I am trying to design a simple buck converter. But it doesn't work as expected. Output voltage should be 19V but it is very close to input with value 48.5V. Also there is a problem with pwm generator. If anyone could help,I will appreciate for it.Thanks.

 

Have you looked at a typical buck-converter circuit and understand how they work?
They have a free-wheel diode (usually a Schottky type) from the output of the MOSFET (cathode) to ground.
This carries the inductor current when the MOSFET is off.

 

If you attach your spice file we could fix it.
P-MOSFET is installed wrong.
Gate drive is wrong.
Missing diode.

 

Have you looked at a typical buck-converter circuit and understand how they work?
They have a free-wheel diode (usually a Schottky type) from the output of the MOSFET (cathode) to ground.
This carries the inductor current when the MOSFET is off.

Sorry, I should have removed it accidentaly. I add new analysis results with diode as reply on post.

 

The MOSFET is on all the time (passing current). Can you see why? (more than one answer)

 

If you attach your spice file we could fix it.
P-MOSFET is installed wrong.
Gate drive is wrong.
Missing diode.

Sorry, I should have removed it accidentaly. I add new analysis results with diode as reply on post.

Have you looked at a typical buck-converter circuit and understand how they work?
They have a free-wheel diode (usually a Schottky type) from the output of the MOSFET (cathode) to ground.
This carries the inductor current when the MOSFET is off.

Sorry for my wrong.
I should have removed diode accidentally.
I attached analysis with diode and file.

 

Sorry, I should have removed it accidentaly. I add new analysis results with diode as reply on post.

 

M1 source and drain are still backward.

And for a 50V input, the MOSFET gate drive also has to go to 50V, which that comparator can't provide.

 

M1 source and drain are still backward.

And for a 50V input, the MOSFET gate drive also has to go to 50V, which that comparator can't provide.

So should I increase the comparators supply , or is there any solutions for it ?

 

V1. Started at 50V. I changed it to start at 0 and take 100nS to reach 50V. Things were blowing up at startup.
S1. The mosfet was backwards. The gate drive was very wrong. So I just put in a switch that can be driven from 0/5V.
Added diode.
OP-amp was fast while the comparator was slow.
C2 to slow down the amp.
Removed the -5V supply.

Give this a try. Good Luck ronS

 

V1. Started at 50V. I changed it to start at 0 and take 100nS to reach 50V. Things were blowing up at startup.
S1. The mosfet was backwards. The gate drive was very wrong. So I just put in a switch that can be driven from 0/5V.
Added diode.
OP-amp was fast while the comparator was slow.
C2 to slow down the amp.
Removed the -5V supply.
View attachment 210221
Give this a try. Good Luck ronS

Thanks for your help ,I will try this .