How to calculate the current needed to drive a MOSFET?

Question

I am designing a motor controller that will be used to control a 210A brushed DC motor. If possible I was planning on using Intersil's HIP 4081A Full bridge FET driver to drive the MOSFETs in my H-Bridge. The FET driver however outputs a max of 2.5A. I know the voltage to fully turn on a MOSFET is 10-12V. Does anyone know how to calculate the current needed to fully turn on a MOSFET. Enough current has to be supplied in order to charge up the internal input capacitor. I read somewhere it is calculated by taking the Gate Drive Voltage/ Internal Gate Resistance. Is this the case?

Answer 1

It's a MOSFET.  It doesn't require any gate current to drive it, only a sufficient Vgs to turn it on.

The gate charge necessary to charge the gate (or to discharge it) is a transient event, not a steady-state current.  Even the transient peak current can be made very small by adding a large series resistor in front of the gate.

The max. current may be used as a source or sink for a resistor to generate a Vgs across the resistance or, it may be used to drive a BJT transistor instead of a MOSFET.   In otherwords, this IC can be used for other things besides a MOSFET H-Bridge .

For a simple MOSFET H-bridge, you need not worry about gate drive current, but it wouldn't hurt to put series resistances in between the output pins of the IC and the gates of the MOSFETS -- anything between 10 and 100 Ohms will do a decent job of protecting the IC.

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Answer 2

under additional details can you add what kind of "control" you want.

pick one or describe a method. i will really help in answering you question.  (A) and (B) will run the FET in a saturation mode. (C) runs in linear mode. sites that show how to build audio amps will give the formulas you need for proper biasing in the case of (C). "tlbs101"s answer applies to (A) only.

(A) - on/ off control

(B) pulses width speed control

(C) analog voltage control

EDIT: a 4081a is a dual class-b push-pull/difference-amp motor driver.... that is a different can of worms. on the bright side, it handles most of the biasing. here, H-bridges and bootstrap does not refer to the biasing of the FET; it refers to the output configuration of the FETs diving the motor (look at it from the load not the source. the motor is in the middle of the "H"). you will want to run it in which ever mode will keep the current draw under 1.8A (75% of2.5A). you can run 1 motor in bridge mode, 2 in bootstrap.  

page two show what appears to be a fullbridge mode running one motor. page 12 shows the  configuration you will need for bootstrap. most of the buffers on the left can be ignored if you dont need the functionality. a quick cross reference of page 7 (fig 4&5) shows you will need at least 8.5 mA to trigger it at those frequencies. this will require at least 12v.  from my brief review of the DIP, it is likely that all you need is a variable PW generator firing a transistor that gives the chip a pulse voltage of 12v -13.5v and 0v-1V. the transistor will need to supply around 12mA to avoid undervoltage conditions.. that is a tough task for TTL, so use a transistor to trigger it (a 7407 is the only inverter/buffer diver in the TTL family that will drive it. look up the current/voltage your circuit can supply before using a buffer or transistor). if you are talented enough to figure out how to hook the thing up, you can figure out how to trigger it with that knowledge. (voltage/current divider AKA rheostat/potentiometer)

you can email me if necessary, but that might be the limit of how much i can help. have fun!

PS what are you building? a class project or a RF bulldozer? i noticed a rheostat on page12 labeled "CW". if CW= clock width, it can be used to for minor speed adjustment (duty cycle). this means if you replace both of them with a potentiometer with the wiper to ground, it can be used to control  a skid-steer robot. if you build one i want the plans.

http://www.alldatasheet.com/datasheet-pd...