• Resolved

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?

  • Paul, 

    I've reached out to our apps team to see if they can figure out what is going on here. If the device is going into repetitive avalanche, that could indeed be causing the device to overheat. We may need to see some waveforms.

    I should have a response for you shortly. 

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

  • In reply to Brett Barr1:

    Can you provide the operation conditions where the failure happened - load current, frequency, duty cycle, etc...?

    Also, any details you can provide regarding the failure will be helpful - was the device damaged? Was there a HS or LS short?

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

  • In reply to Brett Barr1:

    Thank you for your quick response! The frequency is 994 kHz, the duty cycle is somewhere between 0.3 and 0.45, and the load currents are around 1 to 1.5A. I will set up to test the circuit and log the temperature, the output voltage, and the load current tomorrow. I could also record waveforms on the Vsw pin if that would be helpful.
    Paul Norton
    Design Engineer
    Checksum, LLC
  • In reply to Paul Norton:

    Thanks Paul, let me know and I will forward to my apps team to look at. 

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

  • In reply to Brett Barr1:

    The Excel file shows the failure occurring when the temperature reaches 28°C. What is strange is that the current seems to remain constant. The current "Iout drvr1" is being monitored using an INA271 with a 50mΩ resistor. The power supply voltage was 24 volts

    I was running a second CSD97394 with a power supply voltage of 20 volts (Vout Drvr2) and it continued to run. Did not record the current for this device. So at this point I would say that a lower input voltage seems to solve the problem. But why is that the failure happens at elevated temperature?  At 22 to 25° the part can operate for hours at 24 volts, no problem.

    For the CSD97394Q4M in a standard buck configuration what waveforms should I obtain?  

    Paul

  • In reply to Paul Norton:

    Ok Paul, I have forwarded all your info to our apps team. Hopefully I'll be able to get back to you within 24hrs. By the way, I didn't see an attached excel file in that last post

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

  • In reply to Brett Barr1:

    couldn't figure out how to attach the excel file using this blog. When i clicked on the paper clip it sent me to another screen and I could not get back. When I tried to copy/paste it didn't work.

    PaulLog_CSD95394_Fail.xlsx

  • In reply to Paul Norton:

    The first log was flawed because I did not have the thermo-couple right on the device. I discovered today that was the failure! Look at this data and you'll see that just when the part starts to fail there is an exponential rise in temperature.
  • In reply to Paul Norton:

    Paul, 

    Sounds like you are going into thermal runaway, which may be a result of a repetitive avalanche event. Still waiting for a response from our apps. 

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET

  • In reply to Brett Barr1:

    Paul,
    Could you capture wave forms of the PWM signal and VSW?

    Brett Barr

    Product Marketing Engineer, Mid-Voltage FET