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LM74701-Q1: High voltage 54V failed

Part Number: LM74701-Q1


I've tested the LM74701-Q1 with 2 power supplies of 54V and 2 N-Mosfet (IAUZ40N06S5N050ATMA1) with Vds:60V, Id: 40A, Rds On: 5mOhm, Vgs: -20V;+20V, Qg: 23.5 nC

I've connected pin EN to the ANODE and left SW pin floating. When i plugged both of my PSU, i think one of the mosfet (or both) fried because now the Drain and Source of the mosfet are shorted.

I think it's because i'm using a voltage above the Vclamp (43V max). But in the datasheet it allowed 3.2-V to 65-V input range into the Anode. 

My Objective is to use ORing solution instead of Schottky diode to block reverse current and have low forward voltage.

Can you confirm that my problem comes from my voltage being higher then Vclamp ?

Thank you in advance !



  • Hi Noe,

    I suspect the FETs have damaged due to voltage ringing during hotplug being greater than the VDS rating of the FET. During hotplug of PSUs, please capture VIN1, VIN2 and VOUT signals. If our suspicion is correct, then the FETs would not fail at lower voltages like say 45V or 40V, right?

  • Hello Praveen, 

    I'm waiting for some components to arrive and I will run the test again. 

    I will try with a voltage of 25V. I also change my FET to Vds 100V instead of 60V and with the same Rds On.

    I'll keep you update if your suspicion was right.

  • Hi Neo,

    Yes, Please keep me updated on the test results. 

  • Hello Praveen, 

    So I tested with 2 laboratory DC power supply at 25V and it worked perfectly. I had a 22mV drop between Vin1 and VOUT and 17mV between Vin2 and VOUT. 

    After that i tested with 2 EPP-500-54 at 54V, i didnt measure any peak above the normal operating voltage during hot plug. The mosfets are fine but the problem is, I measure 80mV between Vin1 and VOUT and i measure 0mv between Vin2 and Vout. 

    the module 1 should be in Full conduction mode but V(Gate to Anode) is different from V(Vcap to Anode).

    the module 2 should be in Regulated conduction mode but Anode-Gate is 0mV.

    It seems to me that its only the VIN2 who conduct and will supply the current if i put a load.

    What do you think ? 

    Thank you in advance. 

    Edit: I tested the mosfet of Vin2, it is shorted, it makes sense to me since i plugged VIN1 first, the mosfet must have fried at this moment.

  • HI Noe,

    While testing with 54V input voltage have you measured the GATE-ANODE voltage of LM74701-Q1 on module-1 and 2 ? We can comment on the mode of operation by measuring the gate voltage of the controller. 

  • Hello Praveen, I finally convince my coworker that the LM5050 who the chip we needed. I tested the voltage between GATE-ANODE with my DC laboratory up to 30V and it was in accord with the datasheet 

    But when i tried with 54V, only one mosfet was working, i got lucky the second ended up in open circuit and not shorted. 
    Anyway, I didn't resolve my problem with the LM74701 but I'm going to move on with the LM5050. 

    Thank you very much for your help ! Have a good day !