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You can use as many FET gate drivers as you need (what do you mean by "stack"). The concern is driving all the MOSFET gate capacitance. You need to determine the total capacitance from the MOSFET data sheet and then pick a gate driver than will drive as many gates in parallel as practical for the rise and fall times you need.
I think that what he means by "stack" is connect the FETs in parallel to handel higher currents. First I have to ask, did you make sure the FETs you are using can handel the voltage? It won't do anygood to connect a bunch of FETs in parallel to drive a motor if the voltage is too high because then you've just burned out a bunch of FETs. A FET has a near infinite gate resistance so as long as they are all the same gate voltage, they can be connected together in parallel and not have a problem. There are 2 ways to connect them; first you can use a single resistor connected to the PWM driver going to the gate of all the FETs (again as long as they are the same type), or you can connect an individual resistor to each FET and have all the resistors connect to the PWM driver. The resistor is a very good thing to have because it will prevent parasitic oscilations from occuring in the FETs when the driver signal goes high.
perhaps I'm off here, but I believe 'stacking' is what many headphone amp designers use to to achieve higher current output than available from a single chip alone. Literally stacking DIP chips on top of eachother. Not many MOSFET drivers are available in DIP's anymore though.
Just googled your part numberse, they're relatively slow switching bipolar transistors. I imagine they are configured in a 'totem pole' a classic way to drive MOSFETS. Bipolar transistors can be paralleled for this (I think) but as they are essentially current amplifiers, you would need to drive these with more current...which may require yet another stage to give each transistor a high gain.
As crutschow pointed out, driving a MOSFET is all about the gate capacitance so you need to know what these big MOSFETS are.
What FET do you want to use. I'll look it up. Might be best to use a FET driver rather than trying to modify yours. Lets get the FET out of the way, then we can look at drivers.
PS. Is it an H Bridge or just 1 direction with a BIG switch.
What are the part numbers of the FETs. You may be able to drive some better ones with the driver that is there.
Are there big copper traces on the circuit side? 300 amps is a lot for a pc board.
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