Newbie Problem with H-bridge

[brianlojeck]

First off, I am an Electrical Engineering student, but this is not homework.

I'm trying to put together an H-bridge to control a motor on an RC vehicle from a microcontroller that can put out a max of 3.3v at the output ports. Max current draw of the motor is about 6 to 8 amps.

I've put together the following simulation. While I didn't expect it to work right off the bat, the voltage and current through the motor exceeds the power supplies, and I'm not sure what I did wrong.

in the attached diagrams, the logic circuit serves to make the bridge "smoke proof", limiting the circuit states to those that are not short circuits. The 3.3v power rail on the left side of the top level diagram is to represent the chip's output voltage. The 7.2 volt feed is a direct link to the on-vehicle battery, and the 12v feed is a low-current dc-boosted circuit for controlling the mosfets.

the h-bridge itself consists of 5 mosfets, 4 for the "gear shift", and one for accepting PWM commands (the "gas pedal").

I'd appreciate any advice you can offer.

[Ridis]

I looked very quickly to your circuit and I think that you don't have any delay between the falling edge of the current in a MOS and the rising edge in the other MOS of the same bridge leg. This could lead to a short interval during which both of the mos are conducting giving a quite high current spike that increase the average current.

One more question: how you drive the H bridge ? If you use a microcontroller you could be able to avoid the  "N channel fet driver" you use to regulate the voltage to the bridge. The same result you can have driving one MOS per leg in PWM (pulse width modulation).

[OStep]

the 3554 op-amp uses a very simplified model that does not model saturation. Since you're operating it in an open-loop configuration, the output is the difference between the input terminals multiplied by a very large open loop gain. I suggest you switch the op-amp to a virtual one. My other suggestion is to construct your bridge circuit out of ideal switches and diodes (use the SBREAK component in the Switches family) to make sure that the basic waveforms and functionality are there. Once that is accomplished, only then move to a more practical implementation that may use MOSFETs and MOSFET drivers.