Hi I'm designing a PIC based power supply will you have a look at my schematic of the output stage and see if its a decent design. It is going to be a lot more complex and i have got to do the math before deciding on the components. But before i do that i want to know you guys think (it simulates ok). I have found a opamp that will go from 0v to 10v on its output with a 12v supply (i know someone would mention the rail to rail limitations of the opamp ). I have my main power supply this is just for when i need 2.

Thank you all for taking the time to view my post it is greatly appreciated.

 

Q2 makes the circuit non-functional. As shown, Q2 would simply make sure that the MOSFET was turned off; there is no way to turn the MOSFET back on.

Connecting the output of the opamp directly to the gate would make it functional. However, you will be limited by the MOSFET Vgs requirement and how high the output of the opamp can go.

 

Sorry my bad its supposed to be a Ptype MOSFET with a vgs of about 2v.

Thank you for the reply.

 

OK, in THAT case (P-type MOSFET) there is no way for the MOSFET to get turned OFF once turned on, as there is no path for current to get to the MOSFET gate!

Adding a resistor from the gate to 12v provides a current source, but then the gain of the transistor is so high that the output oscillates; the load gets an uncontrolled amount of current.

I'm thinking you wanted the PWM output of the uC to provide a relatively constant level on C1, and for U1 to match that voltage drop across the load.

 

Yes that is correct. Ill look into it some more sketch something up and come back to you.

Thank you for your reply.