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UCC25660: Frequency Limits

William Mcknight
Intellectual 350 points
Part Number: UCC25660

Hello,

We are currently implementing a split cap half bridge LLC, 480W, 48Vout, 390Vin (20 to 25 Vpk-pk ripple). Output will have SR's. Natural convection (no fan).

The output can go from 42V to 60V, which presents a challenge as we will be in burst mode full power for some of the Vout range. We have been testing with ONSEMI's NCP13994 eval board, and when in burst mode there is no way to control the upper switching frequency limit. The upper limit is based on the tank operating point and tank values, so I am seeing based on there design the frequency max limit can get 500kHz or higher. At 100W, 25C ambient, weak fan blowing, the SR's are approaching 75 or 80 degrees. For us, at 480W there is no way the SR's will survive given the Fsw_max we would actually hit, which would be the IC's limit of 750kHz.

We need a solution where we can accurately control the switching frequency limit, especially the max frequency, like you could in a digital design when in burst mode operating at full power. Can the UCC25660 accurately control the max switching frequency (mosfet Fsw - not burst mode frequency) when in burst mode?

I see there is the TSET pin limits the Minimum operating frequency limit for the LLC, as well as the maximum dead-time (so a theoretical max frequency).

Can you please explain if or how this IC sets a minimum frequency limit and maximum frequency limit?  

Thanks

  • 0
    TI__Guru 71605 points

    Hello,

    Your inquiry has been received and is under evaluation.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru 71605 points

    Hello,

    The minimum frequency is limited by the TSET setting.  It is done digitally inside the controller.  The maximum frequency does not have a programable limit.  However, it is limited to 750 kHz by design.

    Regards,

  • 0 in reply to Mike O'
    Intellectual 350 points

    Thanks.

    Please confirm my following statement, but the TSET pin does not actually set the lower frequency LIMIT of the LLC, only the working lower frequency limit for IPPC to control the LLC, otherwise regular current mode control kicks in and still regulates the topology.

    Without upper and lower limit frequency control, like the MPS1002 has, It seems that most analog LLC IC's are supposed to be used with very tight/small vout range, otherwise the loss in the SR's given the condition I described above makes the topology not usable.

  • 0 in reply to William Mcknight
    TI__Guru 71605 points

    Hello,

    The UCC25660 the device has ZCS switching protection on the primary that prevents the converter from operating in the capacitor region. 

    When it comes to using LLC for the highest efficiency possible you are best having a real limited input range.  When the design is at resonance you have ZVS on the primary and ZCS on the secondary.  If you using SRs this means you have no switching losses on the secondary.  Once the design varies from resonance the secondary is no longer ZCS.  The LLC is very popular topology following a PFC pre-regulator for this reason.

    When it comes to controlling the minimum maximum frequency you would be better off using digital controller such as the UCC3138 or C2000.

    Regards,