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UCC28700: UCC28700

Part Number: UCC28700

hello 

In the attached datasheet of 25W smps design using UCC28700 UCC28700 smps design.pdf

In the page number 10 , at the top of the page you mentioned -

" Equation 6 determines if TON(min) exceeds the minimum TON target of 300 ns. Equation 7 verifies that
TDMAG(min) exceeds the minimum TDMAG target of 1.1 μs."

1) how you calculated TON target of 300ns and TDMAG(min) target of 1.1us ?

I did not understand that . explain me ?

Thanks and regards

Rajashekar

  • Hello Rajashekar,

    The minimum Ton time is the leading edge blanking time on the current sense pin and the min discharge time is the minimum value of the secondary rectifier conduction time.

    Regards,
    Sonal
  • Hello Sonal

    Thanks for the reply.

    1) But How do you calculated those values ? i didnot understand.

    2) Those values are fixed for any small power smps design?

    Thanks and regards 

    Rajashekar

  • Hello Rajashekar,

    The leading edge blanking time is set internally to the silicon to avoid sensitivity to the MOSFET turn-ON current spike, this time is determined by looking at the turn-ON delay, the time required for the Coss to discharge and the duration of that current spike. This is an IC parameter and will be the same irrespective of the application.

    Regards,
    Sonal
  • hello

    In the design datasheet of 25W smps using UCC28700, there you mentioned in the pg no :12 , you have directly taken the turns ratio which are obtained from the part number. 

    1) Will you please elaborate how to calculate turns ratio and number of turns on each output ?

    Iam attaching that datasheet8468.UCC28700 smps design.pdf

    and also mention what is the auxiliary supply in that design ?

    2) datasheet attached above ,In the page no:13 , 4.2.7 capacitance on vdd pin , "With a startup resistance (RSTR) of 3 MΩ and a desired startup time (dtCVDD) of 5 s, calculate the CVDD using

    Equation 19." That startup resistance and startup time is same irrespective of any design ? 

    3) How you have calculated NAS auxiliary to secondary turns ratio (i.e, 1 ) and which secondary you have considered for calculating the same ?

    Thanks and regards

    Rajashekar

  • Rajashekar

    The turns ratio is dependent on what the desired output voltage is. See equation 22 on page 20 of the datasheet www.ti.com/.../ucc28700.pdf for recommended value. We also have an excel design calcuator www.ti.com/.../sluc408 that you can use to quickly determine optimal turns ratio with different circuit parameters.

    For the TIDA-00706 test report that you attached, auxiliary supply in the title refers to the target application it is designed for. This reference is designed to be an auxiliary power supply to provide bias power for higher power server, telecom and industrial power supplies.

    The startup time for TIDA-00706 is only applicable if the minimum input voltage is above 120V (as specified in the system specifications). If any of the operating parameters change you will need to modify these values to meet whatever startup requirement you may have.

    For multiple output flyback it is recommended to calculate the turns ratio for the desired output voltage. The following training video goes into more detail on how to design multiple output flyback training.ti.com/multiple-output-flybacks-how-improve-cross-regulation

    Best Regards,
    Eric