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switching frequency for UCC25600 LLC controller

sm3447
Intellectual 860 points
Other Parts Discussed in Thread: UCC25600

Hi,

I understand in the market, LLC design is getting popular and most IC vendors make the controller available for this topology. It seems the reference designs from them are all under about 140k Hz. What is the issue if it moves to above 200kHz or more? 

Of course, the higher  switching frequency, lower the effeciency due to switching loss. Beside this, is any other hidden issue?

Thanks,

M

 

 

  • 0
    TI__Mastermind 25130 points

    There are no issues that I know of. People use them at several hundred KHz.

  • 0 in reply to John Bottrill
    TI__Guru 73595 points

    UCC25600 switching frequency maximum is 350kHz (can be as low as 300kHz) when its burst operation starts. Usually a design will be made with full load switching frequecy around 100kHz to allow frequency variable range enough to adpat high switching frequency need at light load without entering burst opertion too early. If a design with full load switching frequency is made at 200kHz, there will not be much room to make frequency variation to make regulation at light load and the operation will be in burst too early.

  • 0 in reply to John Bottrill
    Prodigy 30 points

    Dear , John

    I have also felt that if we go to higher frequencies in an LLC  converter the mosfets do not heat up , it is the transformer  which starts becoming lossy .

    Even with litz wire,the windings adjacent to the air gap start heating.

    If we avoid copper near the gaps then the problem is somewhat reduced but then  winding space is not optimized. 

    Maybe since  we are using the transformer for dual  purposes, that is for lumped leakage and for isolation ,that

    the transformer  in the llc converter is what becomes a limiting factor  for higher switching frequency operation.

    please guide  if I am going wrong in my transformer design. I have been able to  derive 500 watts using a etd49 core  with a sectional bobbin at 100 khz. with natural cooling Beyond that  core STARTS HEATING. ( I am using n87 grade ferrite from epcos  eq to 3f3 )

    RGDS,

    FEESAH

  • 0 in reply to feesah haseef
    TI__Guru 73595 points

    You may want to reveiw your transformer design to see if your transformer design covers the worst operation conditions of the load and the line. If the transformer gets hot, it means its losses are high. So you may need to check and determine each factor of the losses, such as core, copper, etc then adjust your design accordingly.  If you suspect the frequency is a factor, you may want to check the core material to see if ok to use in that frequency range.

  • 0 in reply to feesah haseef
    Expert 1920 points

    I made a LLC with 97% efficiency at 500W output in a ETD39 core - the core got to 80C (too hot in most designs), but it worked.

    A little bit design changes could reduce my loss to 50% in the core. Make sure your dead time isn't too long.

    Calculate your needed area product: AP_need = L * Ipeak * Irms / (0.4*2.5*10^6*Bmax). Use N97 or 3C95 at 100kHz.