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Odd scope waveform.....aliasing?

eem2am

Aug 3, 2009
429
Joined
Aug 3, 2009
Messages
429
Hello,

Please can you say if the strange oscilloscope waveform of my SEPIC converter’s current sense resistor voltage is real or not….or is it due to aliasing?

Here is the sense resistor voltage……its when the timebase is 100us/div that the waveform looks strange…(going up and down)………..

Current sense resistor voltage waveform (20us timebase on scope)
http://i47.tinypic.com/2q8bymt.jpg

Current sense resistor voltage waveform (100us timebase on scope)
http://i45.tinypic.com/mccdoz.jpg

the above waveforms are from the simulator, as the scope cant give images to USB. I 'fiddled' the simulator to give the waveform that ive seen
Velleman HPS10 oscilloscoper (2MHz bandwidth, 10MS/s)
http://www.farnell.com/datasheets/14532.pdf

SEPIC converter is 5W, vin=6V, vout=5V, Frequency = ~70KHz
 
Last edited:

GreenGiant

Feb 9, 2012
842
Joined
Feb 9, 2012
Messages
842
with a few google searches that seems perfectly reasonable, the SEPIC works based on Duty Cycle... From Wikipedia:
the output of the SEPIC is controlled by the duty cycle of the control transistor.

Based on the 70kHz frequency you should see a period of 14us (looks about right on the pictures) and then depending on the capacitor and inductor sizes that would determine the slope of the peaks.

Now at 10MS/s you have 10 samples per microsecond so that should be more than enough to have an accurate show of what is happening
 
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