I am trying to build a simple H bridge as part of a learning curve. At the moment it is on a breadboard.
I am using 2 P-Channel MosFets (IRF9540) and 2 N-Channel MosFets (IRFZ440N). They are tied to a 3.6v screwdriver motor. I've tested the current draw on this at stall when powered by a 6v lantern battery. The max current I have recorded is 0.8A. When testing on the breadboard it is not under any stalling load.
At the moment I am not using an MCU controller – i'm just jumpering the control wires (going into the gates of each side) straight into the power line.
The P-Channels are getting very hot; which is somewhat to be expected as the P-Channel RDS is relatively high at c200mΩ. (but I have to say that I am surprised by how high the temp is getting. ambient is c.25C.
However this question is more aimed at the fact that the motor is running very slowly. It looks like somewhere in the circuit I am getting serious voltage drop – it appears to be random but sometimes the motor will not turn at all, sometimes it turns slowly and gradually gets faster, but never achieves anything like the speed when directly connected even to a small LiPo 3.7v cell (900mAH).
sometimes I am reading no voltage across the motor pins, sometimes 1-2v. but nowhere near the 6V across the battery pins.
is this all down to poorly balanced fets? how can I really be dropping 4+V?
when the gates on the right hand side of the H bridge are brought high I am getting the following voltage drops across drain and source on each of the fets
Q1 3v
Q2 1v
Q3 4v
Q4 4v
(note annoyingly my normal multimeter is on the blink so I am using one that is granular only as to 1v).
the circuit schematic is here: https://www.dropbox.com/s/ih7bx6hlj40vxtf/Screenshot%202015-05-18%2018.24.49.png?dl=0
a photo of the breadboard assembly can be found here (fwiw): https://dl.dropboxusercontent.com/u/194358/IMG_20150518_183350.jpg
(please forgive the absence of colour coding in the wiring).
I am using jumper cables which for sure can't take multi-amps but they should be ok with smaller currents – none are getting hot in any event.
many thanks in advance for any pointers you can give me.
Justin
Best Answer
Without looking at your schematic I can tell you the problem- the MOSFETs are specified at 10V Vgs and you are giving them a few volts only, so they are not fully turning on. The 6V is probably dropping from the motor too.
You need to find higher voltage drive for the gates to get them to turn on, or (easier) find MOSFETs that are specified to have low Rds(on) with a few volts Vgs. This kind of circuit should also have a UVLO (undervoltage lockout) so if the voltage is too low for them to fully turn on the MOSFETs don't get dangerously hot.