Design PWM to cope with supercapacitor and battery(help)

[shanver]

I need to design an electric circuit which require PWM to let the battery and supercapacitor take turn to supply current to the loads.

stage1: battery charge up supercap and supply current to the load

stage2: supercap voltage increase to a certain level, battery current is cut, supercap supply current to the loads

stage3: voltage of supercap decrease to certain level, again, battery supply current to load and supercap.
http://www.freeimagehosting.net/3eguo

image http://www.freeimagehosting.net/3eguo

my pwm and switch is not functioning well,
the positive input signal undergoes damping oscillation
and the switch (nmos) does not switch off as Vs>Vg

can anyone help me in this project? I will provide any information if require to..

 

[shanver]

http://img43.imageshack.us/img43/8094/projecto.png

here is the problem circuits

 

[berkeman]

I need to design an electric circuit which require PWM to let the battery and supercapacitor take turn to supply current to the loads.

stage1: battery charge up supercap and supply current to the load

stage2: supercap voltage increase to a certain level, battery current is cut, supercap supply current to the loads

stage3: voltage of supercap decrease to certain level, again, battery supply current to load and supercap.
http://www.freeimagehosting.net/3eguo

image http://www.freeimagehosting.net/3eguo

my pwm and switch is not functioning well,
the positive input signal undergoes damping oscillation
and the switch (nmos) does not switch off as Vs>Vg

can anyone help me in this project? I will provide any information if require to..

http://img43.imageshack.us/img43/8094/projecto.png

here is the problem circuits

You mention using an nmos FET, but your schematic shows a pmos FET...

 

[shanver]

You mention using an nmos FET, but your schematic shows a pmos FET...

thank for correcting , should be nmos as in schematic,
it would be fine for pmos/nmos as long as 1 of them are functioning

 

[shanver]

I'm thinking of replacing pwm to comparators.. any suggestion?