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UCC28740: CHANGE SWITCHING FREQUENCY

rajat devagiri
Prodigy 25 points
Part Number: UCC28740
Other Parts Discussed in Thread: LM5021

Hey TI,

I was building a offline LED driver with input : universal (85Vac - 264 Vac) , Output : 20V @ 1.2A , with a DCM flyback 

Wanted to use the UCC28740 as the control IC. The main aim was to maximize the efficiency for my design, for which I wanted to operate the IC at a Fsw lower than 100Khz - went thru the datasheet thoroughly , but it just mentions to operate with required fsw at maximum power.

My question is, is there a way to set the set the switching frequency externally thru some resistor setting?

Any reference or link would really be helpful,

Thank you!! 

Rajat 

  • +1
    TI__Mastermind 24375 points

    Hi Rajat,

    Thank you for the query.

    In short you cannot set frequency with resistor. These controllers are not fixed frequency PWM controllers like LM5021.The control will be based on control law described in page 16 of the datasheet.

    You design the converter so that it delivers maximum power and the operating point lies in the second FM region and it varies as the load changes. Only at this point in CV mode you will be operating at maximum switching frequency. Increasing the load resistance (decreasing load current) will lower Fsw according to control law. Increasing load i.e pushing to cc region also reduces Fsw.

    I request you to start by understanding the UCC28740 EVM in the following link:

    https://www.ti.com/tool/UCC28740EVM-525

    The operation is clealy elaborated here. Also please correlate the design parameters with the excel design calculator as it will give an idea of how the schematic components are chosen to start with.

    Please let us know if you have any questions.

    Regards,

    Harish

  • 0
    Prodigy 25 points

    Got it!! Thanks for the help Harish!! I found this link while I was browsing thru TI's website - https://training.ti.com/getting-started-ucc287xx-and-ucc289xx-family 

    Do you think you could help me with the access, the training would really be helpful to get my converter work ( PS: I am kind of off new to PE)

    Help really appreciated!!

  • +1 in reply to rajat devagiri
    TI__Mastermind 24375 points

    Sure, We can help with that. I will communicate with you. I will close this thread for now.

    Regards,

    Harish