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UCC24612: SR switches ON and off almost instantaneously!

Bhuwenesh Gupta
Prodigy 10 points
Part Number: UCC24612

Tool/software:

Hi,

Based on the evaluation board schematic and layout we have designed the schematic and layout for our custom board. The mosfet we are using is BSC061N08NS5ATMA1. What we are experiencing is that the SR starts and very soon switches off. After reading the forum we noticed that unless the load is added (to make drop of Id X Rdson of 10mV) we added 2A of load (BSC061N08NS5ATMA1 has Rdson of 6 mOhm) but it still gives us the same behavior.

Can someone help?

  • 0
    Prodigy 10 points

    I have replaced BSC061N08NS5ATMA1 (6mOhm) device with BSC117N08NS5 (11.7mOhm) device. But still no difference in circuit behavior!

  • 0 in reply to Bhuwenesh Gupta
    TI__Guru* 85506 points

    Hello,

    I am reviewing your inquiry and will get back to you shortly.

    Regards,

  • 0 in reply to Mike O'
    TI__Guru* 85506 points

    Hello,

    I reviewed your layout and you have long trace to the VD pin of the UCC24612 and the FET's drain.  There are also long traces between the snubber R49 and C24.  Traces add roughly 10 nH of inductance per inch.  There could be ringing causing the issues across the Vd and Vs pin of the UCC24612.

    The first thing you might want to do is study VD, VS, VDD with an oscilloscope to make sure:

    1. VDD is properly biased.

    2. There is not excessive ringing across the VD and VS pins.

    If there is excessive ringing across the VD and VS pin you can use R49 and C24 to snub it.  There is information on how to setup the snubber in the UCC24612 data sheet.

    https://www.ti.com/lit/gpn/ucc24612

    If snubbing does not work you may have to modify the layout.  The data sheet and EVM give layout guidelines and examples that you can use to improve the layout in your design.  The following link will get you to the EVM's user guide that contains information regarding the EVMs layout.

    https://www.ti.com/lit/pdf/sluubp3

    Regards,

  • 0 in reply to Mike O'
    Prodigy 10 points

    Hi,

    This is VDD (Yellow) and VD (Purple) at very start. So it does look like there is ringing here.

    This is VDD (Yellow) and VS (Purple) at very start. There is ringing here too. 

    But the ringing frequency seems too low from above pics. Based on above pics ring freq is around 1kHz!

    What am I missing?

    Regards,

  • 0 in reply to Bhuwenesh Gupta
    Prodigy 10 points

    I have one more query: What exactly does min of 10mV drop of VDS mean which is required for UCC24612 operation? Since we are using 11mOhm mosfet, does that mean that the UC24612 stops working if output current Id is less than 1A? What happens at no load at output situation?

  • 0 in reply to Bhuwenesh Gupta
    TI__Guru* 85506 points

    Hello,

    If the Idson*rdson < 10 mV the gate drive output will go low.  So at 11 mV the gate drive could turnoff.

    Under no load condition the UCC24612 should be in a low power mode and not produce gate drive pulses.  It will depend on the FET body diode for regulation.

    Regards