Dear Mister Baffled,

If we’re talking about making people’s lives easier, you could connect our parts to well-behaved loads with well-behaved sources and we could sell you expensive resistors in fancy packages and make a lot of money.

However, most parts that provide a regulated output have a feedback loop for control and enhanced stability. Sometimes this feedback loop is buried inside the part…in that case we design a control loop we think will work well for a variety of circuits a customer might hook it to. Even a simple device like a Low-Dropout (LDO) regulator has a feedback loop from the output to control the conduction of the pass transistor. This is why, in rare situations, when a load is ill-behaved or a board layout is poor or bypass capacitors are improperly selected-the output will oscillate. In a feedback loop, we’re always trying to balance the aggressiveness of the response (the speed that the regulator will respond to load changes) with stability over temperature, component variation, worst case circuit and device influences. This brings up subjects like Bode plots and phase/gain margin that are uncomfortable for some people.

So, when you look at designing with a part like the FAN21SV04 and see the external feedback loop components, we’re trying to do you a service. You can adjust the feedback compensation to meet the transient requirements of your design.  So I hope you agree - including a feedback loop will make your life easier.

Now if you’ll excuse me, I have to visit my therapist.

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