Hi! I've few questions about Mosfet H-bridge.
1. Does it matter if the current rating of N and P are different? for example N is 70A and P is 100A.
You would never design any MOSFET circuit based on the current limit, you use the on-resistance to estimate power dissipation. If you ever get close to the maximum current
you'll be needing water-cooling and large blocks of copper.... Matching Rds(on) values is
the ideal, but p-channel devices generally have 3 times the on-resistance due to hole-mobility
2. Is it bad having Mosfet that's rated way too much than what the load needs. Ex. 6A stall motor with 70A H-bridge.
Its not bad, its wise - you want some overhead capability if you don't want exploding
MOSFETs on current spikes. As I said you design from the Rds(on)...
3.Can I parallel Two H-bridges? Connect the Outputs together and also the Inputs to the Base for Higher Current.
I read a lot that's its hard to build a Mosfet H-bridge, Why would this be harder than any H-bridges when all I have to do is
copy someone else already working H-bridge/Schematics? Hope you guys understand my English lol. Thanks!
Its not hard per se, its just that it gets harder as the current increases and a lot harder as
the voltage increases. You need protection circuitry that works, you need to understand
the capacitive feedback inside MOSFETs and design so that dV/dt currents don't overload
your gate driver and blow the gate oxide. You need to understand stray inductance so
that the circuit layout doesn't allow the source voltage to bounce too far below ground
(which again can blow the gate oxide).
A 12V 5A bridge isn't too scary, but say 50V 50A and its a different game (there's kilowatts
of power available to destroy things if you get it wrong).
All big H-bridges are n-channel only BTW.