CSD97394Q4M: Buck NexFET Power stage fails over 28°C

Part Number: CSD97394Q4M

I am using the CSD97394Q4M in an application to drive dc cooling fans according to the temperature on the inside of a large chassis. These parts worked famously on bench trials with a varying PWM signal and Vin voltages up to 25 volts, so I desgned it into a backplane to control the speed of the fans. The trouble came when I attempted to run the fully loaded backplane in the environmental oven up to 30°C. My log showed that the CSD97394Q4M failed at 28°. Looking for answers I found on the Absolute Maximum Ratings that showed Vsw to PGND to be 30V max. With 24V on Vin, there is some overshoot to above 30V for maybe 2nS. Next I tried lowering Vin. Even with lower Vin I still get failures when the temperature is elevated.  What do you think the problem is and what can I do about this?

  • In reply to Brett Barr1:

    Hi Brett,

    The attached png scope_5.png shows what the waveforms looked like just as it was failing. The yellow trace is the PWM input, and the green trace is Vsw.  Nothing out of the ordinary. "scope_4.png" was captured as an irregular waveform (took many captures to finally get this, and this is with a good scope ground). The strange thing is that the Vsw goes high before the clock pulse.

    Also attached is the data log showing the exponential temperature rise when the failure was occurring.

    Thank you for looking into this.

    Paul

  • In reply to Paul Norton:

    It seems with this ap, I can only send one attachment, then it won't let me send any more. My email is paul.norton@checksum.com , if you would like to communicate that way.
    Paul
  • In reply to Paul Norton:

    Hmm that is strange. Can you send png.4 and png.5 to me directly at my address b-barr@ti.com?

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

    b-barr@ti.com

    (610) 849 5114

  • In reply to Brett Barr1:

    Paul,
    Copied below is the response from our applications engineer:

    "For customer’ s question on #SKIP pin, here explains how #SKIP works: When #SKIP is 5V, Power Stage works on FCCM mode; while when #SKIP is 0V, DCM model is enabled. With inductor current always higher than the critical current, the driver operations are the same for both FCCM mode and DCM mode. The difference happens at currents lower than the critical current: current can go negative with FCCM mode; while for DCM mode, the driver will turn off LS FET when negative current is detected and the inductor current will stay at 0 until PWM high comes. DCM mode shouldn’t be the reason for the part failure.

    Is Picture "scope_4.png" measured with #SKIP high or low? It looks like DCM operation to me, when LS is turned off, a small negative current could cause HS body diode conduction and generate a waveform as scope_4.png.


    Could you please check with customer the following questions?

    (1) How they control PWM?
    (2) Is the control loop stable at all input voltages? Typically, the control loop bandwidth and phase margin change with VIN voltage. Please make sure the loop is stable at all VIN.
    (3) The customer mentioned that the load current is 1~1.5A, is this the steady state current? is it possible that the current is much higher in some states such as load transient?
    (4) Another possibility could be that the noise is higher with higher VIN, high noise might cause system stability issue as well. Please check if the loop is stable with high VIN, please also check if there are burst of PWM pulses which could cause high current."

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

    b-barr@ti.com

    (610) 849 5114