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LM5050-1: How to buffer LM5050 GATE pin to driver FET gate which has large Qg.

Kazuya Nakai59
Guru 20600 points
Part Number: LM5050-1
Other Parts Discussed in Thread: LM74700-Q1, LM7480-Q1

Hello guys,

One of my customers is considering using LM5050-1 for their next products.

Also they are considering using 4pcs of FET(ISC027N10NM6) with parallel connection as FETs connected to GATE pin of  LM5050-1.

ISC027N10NM6
https://www.infineon.com/dgdl/Infineon-ISC027N10NM6-DataSheet-v02_02-EN.pdf?fileId=8ac78c8c7ba0a117017babb690f45f94

I think that LM5050-1 can't drive 4pcs of ISC027N10NM6 by GATE pin directly and some buffer circuit will be needed to drive the FETs.

Could you please tell me any reference circuit to drive the FETs by GATE pin?

Your reply would be much appreciated.

Best regards,
Kazuya.

  • 0
    TI__Guru 57737 points

    Hi Kazuya,

    LM5050-1 can drive multiple FETs in parallel without any additional external circuit. The gate drive strength of LM5050-1 is only 32uA which is little lower so the FETs will turn ON slowly considering the increase in the total gate capacitance the controller has to drive (due to more FETs in parallel).

  • 0 in reply to Praveen GD
    Guru 20600 points

    Hi Praveen,

    Thank you very much for your reply.

    The FET Qg is 58nC per device. So total of 4 FET is 224nC.
     In this case, transient time of GATE pin from 4.5V to 0V would be calculated with the following formula.

    T=Q/I=224nC/32uA=7ms.

    I think that this means reverse current would be flowed during 7ms.

    I think this is long time. Also the FET would be heated.

    Is there any calculation way for the maximum time of reverse current?

    Thank you again and best regards,
    Kazuya.

  • 0 in reply to Kazuya Nakai59
    TI__Guru 57737 points

    Hi Kazuya,

    The turn ON duration of the FET can be calculated using

    IGATE(ON) = Ciss x dv/dt,

    where,

    • dv= FET Gate-Source voltage
    • dt = turn ON time of the FET. 

    Considering a 50nF of total FETs Ciss and 5V of Vgs across FET gate-source,

    ⇒ Ton = 50nF x 5V / 32uA = 7.8msec

    During turn ON of the FET, the body diode will be in conduction for forward current (current flowing from Vin to Vout). There will be no reverse current flow from Vout to Vin.

    The turn OFF time to block reverse current can be calculated using the same formula,

    IGATE(OFF) = Ciss x dv/dt

    ⇒ Toff = 50nF x 5V / 2.8A = 89nsec

  • 0 in reply to Praveen GD
    Guru 20600 points

    Praveen,

    Thank you very much for your reply and I'm sorry to be late my response.

    I told your reply to the customer. They asked me the following additional question. 
    Could you please give me your reply?

    Q. Can LM5050 be used as parallel because we want to decrease the turn on/off time?
         In other words, is it no problem to use 2 sets of "LM5050+2 pcs of ISC027N10NM6" with parallel connection?

    Thank you again and best regards,
    Kazuya.  

  • 0 in reply to Kazuya Nakai59
    TI__Guru 57737 points

    Hi Kazuya,

    It is not recommended to use 2x LM5050 devices in parallel for faster turn ON of the FETs.

    You can consider using other Ideal Diode controllers like LM74700-Q1, LM7480-Q1 and LM7472x-Q1 for faster gate turn ON.