Tamulmol:
- Does it matter if the current rating of N and P are different? for example N is 70A and P is 100A.
In general, no - just don't exceed the current of the lowest rated device, and you'll be ok.
That said - when sizing MOSFETs for an h-bridge, I've always read to vastly oversize them - that is, you need to control a motor that pulls 25 amps, use a MOSFET rated for 75 or 100 amps (that is, 3 or 4 times more). More on this in a bit...
Tamulmol:
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.
It shouldn't be; while you shouldn't leave the motor stalled (it can cause bad things to happen to the motor, especially in the case where the bridge can supply the current easily) - if the bridge is oversized for the motor, it will be able to happily sit there supplying current.
Tamulmol:
3.Can I parallel Two H-bridges? Connect the Outputs together and also the Inputs to the Base for Higher Current.
In general, you can parallel a MOSFET-based h-bridge - but not a bipolar transistor h-bridge; in practice, it is possible to do -some- paralleling with bipolar transistors, provided that certain protection measures are put in place - but you really should avoid it.
Regarding again #1 above - this is how in a MOSFET-based h-bridge you can design for a larger current load; just double/triple up on your MOSFETs; you can place them together, and bolt them to the same heat-sink. I've seen this done in a Roboteq h-bridge I own...
Also - you speak of "bases" and "outputs" - MOSFETs don't have "bases" - they have "gates", "drains", and "sources" (vs the bipolar nomenclature of "bases", "emitters", and "collectors").
Tamulmol:
I read a lot that's its hard to build a Mosfet H-bridge,
It's a bit easier to build an h-bridge using P and N-channel devices; doing so with strictly N-channel devices, without the proper means of driving the high-side MOSFETs - can lead to much excitement and money lost in smoked parts. Also, even when you do get a working bridge built, there's no guarantee that it will survive long term while under a real load (or application).
Even using combinations of P and N-channel devices can lead to an inefficient bridge; it might work for a small motor, but blow up on a larger motor. H-bridge design can be a very challenging endeavour, which is why most people recommend purchasing vs building such a bridge, especially one for high-current.
Tamulmol:
Why would this be harder than any H-bridges when all I have to do is copy someone else already working H-bridge/Schematics?
As noted, the problem is in finding a working h-bridge; there are a ton of bad schematics and designs out there.
One other possibility for an h-bridge you may or may not have considered, and that is a so-called "hybrid" design.
Basically, you use a couple of SPDT relays to act as the h-bridge (current flow direction control); they can be controlled using MOSFETs as well. Then, on the low-side (between the bridge and ground) you put in your suitably sized N-channel MOSFET.
Switch direction with the relays, change the speed using the MOSFET on the low-side; the only thing you can easily do is braking, and you also can only change direction with the PWM is zero. This is because by changing the direction with the relays while PWM is engaged can cause arcing and welding of the relay contacts (at best accelerating their aging, at worst damaging the parts).
Based on your replies, I think im just going to buy for bigger motors and build my own for cheap motors.