In peak current controlled switching power supplies, there is a phenomenon called “current mode instability” aka “sub-harmonic oscillation”. What is that? Can't seem to get a good explanation of this….

Bonus:

(To mitigate this side effect, they recommend using something called "slope compensation".)

## Best Answer

Rather than going into the mathematics of this, it's quite easy to see this graphically. Consider a peak current mode controller operating at <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.