CSD97394Q4M: Buck NexFET Power stage fails over 28°C

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

b-barr@ti.com

(610) 849 5114

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

b-barr@ti.com

(610) 849 5114

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

b-barr@ti.com

(610) 849 5114

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

b-barr@ti.com

(610) 849 5114

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

b-barr@ti.com

(610) 849 5114