I'm trying to design a Full-Bridge which can handle upto 330 A (@ 12V). I'm paralleling 3 MOSFETs per leg, and I think I've found a low enough RDS(on) MOSFET to make this somewhat practical. Here's a schematic of the parallel MOSFETs in Half-Bridge configuration:

![enter image description here](https://i.stack.imgur.com/wLYy7.png)

All transistors are [AUIRF1324S-7P](http://www.irf.com/product-info/datasheets/data/auirf1324s-7p.pdf) and each resistor in the above schematic can be assumed to be about 5Ω. The power dissipation in each MOSFET for 111A is 20W. I am managing this heat by a surface mount sink and a fan. I've written how I arrive at the 20W figure below, in case it matters.

My main concern now is switching losses. The max. total gate charge of the MOSFET is 252nC - so for each leg the total gate charge becomes 756 nC (3\*252 nC). If I use a run-of-the-mill driver with 2 A output capability, the switch on time is t = Q/I = 750 nC/2 A = 375 nS! My guess is that I will have a lot of switching losses if I drive my MOSFETs this slow. This is where I'm confused: what do I need to do switch these MOSFETs faster? Use a higher current rated driver? 

Assuming I use a 5A rated driver, the time becomes 150 nS. At a frequency of 30KHz, will a switch on time of 150 nS present significant switching losses? If so, suppose I go with even higher rated current driver, how do I ensure that my source (a 12V Lead-Acid batt.) is able to handle current spikes upto 10A?

Essentially, my question boils down to: if 150 nS presents significant switching losses at 30KHz, what do I need to do in order to drive my FETs even faster?

Of course, this all assumes there are no gate resistors! The gate resistor will slow down the switch on even further! But most of the papers on Parallel MOSFETs suggest gate resistors are necessary to prevent ringing.

Conduction Loss Calculation:

The FET's rds(on) @ 175 °C is 1.6 mΩ. With each FET handling 110 A, the power dissipated is ~20W. I want to be able to maintain a temperature of 125 °C on these devices (they are rated for 175 °C) with an ambient temperature of 40 °C. So, (125-40)/(20) = 4.2 °C/W. Considering that the device's thermal resistance between junction to case is 0.5 °C/W, I need a heat sink with lower thermal resistance than 3.7. The heat-sink that I've found provides 3 °C/W at 300 LFM airflow. So I feel I have this area covered (I hope, anyway!).