Toroidal Transformer

Toroidal transformers are the high performers among transformers. They offer the smallest size (by volume and weight), less leakage inductance, 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 transformer has less leakage inductance and less EMI when compared against other transformers of equal power rating. Toroidal transformers with a round core cross section are better performers than toroidal transformers 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. The toroidal transformer also has better winding to winding magnetic coupling because of its toroidal shape. The coupling is dependent on the winding being wound a full 360 degrees around the core and wound directly over the prior winding, hence sector wound windings do not couple as well and have higher leakage inductance. As winding turns are positioned further away from the core less complete coupling will occur; hence toroidal transformers with multi-layered windings will exhibit more leakage inductance.

Toroidal transformers can be used in any electronic transformer application that can accommodate its shape. Although usable, toroidal transformers are not always practical for some applications. Gapped toroidal transformers usually require that the gap be filled with some type of insulating material to facilitate the winding process. This is an extra expense. Split core current transformers can be assembled directly on a conductor while toroids must be passed over a disconnected end of the conductor. A toroid can be split in two, but a suitable clamping mechanism (difficult and costly) is required. Some printed circuit boards are space critical. Mounting a toroidal transformer flat on the board may take up too much precious board area. Some applications also have restricted height so the toroid cannot be mounted vertically.

Generally speaking toroidal transformers are more expensive than bobbin or tube wound transformers. Sufficient winding wire must first be wound (loaded) onto the winding shuttle, then wound onto the toroidal transformer’s core. After that, the best situation, from a cost perspective, is no insulation required over the winding and the next winding uses the same wire size. If the wire is different, then the leftover wire must be removed and the wire for the next winding must be loaded. However, if the winding must be insulated, then if must either be insulated (taped) by hand or the toroidal transformer must be removed and taken to a separate taping machine, then placed back on the toroid winding machine after taping. The shuttle must then be loaded with the wire size and type for the toroidal transformer’s next winding. A toroidal transformer with a single winding (auto-transformer, current transformer) wound on a coated core will probably be cost competitive with an equivalent bobbin or tube wound transformer since the toroidal transformer will not require a bobbin or tube. The cost differential will then depend on the method and cost of mounting the transformers.

Toroidal transformer 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 transformers.

Butler Winding manufactures toroidal transformers in a wide variety of materials and sizes. To ensure quality, Butler Winding purchased two programmable automated testing machines. Most of our production is 100% tested on these machines. For more information on Butler Winding’s capabilities, click on our “capabilities” link.

Need More Technical Information about Electronic Transformers in general?

More information is available on other web pages included in this web site. Saturation and the volt-second product are discussed in the “pulse transformer” web page. An equivalent circuit for a transformer is included in the “power transformers” web page. The “inverter transformer” and “push pull” web pages include some discussion about magnetic “bipolar” and “unipolar” operating modes. There are web pages for various types (applications) of electronic transformers (and inductors). Click on one of the available links.

Also, feel free to contact Butler Winding and ask for technical or engineering assistance.