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TD351 application

Question asked by Jay Matt on Sep 22, 2017

We have used ST PN TD351 for close to 10-years now. Just recently, with the Through-Hole package being discontinued, we switched to the surface mount version of TD351.


Attached, please find a section of our print that shows the TD351 interfaced to driving an IR IRFP90N FET. Also attached, please find a data sheet for that IR FET.


Our application is for the drive of an EDM Machine. There is a physical spark created through electrodes that are separated by a thin film of oil. The oil acts as a resistor and we end up with roughly 8.8A of current flowing through the FET. The current is limited by the 5-OHM resistor just outside of the spark gap.


Since we changed to the surface mount version, we have had some reports of failure of the TD351. We have been running some tests. Attached, please see some waveforms. The image, TEK00001.PNG was taken across the 4.7 OHM resistor, R10. This shows a current spike that calculates to just slightly under 0.85 amps. There is of course a + spike when the transistor turns on, and a – spike when it turns off. TEK00002.PNG shows the spike at a much faster time base. This looks good to us.


Now, look at TD351 CurrentWaveform.JPG. This is an actual photo of the scope. Clearly there is ringing evident.


One of the additional changes we made on the board was adding a fuse to protect the board traces in case of transistor failure. But I am wondering of the inductance of the fuse is causing the FET to ring – taking out the TD351?


I have some specific questions:


  1. Do you believe that TD351 is a good choice to use as a FET driver for the IRFP90N20? If not, could you recommend another ST FET driver that would be a better choice?
  2. Would you agree that the waverform captures in TEK00002 looks good and appropriate?
  3. Would you agree that the Waveform captured in “TD351 CurrentWaveofor.JPG” is dangerous and could blow the TD351 device?
  4. Do you have an opinion about the use of a fuse to protect the FETs? Please find the data sheet attached.