WE Würth Elektronik - Passive Components - Custom Magnetics

Question #1 

Round vs Flat Wire in Magnetic Components

Flat wires provide a greater external surface area than do round wires of an equivalent cross sectional area. Because flat wires provide a greater outside surface area they thus exhibit a lower AC resistance, or equivalent series resistance ( ESR ), than do round wires.

The external surface area of a flat rectangular wire is about 20% larger than that of a circular wire with the equivalent cross sectional area. This allows for greater current flow at high frequencies since the flat wire yields a greater outer surface where the high frequency current tends to flow due to skin effect.

Skin effect really begins to take hold at frequencies above around 10 kHz. It forces the current flow near to the outer surface of the conductor due to eddy currents deeper inside the wire due to the alternating magnetic fields caused by the high frequency alternating currents in the wire. This forces the current to flow close to the surface of the conductor, effectively reducing the cross-sectional area in which the high frequency current will flow.

Skin effect can be mitigated to a great degree by using flat wires with a larger external surface area rather than round wires. Doing so allows more current to flow where it is being forced to flow due to skin effect, near the surface. Creating a lower resistance pathway near the surface of the conductor by using flat wire thus allows an inductor to maintain its inductance as both the level of current and the frequency of the current increase.

For reduction of the DC resistance, a part may use a number of parallel round wires. This effectively increases the cross sectional area of the wire allowing more DC current to flow and thereby lowering the DC resistance. The same effect can be achieved by using a single wider flat wire, whose width may be comparable to two or more round wires, with a diameter smaller than the width of the single flat wire.

Additionally, the flat geometry of flat wires have a higher “winding factor“ than round wires meaning that their flat geometry actually aids in stacking the wires together more efficiently as the turns are wound in the inductor form or transformers winding window. This yields less wasted winding window area. Around 5% for flat wire compared to around 30% for round wires. This gives us a more efficient part in a smaller package.

A choke is a special application of an inductor. The name is typically given to an inductor that is used as a power supply or RF circuit filter element. When used in this application, the inductor is used to filter out or “choke“ off a particular frequency or band of frequncies. Since the current in an inductor cannot change instantaneously, it resists any changes in current flow. This makes it an ideal circuit element for filtering out higher frequency (changining or oscillating currents) such as power supply output ripple currents.

Sometimes we also hear the term “storage choke.“ This term refers to an inductor that is optimized to store energy at a particular frequency in a switch mode power supply. It can be either a single coil indcutor, or a coupled inductor as in the case of a SEPIC design or as what we know as a flyback “transformer.“ In this application the inductor or choke stores energy during the on time of the main switching element, and releases its energy to the circuits output during the off time of the main switching element.

Answer Provided By Jim Lund, Wurth Electronics Midcom FAE 

Question #2 

Magnetic Flux and Flux Density

Magnetic flux is a measure of the amount of a magnetic field passing or “flowing“ through a given area. The symbol is for magnetic flux is the Greek letter F ( Phi; Fee ). The SI units for magnetic flux are Webers.

Magnetic flux density is the amount of magnetic flux per unit area of a cross sectional area, that is perpendicular to the direction of the flux. The symbol for magnetic flux density is the letter B.

The flux density is found by B = F/A : where B is the magnetic flux density in Tesla‘s; F is the magnetic flux in Weber‘s; and A is the cross sectional area of the area that is perpendicular to the direction of the flux. The resulting answer is in the standard SI units of Magnetic flux density, which are Webers/m2 or Tesla.

We also deal with the magnetic field intensity in magnetics design. Physically magnetic field intensity H is equal to the number of turns, N times the current in the conductor, I, divided by the length of the path the current is running through. In mathematical terms H = NI/l. The SI units for magnetic field intensity H are Ampere – Turns/meter (A-T)/m.

Both the flux density and field intensity are important quantities in magnetics design. They are used in sizing the transformer core, calculating core losses and determining the operating characteristics of both inductors and transformers.

Answer Provided By Jim Lund, Wurth Electronics Midcom FAE