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What parameters of the “Skin Depth” affect design?

 

 

1.      Can a change in material affect the Skin depth?

There are no wonderful superconducting materials at ambient temperature, but there are certainly materials to be avoided!At 60Hz

Material

Resistivity (nOhms/m)

Skin Depth

Comment

Copper

17

8.3mm

Copper wire USA Power Grid

Steel

100

0.3 mm

Steel wire, USA Power Grid

Aluminium

28

10.7mm

Aluminium wire, USA Power Grid

 

Hence the steel core in power lines is used purely for strength, with stranded aluminium wires (light, with a better skin depth than copper) being used for current carrying.

 

Copper at 1MHz to 100 MHz

Material

Skin Depth in microns

Comment

At 1MHz, Copper

66

Wire carrying logic

At 100 MHz, Copper

7

Copper track on board

 

The thickness of the base copper on a pcb is normally 1 oz copper, 35microns, or more likely ½ oz copper, which is 17 microns, where fine traces are required.

At 100MHz the max useable size of trace on a pcb is 13x13 microns!

At 1GHz 

Material

Skin Depth

In Microns

Comment

Copper

2.0

Coax cable core

Aluminium

2.6

Waveguide wall

Silver

2.0

Coaxial cable core



2.      What are the design implications of the skin effect?

As always it is at the extremes of the frequency range that designers have to be careful about their designs to make them practical, but cost effective. Power transmissions line therefore need steel to provide the strength, but lots of aluminium or copper wires to provide the current carrying.

At high frequencies, it is exactly the same! Many coax cables for satellite dishes need the strength of the steel core, but use a copper “skin” to carry the signal. Be careful, these cables cannot be used for audio frequencies because the “skin” of copper is not thick enough & the steel resistivity is 6x that of copper.

3.      What can you do if you need the frequency and the current? (for example in audio systems, motors, transformers, inductive cookers & inductive charging !)

This is where Litz wires come into their own. Basically, they consist of small diameter wires (you can calculate the diameters needed from the skin effect at your required frequency) covered in a non-conducting material and then interwoven into bundles.

The insulation material required depends on the application, but varies from a plastic type such as a polyvinyl to silk!

For high performance applications the current carrying capacity of the wires can be optimised by weaving the bundle into patterns. This ensures that the impedance experienced by each wire is the same,  so that each wire carries the same current.

      Summary

For engineers, the skin effect has significant implications on the design of AC electronic and electrical products, infrastructures, electrical power transmission and distribution systems. Designers must balance power, resistance, size, cost and strength requirements in conductors to produce practical products and solutions for each application.

5.      Further reading

If you’d like to check what skin depth your signals are experiencing then Chemandy’s calculator is really useful:-

To calculate the skin depth for a frequency:-

http://chemandy.com/calculators/skin-effect-calculator.htm

Audio Frequencies:-

Audio cables for those of us with good hearing are equally complex. For an in-depth analysis of  audio cables design criteria, Jim, at St. Andrews University, provides a good paper:

https://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/skineffect/page1.html

6.      Reduce your time-to-market

Your time-to-market will be shortened dramatically if you use one of our standard ABS enclosures and have it modified with all the cut-outs needed for your i/o connectors. Vero will also print your logo and id /notation on the product to give you a usable enclosure in a very short lead-time.

Don’t forget to use our pcb test pins to make it easier to fault find problems in the field!

Please contact us if you have questions.

The Vero Technologies team