About Sub Harmonic Oscillation

Hi GARAM,

Hope attached file can help you.

The difference between the average inductor current and the dc value of the sampled inductor current can cause instability for certain operating conditions. This instability is known as subharmonic oscillation, which occurs when the inductor ripple current does not return to its initial value by the start of next switching cycle. Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch node.
For peak current mode control, sub-harmonic oscillation occurs with a duty cycle greater than 50%.  By adding a compensating ramp equal to the down-slope of the inductor current, any tendency toward sub-harmonic oscillation is damped within one switching cycle.
For valley current-mode, sub-harmonic oscillation occurs with a duty cycle less than 50%. It is now necessary to use slope compensation equal to the up-slope of the inductor current.
For emulated peak current-mode, the valley current is sampled on the down-slope of the inductor current. This is used as the dc value of current to start the next cycle. In this case, a ramp equal to the sum of both the up-slope and down-slope is required.
For any mode of operation (peak, valley or emulated), the optimal slope of the ramp presented to the modulating comparator input is equal to the sum of the absolute values of the inductor upslope and down-slope scaled by the current-sense gain. This will cause any tendency toward subharmonic oscillation to damp in one switching cycle.

Slope Comp Greg Smith.pdf

Regards,

Teng