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UCC256301: About Q value

ACGUY
Guru 11170 points
Part Number: UCC256301

Hello E2E,

About a formula (12) in page 49 of datasheet, "Qe=0.15", why does TI set the Qe-value to 0.15?
I think that if Qe is low, resonant wave is to decay early for primary side.
Please let me know the reason for set to "Qe=0.15".

Regards,
ACGUY

  • 0
    TI__Guru* 85946 points
    Hello Acguy,

    Qe is the quality factor and and describes the peaking of the LC tanking circuit. Q=(1/R)*(L/C)^2.

    In general they try to keep this fairly small to reduce the the peaking of Mg(f) from becoming too high and not being able to be reliably controlled.
    The foolowing application note gives an equation for Mg(f) and if you have MathCAD you can and adjust Qe to see what the effects would be of using greater or smaller Qes. www.ti.com/.../technicaldocuments

    One thing you want to pay attention to when using this device is sizing Lm to achieve ZVS. The application note metioned discusses how to do this and this material is not covered in the data sheet.

    Regards,

    Mike
  • 0 in reply to Mike O'
    Guru 11170 points
    Hello Mike,

    Thank you for reply.
    Qe is related to LLC input voltage?
    How does greater or smaller Qe have an effect on the behavior to opeating when Vin decreases?

    I leaning LLC topology AC/DC.
    Please teach

    Regards,
    ACGUY
  • 0 in reply to ACGUY
    TI__Guru* 85946 points

    Hello Acguy,

     

    Mg(f) is the relationship between Vout and Vin.  Please look at equations 19 and 20 for min and max gains in the following application note.

    www.ti.com/.../getliterature.tsp

     

    In regards to Qe refer to equation 22 and figure 5.  As Qe increases the peak Mg(f) will increase.   With recommended Qe the design can handle a 2 to 1 input range without an issue.  For most applications the 2 to 1 range is optimal.  If you try to increase Qe and increase the gain the peaking is too steep and you will not reliability be able to control the output.

     

    Regards,

     

    Mike