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WEBENCH® Tools/LM5175: Flyback diodes across input-lowside and output-highside MOSFETs

Todor Todorov2
Prodigy 40 points
Part Number: LM5175
Other Parts Discussed in Thread: LM5185

Tool/software: WEBENCH® Design Tools

Hello,

As can be seen in the attached report generated by WEBENCH for a LM5185 design, the generated schematic includes 2 flyback diodes across the input-lowside MOSFET and the output-highside MOSFET. The example schematics in the LM5175 datasheet and the LM5185 Quick Start tool do not include these diodes.

Are these diodes some kind of indicator that the 2 mentioned MOSFETs are required to have body diodes? Or are they actual discrete components? And if they should be actual components, what is the criteria for selecting them?

Thanks for your help!

Best regards,

Todor3051.WBDesign5.pdf

  • 0
    TI__Guru* 78270 points

    Hi Todar,

    Thank you for posting. The two diodes in the Webench scheamtic are discrete Schottky diodes.  It is a good practice that designer usually reserve a paralleling Schotty diode to the synchronous rectifier.  This may help avoid using the MOSFET body diode during dead time, because the MSOFET body diode can be slow and incur large ringing when switching.  However, these diodes are good to have but not always necessary.  I would recommend you to reserve the position on your prototype in case you need them after test, hence you can avoid creating another PCB.  If you find they are not needed, you can delete them from your final schematic. 

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Thank you for the clarification. A couple follow up questions:

    1) How do I size the diodes? In my design, the LM5175 Quick Start tool is giving me 22A peak inductor current (11V input, 20V/5A output) - do the diodes need to be able to handle same current or is the max current across them smaller? By how much?

    2) Are only the 2 diodes needed (low side on the input, high side on the output), or should I put diodes across all 4 MOSFETs?

    Best regards,

    Todor

  • 0 in reply to Todor Todorov2
    TI__Guru* 78270 points

    Hi Todor,

    1) Diode are usually selected  by the average current.  The 22A is the peak current, and these diodes will only conduct briefly (<200ns in most cases).  If you switched at 200kHz, for instance, the average current is less than 22A * 200ns * 200kHz = 0.88A.  I would choose a 3 to 5A diode for design margin.  You need to double check the datasheet such that the 22A is well below the specified non-repetitive pulse current. 

    2) you only need to add the diode to the sync FETs, i.e. the low side FET on the buck leg, and high side FET on the boost leg.  If you use parallel FETs, we still recommend to use one diode for the paralleled FETs, although you can add to of them.  We just don't think more diodes would help much because it may may your layout complex and it may reduce the effectiveness of the diode. Note that the diode should be placed close to the MOSFET.

    Thanks,

    Youhao  

  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Perfect, thank you so much for your help!