i am doing MPPT for photovoltaic panel and i connected it to buck converter , when i display voltage of the drain related to ground i got spikes in photovoltaic voltage(VD) i don't know where is the problem here is attached photo of the output voltage

 

Schematic?

 

Very possible that this is radiated noise rather than conducted. The scope probe ground lead acts as a loop antenna at high frequencies.

Schematic?

ak

 

What is the spike amplitude?

Are the spikes a problem or are you just concerned about them?

 

What is the spike amplitude?

Are the spikes a problem or are you just concerned about them?

it isn't just spikes it looks like pwm with spikes but the difference between top and bottom is small , i could see it when i increase sensitivity of oscilloscope .

 

this is the schematic , the analog signal to atmega is coming from data aquisition from pc

 

any suggestion ?

 

You didn't answer my question about whether the spikes are a real problem in you circuit or you just don't like them.

Any switching regulator will generate some spikes during the switching time due to the high currents being abruptly switched and the inductance of the traces and ground. Some of it may be to common-mode current through the oscilloscope ground lead. Look at the ground voltage with the probe while the ground lead is also connected to the ground and see if you still see a spike.

So my suggestion is to ignore them unless they cause a problem in your application.

 

You didn't answer my question about whether the spikes are a real problem in you circuit or you just don't like them.

Any switching regulator will generate some spikes during the switching time due to the high currents being abruptly switched and the inductance of the traces and ground. Some of it may be to common-mode current through the oscilloscope ground lead. Look at the ground voltage with the probe while the ground lead is also connected to the ground and see if you still see a spike.

So my suggestion is to ignore them unless they cause a problem in your application.

as i said before it isn't just spike but the output looks like pwm , it appeared when i increased sensitivity of oscilloscope the output voltage is not straight constant line my application sensitivity is .1 volt so it is important to get straight line

 

as i said before it isn't just spike but the output looks like pwm , it appeared when i increased sensitivity of oscilloscope the output voltage is not straight constant line my application sensitivity is .1 volt so it is important to get straight line

You will always get a small amount of ripple in the output voltage with a switching regulator. Using a larger inductor and/or capacitor will reduce the ripple. How "straight" do you need?

 

You will always get a small amount of ripple in the output voltage with a switching regulator. Using a larger inductor and/or capacitor will reduce the ripple. How "straight" do you need?

when i said "straight" I meant constant value and not curved

 

when i said "straight" I meant constant value and not curved

You can never get it perfectly (ideally) straight, but the larger the output inductor and capacitor the less curved it will be. So you have to make an engineering judgement and decide how much curve (voltage ripple) you can tolerate in your system.