N - channel mosfet H - bridge

[cwok]

hi, everyone
Im tryna build a H-bridge motor controller circuit using only n-channel power mosfets
the mosfets i have (IRL3402) have a drain current of about 85A. Attached is a the circuit i am currently working on from the internet. The reason I am using n- channel mosfets is because P-channel power mosfets is hard to find here in Fiji. I mean no one here sells them. I am tryna use these n - channel mosfets (IRL3402) to replace the two p-channel mosfets (IRF4905) in the circuit without removing the om-amp part of the circuit. How can I do this? Can anyone help me?

 

[MikeMl]

The problem you have to solve is that the upper two NFets will be operated as "Source-followers", meaning that to fully turn them ON, their gates must be pulled to a voltage which is 10V or more positive with respect to their sources, which because they are saturated and have only a tiny voltage from source to drain is effectively 10V or more positive than the V+ in the posted schematic. You will have to construct another power supply, and rewire the op-amp drivers so that they can produce the required gate voltage. Note that the opamps driving the upper NFETs must also be reconfigured to invert the gate signal compared to the way they are configured now.

A lot of work to avoid buying two PFets.

 

[k7elp60]

Relpacing Q3 and Q5 with N channel mosfets will only work if you are able to boast the supply voltage higher as the gate turn on voltage has to be greater than the source voltage. Q3 and Q5 are hi side drivers so when they are on the drain and source voltage is very close to the supply voltage.