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Integrating wireless powered communication networks (WPCNs) into low-power internet of things (IoT) is a promising technique to achieve sustainable cities and society. However, the main design challenge for WPCNs is the low efficiency of wireless energy transfer. To address this challenge, this paper investigates active reconfigurable intelligent surface (RIS)-assisted WPCNs, where the reflecting beamforming gain of active RIS can be utilized to enhance wireless energy and information transmissions. We formulate an energy efficiency maximization problem that jointly optimizes time-slot allocation, transmit power control, and reflection parameters of RIS. Due to the highly coupled variables and fractional structure of the energy efficiency expression, we develop an alternating optimization method to solve it through utilizing the Dinkelbach-type method and block coordinate descent algorithm. Simulation results …