SMPS: What is current mode instability (aka “sub-harmonic oscillation”)?
Rather than going into the mathematics of this, it's quite easy to see this graphically. Consider a peak current mode controller operating at &lt;50% duty cycle. Then you can see below that perturbing the system results in the perturbation decaying and the system returning to steady-state operation.



But at >50% duty-cycle the system does not return to steady state operation. Instead,  it enters a "sub-cycle oscillation" mode.



Simple as that.

See source for a more detailed explanation.