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LM5121

Ron Nash
Prodigy 20 points
Other Parts Discussed in Thread: LM5121, LM5122

We are using the LM5121 for the first time .  Input 24 V , output 90V , 3.5 A  . we are not using the synchronous rect. gate drive port and simply using a diode .    We are experiencing catastrophic failures of the disconnect switch FET as well as failure of the LM 5121 .   Schematically similar to datasheet circuit .

  It appears while the output is rising to regulation the disconnect switch begins to oscillate at a kHz rate , though the value of Cres is 1uf. .   When the oscillation starts ,the output falls near zero yet the current is very high so presumably the current path is through the boost FET which is gated on continuously ( possibly due to failure of the IC  ) .  The boost FET does not fail .

Does any of this sound familiar to anyone .

    

  • 0
    TI__Mastermind 46061 points

    Hi 

    Populating a capacitor between DG and DS may help to minimize the ringing. Please populate about 10 times bigger value than Cgs of the dis-connection MOSFET. Also, please double check SOA of your disconnection switch.

    By the way, I doubt LM5121 can handle 315W. Typical power rage is around 100W. 

    Regards

  • 0 in reply to EricLee
    Prodigy 20 points

    Eric ,   Was wondering what parameters the 100w power range suggestion has to do with since this is only a control IC . ?

    also,  if the problem is with the disconnect switch in general , at this power level ,  is there a way to terminate the DS and DG pins so we can disable and remove the Disconnect switch  from the circuit ? 

    Thx

  • 0 in reply to Ron Nash
    TI__Mastermind 46061 points

    Hi 

    You are right. LM5121 has no strict limit since it is a controller. I was talking about a typical application range. 

    If you don't want to use disconnection switch control, please refer LM5122.

    Regards,