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Original question:

UCC256404: I have a problem with the UCC256404 starting up without a huge soft start cap

UCC256404: I have a problem with the UCC256404 starting up - question on BLK pin and soft-start

Eddie Aho
Prodigy 50 points
Part Number: UCC256404
Other Parts Discussed in Thread: PMP40580, , UCC256402

How do you decide how to setup the BLK pin regarding the voltage level to start?  I thought this should be set for somewhere up around 360 VDC, so the boost would be running.  But, I see on the PMP40580 that it is set to start closer to 84V.  It seems that when I set it to turn on early (lower voltage) that it starts up better.  Otherwise it can pull the bulk down and turn it off and it gets stuck here.  I tried adding more bulk or making the soft-start longer, but it seemed to struggle.  Making the softstart much longer seemed to help.  I saw a note somewhere that suggested placing a resistor at the LL/SS pin to the resistor divider was a method that could help add softstart time.  I don't remember where I found that note.  Can you tell me?

Thank you,

Eddie

  • +1
    TI__Genius 9790 points

    Hi, Eddie

    Normally, LLC can start up before PFC or after PFC, it is depended on system requirement.  I only see TV power need start up LLC before PFC for 85Vac to 264Vac, but the load is very light. UCC256404 is recommended here, since the BLK turn on threshold is only 1V,So there is no issue to trigger OCP protection, since LLC transformer was designed for input voltage>300V and full load. 

    For other applications, it need to start LLC after PFC or input voltage >300V, UCC256402/3 is recommended here because of BLK turn on threshold is 3V.

    Yunsheng