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LMG3425R030: Aux power supply current will increase with larger bypass capacitor

Hao Wen
Prodigy 200 points
Part Number: LMG3425R030
Other Parts Discussed in Thread: LMG342X-BB-EVM

Hello,

I am using LMG3425R030 in a full bridge configuration. I have a bypass cap closed to the half bridge which is 136 nF and I have another larger bypass cap relatively far from the half bridge. When I increase the larger bypass cap to be higher than 1 uF, the 12 V aux power current will increase by 0.1 A. And after I turn on the main power, the aux power supply current will drop back to normal value. With higher switching frequency, the aux power supply current will increase more. However, if there is only the 136 nF close bypass cap, the aux power supply current will always be the normal value. I am wondering what could be the reason for this. Thank you!

  • 0
    TI__Genius 16390 points

    Hi Hao,

    Could you share schematics and layout? Is the bypass cap you are referring to the local decoupling capacitors for the high frequency power loop?

    Regards,

  • 0 in reply to Yichi Zhang.
    Prodigy 200 points

    Hello Yichi,

    Thanks for your reply. I also tried your evaluation board LMG342X-BB-EVM motherboard with LMG3425EVM-04X. When I increase C15 from 0.022 uF to around 6 uF on the daughter board, it has the same issue. Thank you!

  • 0 in reply to Hao Wen
    TI__Genius 16390 points

    Hi Hao,

    Our guess is when using LMG3425 with the ideal diode mode, When you are switching the half-bridge with no Vbus, after the FETs shut off (with no voltage  being driven across them) they have a tendency to detect Vds<0 and then turn themselves back on due to ideal diode mode. 

    They then shut off because the current is too low. The net effect is the devices switch twice for every time they are commanded to switch. This causes the increase in the aux supply current that he sees. When you apply bus voltage, then the Vds does not go to <0 after FET shut-off, so the double-switching stops and the current returns to normal.

    We think the bus cap is somehow changing the details of which fets see the negative voltage when so is affecting when they double-switch. We are not quite sure about the details.

    Hope this helps.

    Regards,