Toroidal transformers are a special type of electrical transformer with a doughnut-like shape core on which the primary and secondary coils are wound. Compared to conventional transformers, toroidal transformers offer greater efficiency, lower energy losses and vibration, and more compact size.

But these advantages and high performance will only be felt if engineers correctly design and specify the right toroidal transformer for a particular application. The overall performance of toroidal core transformers is a cumulative characteristic of the different individual components, including the core, winding and insulation, among others.

This article presents some of the essential design tips and calculations for toroidal core transformers. It will serve as a guide to ensure long component life and optimal performance.

Core design

The core is a vital part of the toroidal transformer as it provides a controlled path for the magnetic flux generated in the transformer. Important factors to be considered at this stage include shape, cross-section area and engineering material for the core.

A suitable cross-section area can be estimated using:

Where:

A = Cross-section area of the core

f = Operating frequency

Bm = Magnetic flux density

Te = Turns per volt

Keep in mind that toroidal cores with a round (or circular) cross-section typically offer better overall performance compared to those with rectangular-shaped cross-sections. This is primarily due to the fact that noise cancellation is more complete with rounded sections than in other cross-section shapes. In addition, circular cross-sections allow shorter turns-per-length and lower winding resistance.

An engineer’s options are near-endless when it comes to material selection. The right material choice will depend on performance requirements and budget. For example, toroidal cores manufactured from premium-grade virgin steel typically perform better than cores made from many other materials. On the other hand, recycled steel (Grade B) is a cheap alternative material for toroidal core design, but it comes with the drawback of low reliability and performance.

Winding

The winding process is just as important as choosing materials and cross-section shape for the toroidal core. A correct winding will not only improve the magnetic flux and reduce leakage inductance but will also lead to a lightweight and cost-effective transformer design.

As a rule, enameled wires should be wound such that they are evenly arranged and leveled around the core. In addition, adjust the winding strength of the machine to prevent perforation of the wires and reduce the risk of short circuits.

The bank winding technique, where portions of the total winding are wound in pie-shaped segments around the toroid, remains one of the best winding techniques for toroidal core transformers. This process minimizes the turn-to-turn voltage gradient on the conductor insulation, providing protection against voltage spikes.

The number of turns in the core shares a mathematical relationship with the voltage of the winding according to the equation:

Where:

E1 = supply voltage of the primary winding

E2 = supply voltage of the secondary winding

N1 = number of turns for primary winding

N2 = number of turns for the secondary winding

Insulation

Insulation is critical to the performance of toroidal core transformers as proper insulation improves the impact resistance, reduces the possibility of short circuits, and extends the service life of the transformer. As a rule for toroidal core transformers, engineers must provide insulation between the core and first winding, on the outside of the last winding and between successive windings.

The specification of insulation material, insulation thickness, creepage distance along the surface of the insulation material and the number of insulation layers must satisfy the requirements of several international safety standards, such as the International Electrotechnical Commission (IEC) standards.

As mentioned earlier, the overall quality or performance of a toroidal core transformer depends on many factors. While this article provides useful information about some of the factors, engineers are advised to reach out to toroidal core transformer manufacturers to discuss their project requirements.

To contact the author of this article, email engineering360editors@globalspec.com