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UCC256404: Option 5 for high start-up frequency but cannot enter burst mode

Chih-Shen Yeh
Prodigy 10 points
Part Number: UCC256404

Hello,

In order to achieve high start-up switching frequency, I set up UCC256404 with option 5. 

The startup switching frequency is around 500kHz and I'm satisfied with it. However, the LLC converter cannot enter burst mode even under no load. 

I suppose this is due to low BMT_H/BMT_L, but I'm not sure how to increase them. The formulas in datasheet 8.2.2.19 assume a Vssinit, which is 0 with option 5. Your excel spreadsheet calculator uses a protected table I have no access to. Can you let me know which direction I should try in terms of adjusting circuit parameters?

Below are my converter parameters and UCC256404 circuit parameters:

Vin = 385V, Vo = 22V, Lr = 42uH, Cr = 13.2nH, Lm = 400uH, Po = 250W

C_VCRup = 10pF, C_VCRlo = 1.8nF, C_LL/SS = 220nF, R_LL/SSup=180kOhm, R_LL/SSlow=100kOhm

Thanks you,

  • 0
    TI__Genius 9790 points

    Hi, Chih-shen

    BMTL defines the output power when enter burst or out of burst mode. The higher BMTL, the higher output power when enter burst mode.  And BMTL=KBMT*BMTH, KBMT=0.6 at option 5. BMTH is defined by R_LL/SSup, R_LL/SSlow and RVCC. BMTH = [(RVCC – 3.5V)/R_LL/SSup – 3.5V/R_LL/SSlow]*100kohM

    So, if RVCC is 13V which is the regular voltage, BMTH is defined by R_LL/SSup, R_LL/SSlow. 

    With your resonant tank, it is better to decrease R_LL/SSup to increase BMTH and BMTL.

    Yunsheng