# problem with buck converter waveforms

Discussion in 'General Electronics Chat' started by abjadi, Mar 28, 2014.

1. ### abjadi Thread Starter New Member

Mar 28, 2014
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0
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?

2. ### crutschow Expert

Mar 14, 2008
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3,361
Which direction of capacitor current is "positive"?

3. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Any sensible answer not dependent on guessing would require you to POST THE SCHEMATIC of this mystery converter.

That includes your measurement points and method.

4. ### abjadi Thread Starter New Member

Mar 28, 2014
18
0

The hall effect sensor is in series with Cout
A 20 ohm resistor is in output

5. ### abjadi Thread Starter New Member

Mar 28, 2014
18
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When I use a small capacitance like 470nF, the problem is solved ! But the ripple is very high. I should use a 100uF capacitance.

6. ### ScottWang Moderator

Aug 23, 2012
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777
You can using 470nF in parallel with 220uf, and measure the ripple again, if the ripple still there and a lot, then it could be the current not enough to offering for 20 Ohm Load as I = 10V/20 ohm = 500mA, show the waveform for ripple.

How is the spec of L1? (current,henry)

7. ### abjadi Thread Starter New Member

Mar 28, 2014
18
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The current in the figure is the -ic
L in a toroid with about 2 cm diameter and with a winding with about 20 turns
the nominal current of L is 5A

Last edited: Mar 29, 2014
8. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Because you are measuring current coming OUT of the capacitor. As the buck turns off the output voltage drops, and the capacitor sources current to the load.

Basically it depends how you connect the "current" sensing probes to the cap. (As others have said).

The best current diagnostic on a buck is to monitor the inductor current.

9. ### abjadi Thread Starter New Member

Mar 28, 2014
18
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the ripples are the same in the following two cases and i is not equal to Cdv/dt:
1) C=100uf
2) C=100uf + 470 nF
but with C=470nF the ripple in output voltage is very high and the relation i=Cdv/dt is satisfied

Last edited: Mar 29, 2014
10. ### abjadi Thread Starter New Member

Mar 28, 2014
18
0
The inductor current has the same wave but with a dc value.

11. ### abjadi Thread Starter New Member

Mar 28, 2014
18
0
Even when I short circuit the current sensor, the waveforms exist but with very lower amplitudes !

12. ### ScottWang Moderator

Aug 23, 2012
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777
You can calculate the output current I=V/20 ohm, how is the V?
You can calculate the V and ignore the ripple temporary.

13. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
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Are you arguing with my point that the best current waveform to monitor is the inductor current?

It does not have the "same wave". It CAN have a similar looking wave shape under certain operating conditions. It can also have a very different wave shape. However it will always show you the very critical buck loop current.