Hi All,

When designing a circuit for switching on/off, e.g. a PWM motor drive, when should one use a FET over a "normal" transistor? Are there any special considerations you need when driving a FET from a microcontroller output? I've not used them much before.

Thanks
FW

 

A transistor requires a relatively low voltage but high current to drive, while MOSFETs usually require at least 4V, if not 10V. The voltage is referenced to the source pin, so it's difficult to get them working on the high side, without a MOSFET gate driver. It's also difficult to get MOSFETs switching very fast without a gate driver, due to the MOSFET's gate capacitance.

 

Transistors can work OK for low current loads. However, you're limited in how much current you can sink or source due to the I/O pin limitations; 20mA source/sink is a common max limit per pin; so 200mA is about your maximum current source/sink for a single transistor. There is also a maximum limit per port and/or IC. You can use that up quickly with transistors.

Darlingtons multiply the available base current, but you wind up with a comparatively high Vce of 0.7v to 1.6v depending on load current and the Darlington in question.

You can multiply the I/O pins' source or sink current by using a transistor as a voltage follower to source current for another transistor's base, but then you get into using several resistors for current limiting, base returns, etc.

There are quite a number of logic-level N-channel power MOSFETs available nowadays. One really nifty series is the IRLDxxx from International Rectifier/Vishay:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRLD024PBF-ND
These little 4-pin DIP N-ch logic level MOSFETS can sink up to 2.4A, have a Vdss of 60v, and Rds(on) of 0.1 Ohm; very handy for breadboarding.

Here's another handy MOSFET:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=IRLU7807ZPBF-ND
Vdss=30, Rds(on)=13.8m Ohms, Id=43A continuous, Qg=11nC
Vdss = the maximum voltage from drain to source
Rds(on) = the resistance from the drain to the source when ON
Id = drain current
Qg = total gate charge in nanoCoulombs

If your PWM frequency is fairly low, you can get by without a gate driver with logic level MOSFETs that have a low gate charge. You could also use a couple of PNP/NPN transistors as a voltage follower to get faster switching speeds, if you want to "go cheap", or use gate driver IC's for maximum performance.

When driving a MOSFET gate, you need to use a resistor between the driver and the gate in order to "snub" ringing that will otherwise occur. MOSFET gates basically act like capacitors, and the wiring between the I/O pin or driver and the gate has parasitic inductance, so you have a series resonant LC circuit that will "ring" at high frequencies, causing the MOSFET to turn on and off very fast. This will lead to high power dissipation in the MOSFET.