problem with buck converter waveforms

[abjadi]

In the following, I put a photo taken from capacitor current and output voltage of a buck converter.

As I know when the capacitor current is positive, the voltage should be increasing.
Can anybody tell me why in the photo, the ripple of current and voltage are almost in negative of each other?

 

[rumpfy]

It's a bit hard to say from the info you give. The waveforms look like they are the same amplitude, and depending on how you get the current waveform, you might be seeing the capacitor voltage minus the capacitor current component. Normally the capacitor current is equal to C . de/dt which means the capacitor voltage should be constant (or in this case, a square wave)

 

[ronsimpson]

You might be measuring the current inverted. Using a current probe it is easy to get inverted.

 

[abjadi]

I measure current with a Hall effect sensor.
The current and voltage do not satisfy i=Cdv/dt
vin=12v, vout=8v
C=100uF, L=100uH, Rload=20 ohm

Moreover I measured the current with a series resistance with capacitor and the same waveform was obtained. I think the figure of current is correct.

 

[abjadi]

When I use a small capacitance like 470nF, the problem is solved ! But the ripple is very high. I should use a 100uF capacitance.

 

[abjadi]

 

[simonbramble]

have you tried measuring the capacitor current with both channels at the same time? This will tell you immediately if the scope time base has been shifted

 

[abjadi]

ronsimson, u r right
The current in the figure is the -ic
but, with 100uF capacitor the i=cdv/dt is not satified

 

[abjadi]

Even when I short circuit the current sensor, the waveforms exist but with very lower amplitudes !

 

[MrAl]

Hi,

So you solved the original problem. Now lets see the waves they way they are now.

When you say "not satisfied" do you mean numerically? What is the value of the capacitor and what are the scales on the new scope picture (new pic to come) ?

 

[abjadi]

Hi,

So you solved the original problem. Now lets see the waves they way they are now.

When you say "not satisfied" do you mean numerically? What is the value of the capacitor and what are the scales on the new scope picture (new pic to come) ?

There is no new pic
only reverse the ic in the photo
I think I measure some noise instead Vout ripples

 

[rumpfy]

In post 2 I had not much to go on. I said that i=c x de/dt. In post #8, Mr Al, abjadi said; i=c x de/dt not satisfied. That was the 'dissatisfaction'.
I like the CRO you are using; looks like a really nice little unit. The trace pic suggests the HF compensation is too much for the 'voltage' waveform.
ajbadi, according to the circuit, when the switch conducts, the rate of rise of current will be a function of the (Vout - Vin). This rate of rise of current will be supplied to the load, which presumably is resistive. As the current rises, the load voltage will rise. The capacitor current should then be C x de/dt, and is a constant.
When the switch opens, the back emf across the coil will cause a circulating current through D1 and Cout and the load current will be discharging the Cout. So what you get is not straight forward.
To measure these ripple currents/voltages, it may be necessary to use the 'AC' setting for the input probe as you have done for channel 1.
Setting up the test probes to make these measurements requires some fiddling for the best spot connecting the earth lead etc, so experiment a bit to get some nice traces.
Many of the switch mode IC manufacturers have excellent data sheets and application notes, so consult them widely.
Frankly, I think you have it pretty much under control. Well done.