H bridge using power mosfet

I am messed up with an abrupt problem in mosfet h bridge driver. I have posted schematic.Voltage I am getting across motor is only 9V instead of 12V. Means one of the ON mosfet takes drain to source drop of 3V. Why is it so? Is there any problem with circuit diagram?

hi,
Measure the Gate/Source voltage on each of the top two FETs..... then check the datasheet for the Vgs threshold.

I measured Vgs for both the top mosfets. I will tell you with reference to cct diagram attached in earlier post. If ip1 is +12V and ip2 is 0V then Vgs for Q1 is 3V,whereas Q4 still has Vgs equal to +12V. And the mosfets Q3 and Q2 have Vgs equal to 0V. Exactly reverse happens if ip2 is +12V and ip1 is 0V. It is always the top fet that takes Vgs=3V and the same takes drain to source drop of 3V in ON state.

p.gholap93 said:

I measured Vgs for both the top mosfets. I will tell you with reference to cct diagram attached in earlier post. If ip1 is +12V and ip2 is 0V then Vgs for Q1 is 3V,whereas Q4 still has Vgs equal to +12V. And the mosfets Q3 and Q2 have Vgs equal to 0V. Exactly reverse happens if ip2 is +12V and ip1 is 0V. It is always the top fet that takes Vgs=3V and the same takes drain to source drop of 3V in ON state.

Click to expand...

hi,
Thats the point I was making, it takes at least 3V Vgs on the top FET's to reach the turn ON threshold voltage, so you will always lose 3V from the 12V supply.
That leaves only 9V for the motor.

Do you follow that.?

I got that 3V is not sufficient to fully turn ON mosfet. But I didn't get this statement- "so you will always lose 3V from the 12V supply." Also I think it will be better if I construct bridge using p-channel and n-channel mosfets rather than all n-channels as in this case. What say? Because in that case source will be at a fixed potential and will never change according to mosfet state.

What say? Say yes.

so you mean to say that H bridge can not be constructed with only n channel mosfets? But I found many H bridge drivers on net consisting of only n-channel mosfets.

Sure they can be N FETs but you need a supply that can drive the gate 10 volts higher than your source voltage. There are several ways do do that. Check out high side drivers on the net.

p.gholap93 said:

I got that 3V is not sufficient to fully turn ON mosfet.

Click to expand...

Correct.
Forget about the "threshold voltage" because that is when the Mosfet is almost turned off with a current of only 0.25mA.
Most Mosfets including the one you are using needs a gate-source voltage of 10V to completely turn on.
Then the gate voltage must be +22V in your circuit because the upper Mosfets are followers.
But then the +22V will destroy the gates of the lower Mosfets.

If you use P-channel Mosfets for the upper Mosfets then when you switch direction then both lower and upper Mosfets will be turned on for a moment causing a very high "shoot-through" current that might kill the Mosfets and/or the power supply. You need a Mosfet driver IC that has some "dead-time" so that one set of Mosfets turn off before the next set turn on.