The pulse-width modulation (PWM) process in DC-DC and DC-AC power converters introduces characteristic nonlinear behavior. When PWM is performed in the presence of noise, specific effects are seen on the duty ratio and consequently on the output of the power converter. These effects depend on the PWM feedback control action as well as on the PWM process itself. Small-signal concepts do not model nonlinear noise issues. Here, the probability density function of the duty ratio is obtained analytically given Gaussian external noise. The results are verified by both simulation and experimental test. For a DC-DC power converter, noise is shown to introduce a DC shift at the output. Subharmonics appear as the noise propagates through the PWM control. The degree of nonlinear effect on a power converter depends on the nature of its control. Current mode control, for example, shows strong nonlinear effects. Noise properties of this and other controls, as well as ideas for noise mitigation, are discussed.