H-bridge sources low current to motor???

[Los Frijoles]

I have created an H-bridge controlled by an ATMega164P which uses P-channel MOSFETs on the top and N-Channel MOSFETs on the bottom. It draws its power from a NiCd battery from an R/C racing car and feeds the motor from said car. Both the N-channel and P-channel FETs have ~33milliOhm RdsOn and the battery is able to source full current to the motor when hooked up directly. However, when I plug the motor and battery into the H-bridge and have it run at 100% duty cycle, the motor takes a few seconds before it even starts turning and then it never gets up to full speed (I have a tachometer attached to the motor).

I am direct driving the MOSFETs from the microcontroller, so the gates are only seeing 5V even though the voltage into the h-bridge is 7.2V. However, according to my oscilloscope the MOSFETs are letting the full voltage through when I insert a 330Ohm resistor in the place of the motor.

I have no real idea as to why the h-bridge is unable to source the current to the motor, so if anyone here as any thoughts I would appreciate them.

 

[dick_freebird]

A low voltage microcontroller and battery may not be
able to give the MOSFETs the gate charge they need to
fully enhance the channel. If you have standard power
FETs, not "logic level", they want 8-10V to hit the spec
RON values and may be subthreshold at 3V.

I don't know this uC byt power MOSFETs want amps of
transient gate drive to turn on in under a uS. Hit it with
a 10mA output buffer and you'll be waiting longer than
your period perhaps, and never even get to fully switched.

Start with the fine print, I guess. That, and oscilloscope.

 

[snafflekid]

Like dick_freebird says probably the mosfets aren't turned on strong enough. You need a couple of half-bridge mosfet drivers to interface the microcontroller to the mosfets. This half-bridge driver can be powered from the 7.2V and level shifts the logic signals from the microcontroller to proper gate voltages.