SparkFun Forums

[roach]

I'm designing a motor-controller board, and was looking at options for back-emf protection using diodes. Some of the other designs I've seen use MOSFETs for this. Since PCB space is more of an issue for me than voltage-drop, I was wondering if anyone can inform me as to the benefits of using MOSFETs instead of a network of diodes?

Also, to start at the beginning, I've only recently figured out how regular (NPN) transistors work. Can anyone point me at a site that explains MOSFETs, and the difference between FETs and regular transistors?

[Kuroi Kenjin]

I don't know how'd you replace a MOSFET with a diode network, but I do know a few things about BJTs vs. FETs.

In order for the BJT to work you need a constant base current. With out it, it goes into cut-off (turns off). Typically to ensure saturation (fully on), a BJT is driven with enough base current to eventually only give a 10x current gain. So if you have a 10 amp load, you need 1 amp of base current to keep it in saturation (or else you hve a voltage drop across collector-emitter and you're burning power).

MOSFETs are nice because you just need to reach the threshold voltage across the gate to source of the MOSFET. The only current draw is very small spike to charge the parasitic capacitance of the gate to the rest of the transistor. Until you discharge this, it's on, and staying on. In fact my Senior Design team was working with these, and since there is usually very little leakage, we would turn on a MOSFET and just disconnect the gate. Also MOSFET's are also more adept at high frequency switching, and cant be driven by voltages larger than the threshold (+ or - polarity). The negative polarity on the gate-source junction just helps it switch faster.

Sorry I don't have a site to point you to. This was covered mostly in my Electrical Energy Processing course (as well as a few others).

That help?

[Philba]

I'm a bit confused. I think you are talking apples and oranges. MOSFETs have "built in" diodes but I wouldn't rely on them. I could see how one could use a MOSFETs and turn it on when BEMF was detected but MOSFETs are fairly slow devices. Everything I've read says that you want BEMF protection to be very fast. I'd just use a schottky diode and be done with it.

Phil

[Kuroi Kenjin]

I agree with Philba.... althought some MOSFET's parasitic diode is beefed up for that purpose, it's better to be safe than sorry. I second the use of a schottkey diode.

[roach]

I'm a bit confused. I think you are talking apples and oranges.

Yep. A bit of digging, and I realized I Was way off base. The MOSFET designs were for reverse-battery protection, not BEMF. Guess I'll stick with those shottkeys

thanks guys!