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UCC256404: UCC256404 won't enter burst mode

JeroenW
Prodigy 20 points
Part Number: UCC256404
Other Parts Discussed in Thread: UCC256304

Dear TI, 

I have an issue with an upgraded UCC256304 design. I've upgraded the design to the UCC256304 design accorfing to the excel calculator, the design seems to run well apart from the burst mode that doesn't run as expected. 

To enable the burst mode I've done the following steps:

  • I've chosen 'option 6' and selected Rbw_lower= 5.36k and Rbw_upper = 30k.
  • Verified the in circuit resistance to be ~4554 Ohms
  • Verified selection occurs (0.25V DC on BW pin) before start of the LLC waveforms (see waveform below)

  • I've selected threshold of the LL/SS pin resistor values to be Rll/ss_upper = 137k and Rll/ss_lower = 56k which should yield a ~3.8V threshold  according to calculation 70 and 71 in the datasheet. 

My problem is that the device will not enter burst mode even when there isn't any load attached, I've further fiddled a bit with the design:

  • I've fiddled with the values  Rll/ss_upper and Rll/ss_lower get a higher burst threshold
  • Used a resistor over the optocoupler to draw extra current from the controller and lower FBreplica
  • Fed a small current to the secondary side output to offset, eventually causing it to shut down entirely without even reaching burst mode

It seems like no burst mode is selected although I think I've selected it correctly, could you please provide input on how to proceed next?

  • 0
    Prodigy 20 points

    The image that did'n make it in the last post

  • 0 in reply to JeroenW
    TI__Mastermind 27085 points

    Hi JeroenW,

    Sorry to hear about the trouble getting into burst mode. If I could explain, Vth is the equivalent voltage on the pin but this is not the direct burst mode threshold value. For the numbers you provided for the resistances, your current BMTH threshold is ~0.77V and BMTL threshold is ~0.44V (taking Vth and Rth and solving for BMTH using equations 74 and 75 in the datasheet. With BW option 6, BMTL is 0.6*BMTH).

    Let me share with you a link to the UCC25640x excel design calculator. It can do all the math for this and may be helpful here:https://www.ti.com/lit/zip/sluc675

     In order for the controller to drop into burst mode, the FBreplica signal needs to drop below the BMTL threshold. Normally what works for me is to load the converter to the point at which I want to have burst mode, look at the peak to peak voltage on the VCR pin, and use this value to set BMTL on the controller. This typically can get you pretty close and then you just need to dial in the BMTL threshold a little bit more to get it where you want it. The excel calculator should be able suggest a value for the LL/SS resistors once you have the BMTL voltage you want to target (since you are using BW option 6 do BMTH=BMTL/0.6 to get the BMTH level to put into the calculator).

    Hope this helps and please let me know if you run into any issues.

    Best Regards,

    Ben Lough