A two-stage AC-DC power converter for powering a load at a substantially constant current, and related methods and systems. The first or front end stage of the AC-DC power converter includes a buck topology power factor correction (PFC) circuit and a PFC controller. The second stage of the AC-DC power converter includes a conventional isolation and regulator circuit configured to receive the DC voltage and DC current output by the buck PFC and then to provide the substantially constant current to the load. By multiplying the rectified input voltage sensed by the PFC controller, the input AC current drawn by the buck PFC circuit has a much improved total harmonic distortion (THD), which is achievable without the need for using an expensive PFC controller. The rectified input voltage sensed by the PFC controller is multiplied using a Zener diode ladder.