In SMPS , why does peak current mode control have lower gain for inner loop compared to average current mode control?
Te real reason is : There is always peak to average error for peak current mode control. You are controlling the peak current not average current. At the zero crossings, due to the big inductance ( used for CCM), the slope V/L = di/dt , V is very low and inductance is very big,therefore the slope of the current will be slow. Which also means that the peak current tries to catch up to hit the reference set point, but it cannot because V is very low and in doing so, the average input current is distorted at zero crossing. Therefore with peak current mode control you have more crossover distortion. And remember there is a ramp in peak current mode control, which actually limits the duty cycle at the zero crossings ( the ramp is not helping actually, it is working against us). Llyod Dixon talks about this in Unitrode seminar notes. Also Christophe Basso talks about this in his textbook. The solution to remove the peak to average error is to make the inductance bigger so that the current ripple is lower. But, if you make L bigger at the zero crossing, the di/dt will be even slower. PCMC was abandoned for this reason in PFC. Average current mode control (CCM) and constant ON time boundary mode are quite famous for Power factor correction