Mosfet drive with opamp

[rsklan]

"I am designing a brake circuit for a motor driver. This circuit must use an op-amp and a MOSFET. A comparator is used to detect the back EMF and the bus voltage, and based on these conditions, the MOSFET is supposed to turn on or off.


I have completed the simulation and circuit design, but I encountered a problem. When the back EMF voltage is greater than the bus voltage, the MOSFET turns on as expected. However, when the back EMF drops below the bus voltage, the MOSFET does not turn off. I can't control the gate voltage of the MOSFET.


I have experimented with pull-up and pull-down resistors, and in the simulation, I get the correct result. The calculations are accurate, and the circuit design is correct, but I can't get it to work properly in practice. Can you help me?"

 

[FvM]

My crystal ball is broken, can you show a schematic for forum members who are not so good in mind-reading?

 

[D.A.(Tony)Stewart]

So you mean your common mode input range is not enough for the task.
How would you define those specs? (BEMF and Vcm range)

If you have LTspice files , zip up the active parts and .asc file.