What is Skin Depth?

When an AC current is applied to a conductor, the current concentrates near the surface of the conductor and its strength decreases as you go towards the center of the conductor. The depth till which current flows in a conductor is called as Skin Depth. The figure shows the cross section of a cylindrical conductor, the intensity of the red color represents the intensity of the current in a cylindrical conductor.

The Skin Depth is dependent on the frequency of the current/signal and the resistivity of the material. It inversely proportional to the frequency and directly proportional to the resistivity.

The Skin Depth can be calculated using the following formula:

everything RF has created a calculator which enables you to easily calculate Skin Depth for a particular material at a particular frequency. Click here to Calculate Skin Depth using a Calculator.

The Skin Depth of a conductor is a product of the Skin Effect. This phenomenon occurs when high frequency electrons are traveling through a conductor, and instead of being evenly distributed through the conductor as with DC current, tend to migrate toward the surface of the conductor as a function of frequency. Hence, alternating current through a conductor creates a current density greater at the outside walls of the conductor and lesser in the center of the conductor. The skin effect is a product of the frequency dependent resistance of the conductor being greater at the center of the conductor and less at the outside edges.

The skin depth is a measure of the current density, and is defined as the distances from the outer edges of a conductor to the point at which the current density falls to 1/e of the value at the surface. In a layer four times the skin depth from the surface of a conductor, approximately 98% of the current will flow in a conductor.