In this letter, we investigate the joint power control and time allocation scheme in a wireless powered communication network, in which wireless sensors harvest the energy and then transmit information with a time switching protocol. Unlike the time-division-multiple-access scheme, the time switching protocol allows sensors transmit information signal simultaneously. Our purpose is to maximize sum of the user-centric energy efficiency (EE) of the system. However, due to the coupled time and power, the user-centric EE optimization problem is non-convex, which is difficult to be solved directly. We propose an iterative resource management scheme to solve the formulated problem. Numerical results are presented to illustrate the fast convergence and to show the advantage of our proposed algorithm compared with the previous studies.