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UCC28710: UCC28710 high-voltage buck

Kevin Zhang
Expert 3720 points
Part Number: UCC28710
Other Parts Discussed in Thread: UCC28910

Hi all,

This is a high-voltage BUCK made with UCC28710. UCC28710 works in DCM mode. The input 30-150V, output 12V/3A.

1、I would like to know how this frequency is calculated. Is the calculation of the inductance value the same as the formula under CCM? If not, how to calculate it.

2、In DCM mode, the switching frequency will change with the load current. Does it have PSM mode at light load?

  • 0
    TI__Expert 3720 points

    Hi all,

    Can the Flyback formula in the data sheet be used to calculate the BUCK circuit? Thanks.

  • 0 in reply to Kevin Zhang
    TI__Mastermind 24365 points

    Hi Kevin,

    Thank you for the query on UCC28710.

    Yes, the formula can be usd for buck circuit also. But be sure to verify the chosen inductor is sufficirnt to meet the minimum on time requirement of the IC. 

    DCM CC_CV Buck calculations(3).pdf

    The above is design flow for UCC28910 but should apply for UCC28710 as well.

    Regards,

    Harish

  • 0 in reply to Harish Ramakrishnan5
    TI__Expert 3720 points

    Hi Harish,

    Thanks for your reply. I am a little confused about this CV control on below picture:

    1、The frequency will change with the output current. What is the relationship between them? Is it proportional?

    2、Will the duty cycle change with frequency during regulation?

    3、How to determine the frequency range based on the maximum output current of 3A？

    Many thanks.

  • 0 in reply to Kevin Zhang
    TI__Mastermind 24365 points

    Hi Kevin,

    Thank you for the reply.

    1. The control in CV mode is as per control law and the equation P = 0.5LI^2Fsw. Moving from left to right of the chart (which is analogous to increase in load), the control traverses frequency modulation and Amplitude modulation zone and then FM again to control output power.

    2. Duty cycle will change (on time/ peak current change) in the middle amplitude modulation zone where the Fsw is constant at 33khz. Above this peak current is kept constant and frequency is increased (upto allowable maximum as design permits). When it hits CC mode Tdmag as a fraction of Fsw is kept at 0.43 where Fsw starts decreasing.

    3. I think 36W is going to be challenging for buck design. But the idea is to start design for AM range where the Fsw is known and CS pin max voltage is chosen from which Ipp can be estimated. Then calculate the inductance from here. 

    From here verify the Ton and Toff by V = Ldi/dt to satisfy for minimum on time criteria and see if this can be optimized for higher switching frequecy iteratively.

    Regards,

    Harish