A nonlinear analysis of a DC-DC buck converter is presented in context of its harmonics in a power delivery network (PDN). The closed-form relationships of the harmonic distortions associated with the DC-DC buck converter are derived by Volterra series to estimate the effects in a practical environment using S-parameters. For the validation of the proposed method, the buck converter is designed in a 180 nm LDMOS technology and three test cases are considered by using three different practical PDNs. A close matching is seen between the results obtained by proposed and conventional methods with a significant speedup reported. © 2018 IEEE.