I used the attached circuit to drive Buck converter but when i connect the power at the input terminal of the dc converter the pulses at the mosfet transistor gate is shifted as seen. the load is dc motor that draws almost 0.15a at 20v, switching frequency is 5khz

 

the pulses at the mosfet transistor gate is shifted as seen

Where was your scope probe connected?
Where is the ground reference level in your scope shot?
Was the scope AC or DC coupled?
Why are you high-side driving the motor (low-side driving is more usual and would use fewer components)?
C1 seems redundant.

 

Where was your scope probe connected?
Where is the ground reference level in your scope shot?
Was the scope AC or DC coupled?
Why are you high-side driving the motor (low-side driving is more usual and would use fewer components)?
C1 seems redundant.

the scope probe was at the mosfet gate and the ground was connected to the ground of the arduino board.
the scope was default i didn't change it and i can't remember where it was AC or DC coupled.

 

where should I put the scope?

 

Looks like the output you are getting is correct. But what Are you trying to achieve. You call it a buck converter, but you have no feedback in order to regulate the voltage.

 

Yes I will add feedback after I get correct results

 

Where was your scope probe connected?
Where is the ground reference level in your scope shot?
Was the scope AC or DC coupled?
Why are you high-side driving the motor (low-side driving is more usual and would use fewer components)?
C1 seems redundant.

I got this good pwm when I replaced the dc motor with resistive load

 

Yes I will add feedback after I get correct results

But you are getting correct results. What are you expecting the waveform to be.? That is do you know what the waveform should be Other than what your scope shows.

 

Could I use freewheeling diode with load? Will that affect the buck converter?

 

You already have a freewheeling diode in D2.

 

C1 seems redundant.

The switching frequency is only 5khz. The capacitance at this point needs to be large. 22uf + 0.1uf is a good place. I did not go through the formula for frequency vs capacitance but off the top of my head I know it should be much larger than what I use a t 200khz.

You are switching slow 5khz and the 100uH coil is much too small.
When the MOSFET is on it pulls up to "supply" and stores energy on the inductor. When the MOSFET first turns off the energy in the coil pushed down against ground ( negative one diode drop) until the energy runs out. Then the voltage sits at the motor voltage.

 

Sorry the coil is 460uh do you think that it is a frrquency issue?

 

I increased frequency up to 50khz and it it works good
Thanks