My flyback is running at 250 KHz, and on a few PCBAs from a lot I'm seeing about a 5 Vpp 250 KHz sinusoidal ripple between GND and GNDI, and the expected 50 mVpp copy of the dutycycle is riding on top of this. I can reduce this 250 KHz ripple by adding more capacitance from GND to GNDI. The attached document shows the difference.
Does anyone know how the flyback is generating a sinusoidal return current at the flyback frequency? Is this a sign of a stability issue?