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Al Williams

Dr. Dobb's Bloggers

My Favorite MOSFET

December 30, 2009

Everyone has their favorite parts that they just use without even hardly thinking about it. For me, one of my favorites is the IRL series MOSFETs. These are almost the perfect switch. I have a bag of IRL520's around. They cost about a buck in small quantities. They handle over 6A at 100 degrees C, break down at 100V, and turn on with way less than a half ohm with logic level drive. What's not to like?

Sure, for just turning on a relay or something, you can always use the ever-popular and very cheap 2N2222 or 2N3904 (more favorite parts). But for just getting one digital output to switch, the FET sure is handy and only a few cents more (now if I'm designing something for mass production, that's another story, of course).

Today I needed an H-bridge to turn a 3V motor one way or the other. My entire circuit wound up being 4 IRL520s and a 3V battery holder. The 5V logic output is plenty to drive the 3V high side. The FET's internal diode structure saves it from inductive spike. I just drove the gates with 4 outputs from the CPU. I finally bit the bullet and put 4 100K resistors to ground, one on each gate, to make sure stray charge didn't zap the gates, but it worked fine without them.

So my H-bridge cost about $4.10 and took 5 minutes to breadboard. Sure, the FETs are overkill for the little motor, but for $4, I don't care. The crossover switching isn't too efficient – it isn't a paragon of design virtue. But it was cheap, quick, and easy and it got the job done. If you've ever wanted some dark secrets about H-bridges – or you haven't run into them before – Modular Circuits has a few pages that are a good read.

Just for fun, I simulated the H-bridge after I built it so I could examine the effect of some of the circuit's shortcomings (yeah, my idea of fun is a little strange). LTSpice was up to the challenge (using an inductor to model the motor; see below, or click the picture for the full view).

Got a favorite part? Let me hear about it.


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