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LM5117 design from WebBench does not work as expected. Any suggestions?

David Kepley
Prodigy 10 points
Other Parts Discussed in Thread: LM5117

We have implemented a test circuit with the LM5117 for the following Design Inputs:
#    Name       Value            Description
1.    Iout           3.0 A             Maximum Output Current
2.    Iout1         3.0 Amps    Output Current #1
3.    VinMax      55.4 V          Maximum input voltage
4.    VinMin      18.0 V          Minimum input voltage
5.    Vout           12.0 V         Output Voltage
6.    Vout1         12.0 Volt     Output Voltage #1
7.    base_pn   LM5117      National Based Product Number
8.    source      DC               Input Source Type
9.    Ta             30.0 degC   Ambient temperature

 We modified the WebBench circuit to add a Low Pass filter to the LM5117 Vin pin (consisting of a 4.99 Ohm series and a 1uF Ceramic to ground) and an additional power input bulk Capacitor (68uF 100V).

We started testing the circuit and the regulation looked good at lower Vin (<26V), but as we increased the Voltage Input, the circuit fails.  The high side MOSFET, LM5117, and Cout has failed on several boards (or some combination of the three parts).

We did add (5 Ohm) series resistors to the gate drives HO and LO and it seems slightly better, but still fails.

Any help or suggestions?

Thanks,

Dave
  • 0
    TI__Guru 53011 points

    Hi

    Since you mentioned that it is slightly better by increasing gate resistors. By adding strong snubber, you may be able to see an improvement.

    HAve you check the voltage spike at switching nodes ?

    Is there no failure if you remove the low pass filter ?

    Regards,

    Eric

  • 0 in reply to EricLee
    Prodigy 120 points

    Eric,

    I believe the one of the diodes specified by Webench for this design (Toshiba CMS06  VRRM= 30.0 V)  does not have sufficient VRRM. The VRRM should be around 60V. I think there is abug in Webench regarding the LM5117 design.

    Kevin Jackson

  • 0 in reply to Kevin Jackson
    TI__Intellectual 2930 points

    Kevin,

    You're right this is a bug in the WEBENCH Design. I will makes sure this is fixed immediately.

    Thank you for pointing it out.

    Best regards
    Tommy 

    Also Here are some diodes that should be able to replace the Toshiba part:

    BAT41ZFILM      
    SS2PH10-M3      
    MBRS3100T3G      
    10MQ100NTRPBF      
    B1100LB-13-F      
    MBRS1100T3G      
    BYS11-90-E3/TR      
    BYS12-90-E3/TR      
    DFLS1100-7      
    50WQ10FNPBF 
  • 0 in reply to Tommy Jewell - WEBENCH Design Center
    Prodigy 120 points

    Tommy,

    Thanks for the quick reply.

    Kevin