Re: HomeMade Motor Controllers
 

I'm sure that power MOSFETs do have some differences in construction, but that diode is in all MOSFETs; it's part of the way they're built. This diode is called a parasitic because it exists, but isn't there intentionally. Attached is a drawing of a MOSFET with the parasitic diode in red. Note that the diode only exists when the source and body are connected (as it usually is).

Re: HomeMade Motor Controllers
 

Regarding the original post...

Since you mentioned you are more into microcontrollers now, are you looking to build a full-power Victor equivalent, or maybe a smaller version of some kind of pulse-width modulated motor controller for learning and experimentation? There's some magic going on inside a Victor which converts the 1 to 2 ms pulse into the H-bridge drive, which you can skip if you just want to build an H-bridge and drive it straight out of the microcontroller - more accurately, you can pull that bit into the micro-c and play with it. Instead of ganging up power transistors to get to 50 amp current capability, you can start out with a single transistor in each leg and a smaller motor or lower voltage so you go through less cash replacing blown-up parts.

Assuming, that is, that you're still thinking about building this thing...

- Steve Janesch

Re: HomeMade Motor Controllers
 

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

Re: HomeMade Motor Controllers
 

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.

Re: HomeMade Motor Controllers
 

I asked the questions 2 years ago............But thanks. Even though I have learned the answer in college by now.

-John