Toroidal Transformers


Application Notes > Custom Transformers > Toroidal Transformers

Gowanda designs and manufactures toroidal transformers for a variety of power supply and conversion applications. A toroidal transformer is a transformer that is designed on a doughnut shaped core. Toroidal transformers offer small size, less leakage inductance and lower electromagnetic interference (EMI).

Since toroidal transformers are a sub-category of power transformers, click here for a link to the Power Transformer Theory page and here for a link to the Power Transformer Operating Theory page.

Toroidal Transformer Design

      • It is typical for the power rating (expressed in VA), number of primary and secondary windings and their respective voltage and current levels of a toroidal transformer to be specified as a minimum requirement.
      • The operating frequency should be given to allow proper core calculations and sizing. Proper wire sizing is accomplished by specifying the voltage and current levels across and passing through the toroidal transformer windings.
      • Toroidal transformer designs vary widely in terms of power rating, inductance, voltage level (low to high), operating frequency, size, impedance, bandwidth (frequency response), packaging, winding capacitance, and other parameters. Silicon steel and nickel iron are available as tape wound cores or laminated pieces.


      • Computer power supplies
      • Radio transmitters
      • Electromagnetic Interference (EMI) sensitive circuits
      • Any application requiring a transfer or conversion of electrical energy

Ferrite Core Toroidal Transformer

Gowanda can design and manufacturer high performance ferrite core toroidal transformers for electronics. A 360 degree wound ferrite core toroid (and toroids in general) 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 coils of equal power rating.

      • Operating Frequency: above 1 megahertz at low gauss levels
      • Sizes: 0.1 to 5.5 inches O.D.
      • Weight: fraction of an ounce to 1.7 pounds

Note: Specialty ferrite core toroidal transformers are available with several voltage ratings and can be purchased with or without an insulating coating.

Use of a ferrite core toroidal transformer combines the performance features of the toroidal shape and the low loss feature of the ferrite core material. In high frequency applications a ferrite core toroidal transformer can offer smaller size (by volume & weight) and lower losses. Core materials have been developed for power applications and filtering applications. Some have a temperature coefficient designed to offset capacitor temperature drift for tuned filter applications. Core material initial relative permeability can range from 750 to 15000. Materials for the highest frequencies usually have lower permeability.

Tape Wound Toroidal Transformer

Tape wound core toroidal transformers are made by wrapping thin long strips of magnetic material around a winding mandrel. Originally, tape wound core toroids were developed to replace vacuum tubes. Vacuum tubes were fragile and required frequent replacement. The tape wound core toroidal transformers were more reliable. Magnetic coupling permitted the mixing of signals while maintaining electrical isolation between circuits. Tape wound core toroidal transformers also developed along another path. Early toroidal transformers used thin ring shaped laminations stamped from electrical steel. The steel from the center was waste material. A core was made by stacking these rings to the desired height. The laminated stack reduced core eddy currents. Lower eddy currents result in lower core losses. The thinner the laminations, the lower the losses were, but the more time it took to process and stack the laminations. Designers adapted the tape wound core process to general-purpose transformers as well. Winding tape wound core toroidal transformers were much faster than stacking toroidal cores; hence use of thinner material became more practical. The process was then adapted to rectangular cores known as C cores.

Today, tape wound core toroidal transformers can be made with strip as thin as 0.000125 inches. They are available in alloys of silicon steel, nickel-iron, cobalt-iron, and amorphous metals. Some materials are processed to enhance square loop properties. With appropriate gauss derating, the thin strip extends the useful frequency range up to 10 to 20 kilohertz depending on the type of material. Ferrite cores have lower core losses and cost less per unit weight, but their saturation levels are much lower. Low weight and minimal space are desired features for aviation and aerospace applications. Consequently tape wound core toroids are usually preferred over ferrites for these applications provided the operating frequency is not too high.

Tape wound core toroids wound with nickel-iron alloys are particularly sensitive to shock and vibration. These cores need to be place in a protective box with a damping medium such as silicon oil. Silicon steel alloys are the least sensitive. Silicon steel is frequently used without a protective box. It depends on the particular application.

Photo Gallery

This Bobbin Wound Power Transformer was built to mount on a chassis panel.

This image shows the soldering of the lead terminations which make the connection on the printed circuit board for this small toroidal transformer. This transformer was for a medical application.

Each individual part is tested. In this image the soldered toroidal transformers are tested to see if they meet the client’s specifications. All data is saved for later use.

A small hook winding machine is used when the core is extra small. This machine winds directly around the core itself creating a toroidal transformer or inductor.