Toroidal Transformers

Toroidal transformers are built around a donut-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer’s efficiency by reducing the core’s reluctance. The closed donut shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the toroid core is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core’s magnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size, lower weight, less mechanical hum, lower exterior magnetic field, low off-load losses, single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited rating.

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to a megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal transformer construction is the higher cost of windings. As a consequence, toroidal transformers are uncommon above ratings of a few kVA.


  • Smaller size and lighter weight than conventional laminated designs
  • Low form factor, flat design
  • Single bolt mounting
  • Lower losses and higher efficiency
  • Reduced audible noise
  • Inherently low magnetic flux leakage
  • Shielded designs are readily produced to meet low leakage current or EMI requirements

Typical Applications

  • Power supplies
  • Battery chargers
  • Inverters
  • A-c filter inductors (reactors)
  • D-c smoothing inductors (chokes)
  • Autotransformers
  • Isolation transformers

Ratings and Sizes

All of General’s products are custom designed for specific applications. Available ratings depend upon a number of factors including power requirements, maximum voltages, currents, and frequency. We can produce toroidal isolation transformers with ratings from 20 va to 4 kva for 50/60 Hz, operation, autotransformers to 8 kva, and transformers for 400 Hz operation to 20 kva. Please do not hesitate to inquire as to your requirements.

Please call us at 972-442-1285 or contact us for your custom toroidal transformer application.