Basic voltage/current regulation with MOSFET, cannot fully close current controlling MOSFET

Question

I tried to decrease value of Rg resistor on the gate of the MOSFET (the one on the right) but then the MOSFET does not open enough and i get low currents trough it, i have this setup on breadboard and results are very close to simulation, when i have voltage at 10V and current fully closed i read 15-20mA on the DMM.

I use IRF630 for MOSFETS and the circuit is powered off laptop charger 19V.

Thanks.

Edit: There is also error in regulation, preventing the mosfet to close fully as there is 3.2V at gate when the voltage is closed, i guess that is the reason it does not close.

Link to simulation: Falstad Simulation

Answer 1

In Falstad’s sim, you must define the Beta (RdsOn) and not just Vt . You had the default beta=0.02 which is a high Ron like 4000 series CMOS. The properties allow you compute RdsOn for the Vg, Vgs(th) and Beta value.

Also all IC’s are ideal voltage sources, so to simulate current limit add 220 to output or if CMOS 1k and inside if using a closed loop.

Logic would be 300 ohms in series for 4xxx 50 Ohms for 5V logic and 25 Ohms for 3.3. logic family. if you cared about load capacitance and ESR etc, you add those if that is important.

Normally I might choose Vout for 15V max and not divide 19 with 10k series to 2k shunt. The FET has a body diode option which don’t need to add since you defined

So you can model any RdsOn by selecting Vgs, Vgs(th) and use Beta somewhere between 1 and 50 for starters.

Answer 2

Actually you are wasting your time on that circuit. A high side N-MOSFET shall have an additional isolated power source (like in my schematics 0-15V) to be able to control the load current/voltage. There are charge pumps for that, if you don't want to use a separate power.

So, forget it or change everything.

^{simulate this circuit – Schematic created using CircuitLab}