Why do SEPIC Converters produce resonant oscillation at low Vin?

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Joined Mar 9, 2020

I have a SEPIC converter powering a 12V/6A load off a smart battery. The battery voltage fluctuates between 10V and 17V. When the battery gets to low capacity, ~10-20% (or ~11V input voltage to the converter), and I am continuing to draw full load, a resonant oscillation appears, and grows, in the the 10kHz range until the sound is audible from the inductors in the converter. I was wondering why this is happening? At lower current draws the oscillation does not appear and in an earlier design that used coupled inductors this did not occur. I added a damping filter across the SEPIC coupling capacitor with R ~ SQRT(2L/C) and Cdamp > 2*Csepic and that removed the oscillation almost completely. I get the idea that the damping circuit is basically replacing the inductor coupling that is lost, but I'm not sure why this use case introduces current oscillation through the inductor. I've attached my schematic of this section with the damping circuit circled. Any insight or reference someone could point me to for educating myself further would be very much appreciated!


And here's a scope screenshot of the current oscillation (blue, AC coupled, DC offset ~3A), the input voltage (yellow, oscillations are present but minor compared to offset) and output voltage (green, stays stable throughout discharge)