This slide will focus on the frequency dependent AC resistance. Because of eddy currents in the wire, which is a result of the AC-current, the current will be pushed to the outside skin of the wire. This effectively results in a reduction of the area that current can flow. Pointing out the equation of the skin depth, the higher the frequency the lower the skin depth. Coming back to the equation of calculating the resistance of a wire, the smaller the conductive surface, the higher the resistance. Since this resistance will only appear when driving an AC-current through wire, like the ripple current in a switch mode power supply, this resistance is called AC-resistance.