Thanks for the help Oddbot!
First off, thank you very much for taking time from your day to give me a hand here Russell, I greatly appreciate it.
I am not trying to switch a 240A load with my Arduino, and that is exactly the point. I am building a motor driver, and I expect it to be handling about maybe 30 amps at the most, but MOSFETs are cheap, and I figured that if I run 8 of them in parallel for each channel, the load on each will be greatly reduced, making it possible to get away with far less heatsinking, and as well just generally not driving the MOSFETs anywhere their capacity, increasing longevity, reducing the chance that Ill fry them all. It doesn’t seem to have a downside that I can think of to have more than necessary in parallel.
Im not 100% sure if you just typoed there and typed transistor rather than resistor, but I don’t think that I need a seperate gate transistor for each FET, I was under the impression that I needed a seperate gate resistor for each FET. Where I got that impression was from doing some googleing before I posted here, in particular this page: http://www.digikey.ca/techxchange/message/9251 (the post right at the bottom, and third post from the bottom, relevant sections copied here for your reading ease)
From the information above it would seem that simply using a single resistor between the gates and the driver should work, and in some cases it does however there is a good reason for separate resistors. Using the shortest possible path between the gate resistor and the gate of the device will result in less stray capacitance from surrounding circuits. With a separate resistor for each gate the individual resistance can be higher thus providing more protection for each transistor from issues mention above. You may also consider using a zener diode from the gate to the source of the transistor. This will protect the transistor from the easiest way of killing a MOSFET which is to the exceed the gate breakdown voltage. When switching large currents ground bounce can cause the potential at the source of the transistor to not match the ground voltage of the driver circuit. This could result in a much larger voltage from gate to source than intended. The zener diode protects the gate from breakdown and the gate resistor protects the zener from conducting excessive current.
A “secret” to successful paralleling of MOSFETs in linear mode it to keep things symmetrical, otherwise some of the FETs may oscillate. Since you’re driving many MOSFETs from an op amp the turn-on time even in switch mode (FETs on hard) is long enough for one or more to build up a destructive oscillation. So, don’t daisy chain the gate drive from FET to FET, run the gate drive traces from the op amp in the center of the MOSFET array and place a 100 ohm series resistor right at each MOSFET gate. Reducing the driver (op amp in this case) output impedance can help. A driver such as the LT1010 can help. I learned all of this the hard way in more than one design.
And this page: http://www.eevblog.com/forum/beginners/how-do-i-parallel-n-channel-power-mosfets-together-for-higher-current-rating/ (again most relevent section copied for your reading ease)
I’ve taken apart many hobby king RC plane speed controllers.
They have 6 mosfet banks (one for each of the 3 phases plus the high and low parts of the hbridge) and between 3 and 10 fets in parallel for each.
The drains and sources are directly in parallel but the gates for each have a 100R resistor to the fet driver IC.
I’m not saying its the right way to do it, just that it works fine for cheap china products.
They switch between 20 and 100A depending on model.
http://www.etotheipiplusone.net/pics/kewlmoter/hk_18_esc_5.jpg
And lastly this page: http://www.candlepowerforums.com/vb/showthread.php?79996-Multiple-mosfets-in-parallel (and ditto)
Parallellllllling multiple mosfets isnt that hard.
The drains and sources are tied together, while each
gate gets its own drive resistor (like 10 or 15 ohms).
That’s the trick…make sure each device is driven with
its own gate resistor.
Im not sure if I am getting confused with BJTs, I do understand that FETs are voltage controled devices while BJTs are current controled, and yes I understand that large resistor values shoulld not be used both because of the slightly different characteristics of each FET will cause one to turn on faster than another and thus carry more of the load and that FETs generate more heat while they are in the linerar part of their cycle on. Both not good things as you say.
I respect your opinion greatly, I know that you have been at this much longer than I have, but I am hesitant to connect all my gates together and use only one resistor, after reading all of the above material. I have tried connecting said FETs together as you suggested, using a shared resistor for both of them in a previous iteration of my motor controller, and I am not sure but I think that may have at least contributed to them failing. They did work for a short period of time but failed quite quickly. (Of course the resistors were surely not soley to blame, I did not properly account for back EMF in that design either) Anyways, I am curious if your resistor suggestion would stay the same if I decided to have one for each of my FETs and if you still think that the Arduino would be able to sink the required current for all 8.
Once again, thank you so much for all of your help.
