Why did my FET fail?
August 25, 2009
-By Dr. F.A.E.
We’re introducing a new blogger to the site: Dr. Fred A. Engleberry. Dr. F.A.E. holds a PhD from MIT (Muckton Institute of Talknology) and has several months of valuable experience with applied technology. We are pleased to have Dr. F.A.E. available to answer questions collected from customers around the world.
Dr. F.A.E, “Smoky” from General Specifics Inc. sent a question…”Why did my FET fail?” Without further ado, we’ll turn the session over to Dr. Fred.
First of all, Smoky, you’re probably expecting a lot of annoying questions about your design. Such as, what frequency you’re running at, what the gate drive circuit looks like, what the load is and what supply voltage is present. Some design engineers might try to determine whether there is an avalanche condition beyond what the device might be reasonably expected to tolerate, whether the gate drive is insufficient or oscillating, whether the load is inductive, whether voltage spikes creep too close to breakdown voltages on the gate or drain, or whether the total package dissipation is being exceeded.
However, let’s say in this instance I could see your schematic and Bill of Materials (BOM). Your gate resistor, R42, as noted on the schematic, should be one ohm, but the BOM shows 1,000 ohms. Replace this resistor with the proper value and you will find that your FET turn-on and turn-off rise and fall times will become reasonable and you will avoid gate oscillation – and your FET design will become robust.
Want more information on FETs? Check out our website for MOSFETs at http://fairchildsemi.com/products/mosfets/index.html
About the author:
Dr. Fred A. Engleberry (also know as Dr. F.A.E.) holds a PhD from MIT (Muckton Institute of Talknology). Fred has a world-class intellect and rarely neglects an opportunity to mention it. Argumentative and contentious, he's often irritatingly correct when answering questions and discussing semiconductor technology, real world design trade-offs and engineering issues. Dr. F.A.E's core competencies include using big words when small ones would suffice and providing long-winded, indirect answers to simple questions.
Tags: Dr. FAE, FET designs, FET failed, gate oscillation, MOSFETs
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