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UCC28700-Q1: Not getting max load (frequency limitation)

rob van hoeij
Prodigy 40 points
Part Number: UCC28700-Q1
Other Parts Discussed in Thread: UCC28700

Hi,

I use the UCC28700 for a flyback 12V3A and the WE 7508112341 trafo. 

i cant reacht the max output current, after 1.6A the voltage drops slowly to 0V, it seems like a current limitation, the frequency is not increasing enough (55kHz).

Do you have any idea why it stops at 55kHz?

Rcs = 0.43R (Rcl = 1k, 20pF)

Rs1 = 121k + 5.62K

Rs2 = 60.5k + 5pF

  • +1
    TI__Guru 70515 points

    Hello,

    Your waveform looks like your design is operating deep into DCM and may be running at lighter loads.  If you are operating at 55 kHz at this load to get the design to work at higher frequency you may just need to increase the load.  

    The waveform you attached looks like the switch node and it looks quite noisy.  This noise will couple to the aux winding and will most likely trigger an OVP.

    I have attached an application note discusses primary fault sensing common issues and how to resolve them.  The UCC28700 has PSR fault sensing and you should be able to resolve your issues with it.

    5700.SLUAAC5 8 31 21.pdf

    Regards,

  • +1 in reply to Mike O'
    Prodigy 40 points

    Thanks Mike, i will check this!