• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 192 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UCC28950: About the derivation of the formula

Hirotsugu Kobayashi
Mastermind 9540 points
Part Number: UCC28950

Tool/software:

Hi team, I have questions about the derivation of the formula in the datasheet.

Q1

In the formula for a1, is it correct that the numerator Vrdson is the primary side SW FET, and the denominator Vrdson is the secondary side synchronous rectification FET?

Q2

Is the formula for LMAG derived from V = L*di/dt in the commutation state when the low-side FETs of the leading and lagging legs are on?

If so, di is ΔILOUT/(2 * a1).

If the transformer primary winding is n1, the secondary winding is n2, and the transformer winding current change is ΔIP, then ΔILOUT/a1 should be derived from the formula n1 * ΔIP = n2 * ΔILOUT.

Why is there a coefficient of 1/2 multiplied to ΔILOUT/a1?

Q3

When LMAG is derived from the formula for the commutation current, I think LMAG is the sum of the transformer Lp and the resonant coil L1. Is that correct?

If so, please tell me why only the transformer value is listed.

Is it because it can be relatively ignored?

  • 0
    TI__Guru* 86266 points

    Hello,

    Your inquiry has been received and is under review.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 86266 points

    Hello,

    Please see my comments below.

    Hi team, I have questions about the derivation of the formula in the datasheet.

    Q1

    In the formula for a1, is it correct that the numerator Vrdson is the primary side SW FET, and the denominator Vrdson is the secondary side synchronous rectification FET?

    >Yes

    Q2

    Is the formula for LMAG derived from V = L*di/dt in the commutation state when the low-side FETs of the leading and lagging legs are on?

    > YES

    If so, di is ΔILOUT/(2 * a1).

    >Correct

    If the transformer primary winding is n1, the secondary winding is n2, and the transformer winding current change is ΔIP, then ΔILOUT/a1 should be derived from the formula n1 * ΔIP = n2 * ΔILOUT.

    Why is there a coefficient of 1/2 multiplied to ΔILOUT/a1?

    > This is to avoid running in voltage mode control.

    > This is described in application note slua560 that can be found in the following link.

    https://www.ti.com/lit/pdf/slua560

    Q3

    When LMAG is derived from the formula for the commutation current, I think LMAG is the sum of the transformer Lp and the resonant coil L1. Is that correct?

    > Lmag is selected to avoid voltage mode control.

    Thank you for interest in Texas Instruments (TI) products.  If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

    Regards,