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Original question:

LM74900-Q1: Secondary/Aux LM74900-Q getting hot when using Qty 2 LM74900-Q1 to create a redundant 48V PSU

LM74900-Q1: Secondary/Aux LM74900-Q getting hot when using Qty 2 LM74900-Q1 to create a redundant 48V PSU

Andrew Correnti
Prodigy 20 points
Part Number: LM74900-Q1

Tool/software:

I finally have more boards to check and now I am not getting anything on the Secondary except for the fault light.

I am getting 60 volts on VCAP on the Secondary, and I am not sure where that is coming from. 

Any ideas would be greatly apricated

  • 0
    TI__Mastermind 49375 points

    Hi Andrew,

    UVLO is not supposed to be left floating. Please pull it high and FLT light should not glow.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 20 points

    Hello Shiven thank you for your help,

    to make the system work I ended up changing a few parts:

    I populated R25 and R27 and removed R28.

    I would like to understand how those changes will affect the overall current protection of the circuit and anything else I should be aware of.

    If it would be easier to talk I am open to a phone call

    Thank You

  • 0 in reply to Andrew Correnti
    TI__Mastermind 49375 points

    Hi Andrew,

    HGATE has a control signal that depends on UVLO and OV

    R25 and R27 decided the UVLO threshold for you and with R28, OV was permanent high. This will not let the HGATE turn-on.

    Since now OV is permanent ground and UVLO threshold is set to some value. FLT/ will not be triggered permanently. FLT/ depends on the following faults.

    These resistors do not influence over current protection settings.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 20 points

    So do you recommend to reinstall R28 to keep the Secondary off until the primary is lost?  

    Do you recommend to keep R25 and R27 installed or remove them and tie UVLO to the AUX input power directly?

    For my understanding:

    What is the purpose of adding the transistor to the output of HGate on the Secondary?

    Thank you

  • 0 in reply to Andrew Correnti
    TI__Mastermind 49375 points

    Hi Andreas,

    The decision of your resistor selection depends on your system requirements. From device point of view, they just have a comparator internally to decide if HGATE should be ON or OFF

    Purpose of adding transistor to outout of HGATE of secondary is to have very high speed turn-on.

    Usually the HGATE is driven from 55uA source. But with transistor, we drive it from CAP pin and can get higher strength.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 20 points

    Hello Shiven,

    I am still having issues with this circuit.  I have installed R28 and removed R27 and R25.  I then tied UVLO to +48V_CONN_2.

    So I now have:

    SW = floating

    UVLO = +48V_CONN_2

    OV = +48V_CONN_1

    When OV is at +48vDC Hgate is at +48volts and when OV is 0vDC Hgate is 0vDC

    Do you have any recommends on what I can try or what I am doing wrong?

    Thanks

    Andrew

  • 0 in reply to Andrew Correnti
    TI__Mastermind 49375 points

    Hi Andrew,

    OV is an input pin which decides the functioning of HGATE.

    If OV is high ( > 0.6V), HGATE will be pulled to OUT pin.

    If OV is low (<0.6V), HGATE will be driven at OUT + 12V.

    Please ground the OV pin for now by removing R28 and verify if the circuit works.

    Regards,

    Shiven Dhir