Showing SEPIC capacitor overvoltage spikes on scope...need diff probe?

[Flyback]

Hello,

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
https://www.coilcraft.com/pdf_viewer/...re:msd1583.pdf

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?

 

[crutschow]

Post your circuit diagram and indicate which capacitor you are measuring.

 

[simonbramble]

You shouldn't need a differential probe, but you need to buy a scope probe socket (or make your own) as the scope earth lead will pick up interference. Remove the earth lead of the probe, wind a piece of wire around the earth collar of the probe and connect this to one side of the cap, making sure this earth lead is only a few mm long. Probe the other end. This will keep the pickup to a minimum and get you a true reading.

yes there will be some ringing across this cap on switch on as you have a tuned circuit with the input and output inductors and the coupling cap. try making the coupling cap smaller (1uF) - it is only conducting a square wave so its value does not have to be too high.

Failing that, try a 4 switch buck boost converter (LT3791 for LED driving)

See my website for more info on the waveforms you should be seeing (and for example LTSpice circuits you can try)

 

[schmitt trigger]

Simon;
I hadn't seen your website...very useful!! I have bookmarked it.

Completely agree on a very short grounding wire required for PSU waveform analysis. I cringe when I see a tech analyzing PSU waveforms with the grounding wire further attached to a 10 cm alligator clip...it is a receiver antenna!!

 

[Flyback]

Schematic of sepic...
https://i48.tinypic.com/nqd3bp.jpg