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A small signal model for average current mode control based on equivalent circuit is proposed. The model uses the three-terminal equivalent circuit model based on linearized describing function method to include the feedback effect of the side-band frequency components of inductor current. It extends the results obtained in peak-current model control to average current mode control. The proposed small signal model is accurate up to half switching frequency, predicting the sub-harmonic instability. The proposed model is verified using SIMPLIS simulation and hardware experiments, showing good agreement with the measurement results. Based on the proposed model, a new feedback design guideline is presented. The proposed design guideline is compared with several conventional, widely used design criteria to highlight its virtue. By designing the external ramp following the proposed design guideline, quality factor of the double poles at half of switching frequency in control-to-output transfer function can be precisely controlled. This helps the feedback design to achieve widest control bandwidth and proper damping.