One more consideration is switching time. Usually the higher current FET devices are slower than the lower current ones. This matters if the power dissipated in the devices will be significant because of PWM control doing a lot of on-off switching. One more area of engineering trade-offs, and more tedious analysis in order to keep the cost down.

 

Assuming your DC bus is 140 V, a 3.5 HP motor would require about 19 amperes, though the startup current could be much higher. The FET you selected could very easily handle that much current, even with a rather small heatsink. Actually, if you are going to use 120 VAC, as indicated in another thread, you could probably save some money and/or get even lower ON resistance by going to a lower voltage rating. 200 volts probably doesn't leave enough margin for 120 V +10% (185 V peak), but there are lots of FETs in the 250 to 500 V range. A rating of 650 V is considerably higher than would be typically used in many applications, so it is probably more than proportionally more expensive.

e.g. IXFH80N25X3 from IXYS is rated at 250 V, somewhat cheaper and 16 milliohms vs. 26 milliohms for the Siliconix part. I didn't look at the gate charge, which may be less favorable. Low gate charge means low gate current, which aids in accomplishing fast switching for low switching losses.

The TO-247 package is my favorite for moderately high power for through-hole. It is very easy to reliably mount to a heatsink, though I strongly recommend mounting it to the heatsink, even if just temporarily, before soldering to assure that it is properly aligned and parallel to the sink.

 

Assuming your DC bus is 140 V, a 3.5 HP motor would require about 19 amperes, though the startup current could be much higher. The FET you selected could very easily handle that much current, even with a rather small heatsink. Actually, if you are going to use 120 VAC, as indicated in another thread, you could probably save some money and/or get even lower ON resistance by going to a lower voltage rating. 200 volts probably doesn't leave enough margin for 120 V +10% (185 V peak), but there are lots of FETs in the 250 to 500 V range. A rating of 650 V is considerably higher than would be typically used in many applications, so it is probably more than proportionally more expensive.

e.g. IXFH80N25X3 from IXYS is rated at 250 V, somewhat cheaper and 16 milliohms vs. 26 milliohms for the Siliconix part. I didn't look at the gate charge, which may be less favorable. Low gate charge means low gate current, which aids in accomplishing fast switching for low switching losses.

The TO-247 package is my favorite for moderately high power for through-hole. It is very easy to reliably mount to a heatsink, though I strongly recommend mounting it to the heatsink, even if just temporarily, before soldering to assure that it is properly aligned and parallel to the sink.

Yes, I'm going to use 120VAC as indicated in a previous post. I'm just going to get away with one Mosfet. I don't think I should complicate stuff so much. I will consider what you said about selecting a cheaper one by getting a lower ON resistance.

 

Mosfets share well because as they heat up the channel Resistance increases throttling back in exactly the way that external ballasting resistors would.. it only really works if the FETS are in close thermal contact so they can reach thermal equilibrium. make sure your driver can handle the extra gate current, you must operate them in perfect unison. What about a nice big IGBT they deserve our love too.

Armatures And Coils?
American Anglican Conspiracy?
Auto Adjusting Clutch....
Whats it all about?
I don't know yet, but I'm getting there.....

 

I have opened this post to talk about this project in specific. https://forum.allaboutcircuits.com/forums/the-projects-forum.6/