The Proximity Effect increases the AC-resistance as well. A current driven wire creates a magnetic field around the wire which also creates eddy currents. By using two wires in parallel, the magnetic field of both wires influence the eddy currents resulting in the current not being pushed to the entire skin of the wire. This causes the proximity effect to increase which results in a much higher AC resistance. To summarize, the higher the frequency, the smaller the AC current driven surface, the larger the AC resistance. The higher the AC resistance, the higher the copper losses. To somewhat mitigate this problem, the WE-PDF uses a flatwire where the AC copper losses are minimized in comparison to inductors using one or multiple roundwires.