[Al Skierkiewicz]

Originally Posted by Qbranch

The main difference between your PNP/NPN transistors and P-/N-Channel (MOS)FETs is that standard transistor are "voltage" effect while Field Effect Transistos are more of "current" effect devices.

FET devices function much like variable resistors... their spreading resistance allows them to evenly share currents unlike transistors which, when paralleled, tend to put much more (or all) of the current on a single device while the others sit idle. A FET, since you can think of paralleling FETS like paralleling resistors of equal value, transmits curret equally between all of the devices in parallel.

One disadvantage of FETs is that they have strange drive characteristics... they are difficult to turn on and off. Thankfully FETs have become much more civilized and inideof most FETs many of the suppression diodes that were once external are now internal... (i suggest IR for FETs, they make the most friendly ones).

IR(InternationalRectifier) also makes some drive ICs which can handle driving your gates on and off for you, saving some circuit knowledge.

Also, using a PIC to drive your circuit helps as well. Many 16/18 series PICmicros incorporate ECCP peripherals which can directly drive (in hardware) the four lines for a full H-bridge.

If you want more detail... just let me know. Hope that answers some ?'s.

-q

Alex,
Bipolar transistors are current controlled devices while FET are voltage controlled. I think of the FET as having an insulated gate so no current can flow in a FET. Another reminder is a bipolar transistor must have current flowing through a resistor to bias the base on and cause collector current to flow. FETs do function more like a variable resistance but they are easier to parallel becuase of the very low impedances involved. Both transistors and FETs are best paralleled using a small series resistance in series with one of the output elements to balance the currents in each device. As sciguy Phil, pointed out earlier, the construction technique of power MOSFETs (HEXFETS) causes the diode to be a natural (and usable) byproduct of production.