What’s the difference between conventional and planar switching power transformers?: Page 2 of 4

July 22, 2019 //By Dennis Earley
planar switching
Escalation in the use of higher frequencies for today’s power-conversion applications has designers increasingly looking at planar magnetics devices over traditional transformers.

Higher frequencies = smaller magnetics components? 

The first-order effect of the increase in switching frequency is the reduced inductance of magnetic components. As frequencies continue to rise to several hundred kilohertz and into the megahertz range, other factors emerge that can impact the size-reduction benefits of lower inductance. 

Of significant importance is skin effect. Skin effect is the tendency of an alternating electric current (ac) to become distributed within a conductor, whereby the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.

Since resistivity is a function of the cross-sectional area of the conductor, the result of skin effect is higher resistance at higher frequencies.

This can be resolved, when using a conventional wire-wound-on-bobbin transformer, by increasing the diameter of the winding wire. Another way would be to bundle multiple, smaller gauge wires. Both add more conductive capacity, but they also add to the bulk of the windings. This, in turn, can result in increasing the core size, which results in higher core losses. A planar transformer’s windings, made up of thin copper foil patterns, are less susceptible to skin effect.

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