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LM5156H: DC/DC Converter flyback isolated input 24Vdc output +75V 240mA

THIERRY PREVOTEAU
Prodigy 70 points
Part Number: LM5156H
Other Parts Discussed in Thread: LMV431

Hi, I need a converter with an isolated output of 75V 240mA. For this, I used the TI power designer tool that suggested the LM5156H.

to accelerate my design I bought an LM5156H evaluation module and modified it with the specifications given by the tool.

The converter seems to be working but the voltage output is only 37V. I verified the transformer and after redoing it several times to be sure the result is always the same.

Could you help me please ?

Regards

Thierry

  • 0
    TI__Genius 11215 points

    Hello Thierry, 

    Thanks for reaching out. Do you have a schematic or a list of all the modifications you made on the EVM? Did you also check the turns ratio of the transformer, the sense resistor and the compensation needed? As first thing, I would use the Excel Quickstart calculator to check if all the parameters have been modified properly (https://www.ti.com/lit/zip/snvc240 ). Let me know if I can help you furthermore. 

    Kind regards,

    EM 

  • 0 in reply to EM
    Prodigy 70 points

    Hello EM,

    Thanks to help me.

    Here is the modification of the BOM on the EVM according to webench power design:

    • M1 = CSD19538
    • R16 = 7.87K
    • D5 = REMOVE
    • R20; R21 = 105K
    • C24; C26 = 6.8nF
    • R12 = 0.025R
    • R14  = 28.7K
    • R13 = 15.4K
    • C20 = 56µF
    • R22 = 1.02K
    • D4 = BAT54
    • D1 = CSD01060E
    • T1 Lp = 40µH transfert ratio Np:Naux:Ns = 1:0.7:5

    Yes the transformer has been checked.

    Kind Regards

    Thierry

  • 0 in reply to THIERRY PREVOTEAU
    TI__Genius 11215 points

    Hello Thierry, 

    Thanks for your feedback. Can you please also share the specs of this design as well (Vin, Vout, desired switching frequency)?
    In addition, I already have some comments to share with you: 

    • The secondary side soft start diodes (D4) must be rated for the output voltage desired 
    • Your feedback divider is not set correctly: in an isolated flyback, you need to calculate the FB divider with respect to the reference. In this case the LMV431 is a 1.24 V reference, in your case with R22= 1.02k you get as output voltage Vout = 1.24V / 1.02 * (1.02 + 30) = 37.7 V . You should change the FB divider in order to get your desired output (for example you might choose 100k as top resistor and 1.68k as bottom one). Please keep in mind that all the times you change the FB divider, you should also check the compensation components.
    • R13 is the resistor of the secondary side soft start, it is ok to leave it at 49.9k 
    • R14 is your optocoupler resistor, please check its value 
    • I see that you changed the compensation components (R20, C26, C24 and R21). Please check the stability of the design with the Excel Quickstart calculator

    I would recommend to use the Excel file to check all the parameters (especially the Bode Plot, https://www.ti.com/lit/zip/snvc240 ). This app note might be helpful as well in the design of an isolated flyback and can be used for the LM5156H too: https://www.ti.com/lit/an/snva866/snva866.pdf 

    Let me know if further support is needed. 

    Kind regards, 

    EM

  • 0 in reply to EM
    Prodigy 70 points

    Hello EM,

    Many thanks for your help !!!

    Now all works fine.

    I made a mistake due to the reference voltage that I thought at 2,5 Volts instead of 1,24 Volts

    Thanks again

    Kind regards