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Demonstrating SEPIC capacitor overvoltage?


Aug 3, 2009
Aug 3, 2009

Please assist in measuring short term overvoltages on my SEPIC capacitor......

SEPIC capacitor is 22uF, X7R, 1812, 16V

I have done a SEPIC converter.
Max output power = 5W.
Input voltage = 5 to 8.5V
Output voltage = 5V or 40V (depending on luminaire)
Switching frequency = 40KHz to 110KHz
Constant off time control

The SEPIC coupled inductor is coilcraft MSD1583-473

When i looked for overvoltages across the sepic capacitor on the Scope (200MHz bandwidth scope) , i merely placed the scope probe across the SEPIC capacitor.

However, the scope probe ground will have an associated capacitance to the ground, and so i fear that i may have unwittingly filtered any overvoltage spikes away?
-Must i use a differential probe to look for the overvoltages on the sepic capacitor?
I am wondering if there will be overvoltages at start up, due to the ringing with the leakage inductance of the coupled SEPIC inductor?
Also, as the FET switches, i fear that there may be overvoltages on the SEPIC capacitor at these intervals also?

Do i need the differential probe?, and what bandwidth Diff probe must i purchase?