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This paper presents a simplified envelope model to study the dynamics of a phase-controlled resonant converter suitable to drive light emitting diode (LED) lamps. The output current is controlled, at constant switching frequency, through the phase displacement, Ψ, of the control signals of each class D section of the resonant inverter stage. The envelope model of the resonant network is reduced to an equivalent L-Cp circuit. The approximate closed form of the transfer function is obtained using the new reduced-order model. The model predicts that the current gain is set by the resonant inverter stage while the bandwidth is mainly defined by the output filter and the LED's dynamic resistance, providing the whole system with excellent dynamic behavior. The fast dynamics of the resonant converter ensures an operation free of instabilities and flicker effect with a simple integral controller. A 120W prototype has been built to validate the study.