Hello..
I'm trying to design a dc dc step down converter in Proteus..
When im using a pulse generator instead of a mosfet; there is no problem, my filter works. But when start to use mosfet as a switch, it all goes wrong. The output voltage gets as high as possible (app. supply voltage.) whatever the gate PWM duty cycle is.
I try to design the converter by including the drain-source capacitance but it does no help.
Does any have any idea about this problem..
Thanks for help..

P.S.
Circuit Diagram :

PWM Freq. : 20 kHz.

 

Try putting a sufficient load (resistor) at the output node. The difference is that with a pulse generator, the input voltage is switching between either Vsupply or ground (grounding one side of the coil thus making it works with very small load), and with MOSFET you get either Vsupply or floating which is really what you want for normal operation.

 

you mean putting a load at the source of Mosfet?
or
at the input of the capacitor ( output of the filter )
i tried them both; but it didnt work

Edit :

i usually tried 1Mohm at the source Mosfet for low power consumption (12/1mohm * 12); but i think i was wrong.. now im trying 200ohm.. and it almost works.. i get a nonlinear output like %25 DC => 7 V %50 DC => 10 V %75 DC => 14 V..

 

At the DC-DC output which is parallel with the output capacitor. As you've found out now, DC-DC needs minimum load at the output, otherwise it would go haywire (no regulation). The minimum load is design related, as a rule the bigger the DC-DC power is the bigger minimum load you'd need. This can be improved if the controller is able to skip pulses.