lpm=4mH and Nps=10.

Although the tow applications are very similar the designs differ considerably.

Is my design correct and optimum?

Why the reference design differs to my one?

Regards, Ras

  • 0
    TI__Guru 71435 points

    Hello Ras,

    TI does not generally support modified password protect excel design tools.  However your equation from Ippk needs to be set for the minimum input voltage.

    =Pout*2/(Eff*(Vinmin*SQRT(2)-Vrip)*Dmax)  = 5W*2/(0.74*(380V-23.5)*0.495) = 0.077A

    fsw = 74 kHz, Lpm = (380V-23.5V)/(0.077A*74kHz) = 63 mH

    Please note if this was an offline design of 85V to 265V the minimum input bulk voltage would be 72V.

    That being said at 42 kHz the primary magnetizing inductance at 72 kHz would be 2.56 mH.

    The tida design was design for 175V AC to 265 V AC.  The minimum input voltage that you entered in the tool is high than what you enter and could account for the difference in the Lpm.

    Nps is based on voltage second balance and will vary depending on the what the minimum input voltage.  This design has different input requirement than yours.  So that should explain the turns ratio difference as well.

    Regards,

  • 0 in reply to Mike O'
    Genius 3455 points

    Hi Mike,

    Thank you for the reply.

    How do you recalculate the Lpm for 42kHz?

    In my calculation it results in (42k/72k)*4m=2.33mH.

    Regards,

  • 0 in reply to Ras Sharif50
    TI__Guru 71435 points

    Hello Ras,

    You can double check this with the excel design tool.  There is also an equation in the data sheet that you can use to double check the Lpm.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 71435 points

    Hello Ras,

    I hit send to quickly.

    Equation 18 in the data sheet can be used to calculate the primary magnetizing inductance (Lp).

    The following link will get you to an Excel design tool for the UCC28722 that you can use to calculate Lp as well.

    Regards and thankyou for choosing TI,