Toroidal Inductor Theory

Toroidal inductors / transformers are the high performers among inductors. They offer the smallest size (by volume and weight) and lower electromagnetic interference (EMI). Their windings cool better because of the proportionally larger surface area. A 360 degree wound toroidal transformer has a high degree of symmetry. Its geometry leads to near complete magnetic field cancellation outside of its coil, hence the toroidal inductor has less EMI when compared against other inductors of equal power rating. Windings that are less than 360 degrees exhibit more EMI.

Toroidal inductors with a round core cross section are better performers than toroidal inductors with a rectangular cross section. The cancellation is more complete for the round cross section. The round cross section also gives a shorter turn length per unit of cross sectional area, hence lower winding resistances. Good turn-to-turn coupling is dependent on the winding being wound a full 360 degrees around the core. As winding turns are positioned further away from the core less complete turn-to-turn coupling will occur. Turns on the outer layers see a core cross sectional area that includes some non-magnetic area (air, insulation, copper). This added area generates some leakage inductance that adds to the inductance expected from the core.

Toroidal inductors can be used in any inductor application that can accommodate its shape. Although usable, toroidal inductors are not always practical for some applications. Gapped toroidal inductors usually require that the gap be filled with some type of insulating material to facilitate the winding process. This is an extra expense. Powdered cores have an effective distributed gap. These are usually preferred over a filled gap because of lower cost and reduced gap losses. Some printed circuit boards are space critical. Mounting a toroidal inductor flat on the board may take up too much precious board area. Some applications also have restricted height so the toroidal inductor cannot be mounted vertically.

Generally speaking toroidal inductors are more expensive than bobbin or tube wound inductors. Sufficient winding wire must first be wound (loaded) onto the winding shuttle, then wound onto the toroidal transformer€™s core. (For bobbin/ tube wound wire is continuously de-reeled from a spool of wire.) After that, the best situation, from a cost perspective, is no insulation required over the winding. If the winding must be insulated, then it must either be insulated (taped) by hand or the toroidal inductor must be removed and taken to a separate taping machine. Some inductors have more than one winding. If additional windings are required, then the toroidal inductor is placed back on a toroid winding machine after taping. The shuttle must then be loaded with the wire size and type for the toroidal inductor€™s next winding, thereby adding most cost to the inductor. Toroidal inductors with a single winding wound on a coated core may be cost competitive with an equivalent bobbin or tube wound inductor since the toroidal inductor will not require a bobbin or tube. The cost differential will then depend on the method and cost of mounting the inductors.

Toroidal inductor cores are available in many materials: silicon steel, nickel iron, moly-permalloy powder, iron powdered, amorphous, ferrites, and others. Silicon steel and nickel iron are available as tape wound cores or laminated pieces. Non-magnetic toroids are also available to make air core toroidal inductors.

Butler Winding can make (and has made) toroidal inductors (and transformers) in a wide variety of materials and sizes. Butler Winding also does €œbobbin wound€ and €œtube wound€ inductors. Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have experience with foil windings, litz wire windings, and perfect layering. For toroids, we can (and have done) sector winding, progressive winding, bank winding, and progressive bank winding. Butler winding has a variety of winding machines, bobbin/tube and toroid. That includes two programmable automated machines and a taping machine for toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation and can also encapsulate. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines.

If you need assistance with your design, please contact Butler Winding and ask for Engineering.