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Original question:

LM5156: Question on flyback quick start calculator

LM51581: Pri Inductance of Transformer calculation shows CCM but the value chosen is for DCM(snva994a)

Naveen Gupta
Prodigy 130 points
Part Number: LM51581
Other Parts Discussed in Thread: LM5157

Tool/software:

Hello,
in snva994a application note page 4, the transformer's PRI inductance is calculated as 13 uH, and it says for normal operation they are going to use CCM, but when selecting a value for CCM operation, they have to choose 8 uH. Why is that? Won't that push the converter in DCM?? 

Also on the same page, it says this - ILPEAK is used to properly size the current sense resistor. 
Where is the circuit current sense resistor used? Am I missing something? I thought it was internal?

Thanks and Regards,
Naveen

  • 0
    TI__Expert 4090 points

    Hi Naveen,

    Thanks for using the E2E forum.

    The inductor value used is the derated inductor value. At peak inductor current the value of the inductance is reduced; therefore, 8uH is used. This information is given in the inductor datasheet.  

    Regarding the current sense resistor, the Lm5157/8 devices do not need external current sense resistors. The App-note is discussing the general relation between the selection of current sense resistor and the peak inductor current based on derated inductor value.

    Best Regards,

    Hassan 

  • 0 in reply to Hassan Jamal
    Prodigy 130 points

    Hello Hassan,
    Thanks for your insight.
    Is it possible to share the datasheet of the selected transformer? I tried searching for it, and it seems this is custom-made.
    I just want to understand it better so what you mentioned makes sense that the chosen value is much higher (higher than 13uH) and when at high voltage input the system will be in DCM at that time and we can observe the indcutance value falling to 8 uH? Please correct me if I am thinking wrong.

    Thank you again.

    Regards,
    Naveen

  • 0 in reply to Naveen Gupta
    TI__Mastermind 30390 points

    Hi Naveen,

    I checked in the web and internal files, but I did not find the specs for the transformer either.
    Here is another thread which ask the exact same questions, but was not able to get all files from coilcraft either.
    https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1427189/lm5157evm-bst-transformer-specifications-coilcraft-zb1324-al

    Instead of comparing inductor specs, I would recommend to look into the general influence of inductance for the flyback switching behavior.
    Have you already used our power stage designer tool?
    https://www.ti.com/tool/de-de/POWERSTAGE-DESIGNER
    You can select the flyback topology, enter all parameters and double clean on each component in the schematic to see voltage and current flows depending on Load and VIN. You will also see if the device is operating in CCM or DCM, so I think it is a very helpful tool.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 130 points

    Hi Niklas,
    Thanks for suggesting this tool; I will check it out.
    It seems it will help me understand this better.

    Thanks and Regards,
    Naveen