Toroidal Transformer vs. Standard Transformer: Which One Is Better?

Here’s a detailed toroidal transformer vs. standard transformer comparison.

Transformers are electrical devices that transfer electrical energy from one circuit to another while varying the electrical current or voltage. 

These passive components are vital in electrical systems, and we’ll look at the two most popular types. 

What Is a Standard Transformer?

Standard transformers feature a laminated steel core with interleaved E-shaped steel sheet stacks capped with I-shaped pieces. 

These E-I pieces form a loop and give the transformer its name (E-I transformer).

The cores in these conventional transformers contain high permeability steel that minimizes the magnetizing current. 

Also, they confine the flux to a path that closely couples the windings.

This design improved efficiency from the earlier solid iron cores because they had prohibitive eddy current losses. 

The laminations insulate the flat sheets from the neighboring ones, confining the eddy currents to highly elliptical paths. This confinement reduces the magnetic flux magnitude.

The internal structures of core and shell-type laminated-core transformers

The internal structures of core and shell-type laminated-core transformers

You can reduce the losses further by using thinner laminations, but these are more expensive and labor-intensive to build. 

So these optimizations are typical in high-frequency transformers because they can handle up to 10KHz.

The primary advantage of standard transformers is they are economical to build. However, they have more losses than the toroidal type. 

Their main application is in electrical energy transmission as step-up and step-down transformers.

A laminated-core transformer

A laminated-core transformer

What Is a Toroidal Transformer?

The only noticeable difference when looking at standard and toroidal transformers is the shape of the core. 

A conventional transformer looks rectangular, but the toroidal type has a doughnut shape, hence the name.

And depending on the operating frequency, the hollow ring can consist of a long wound permalloy or silicon steel strip. 

It can also consist of ferrite or powdered iron.

A toroidal transformer

A toroidal transformer

This difference might seem cosmetic, but it presents a lot of positive improvements from the standard type. 

For starters, the closed ring design eliminates the air gaps present in the E-I transformer core.

Another thing you might notice is the primary and secondary coils cover the entire core in a concentric winding. 

This winding reduces the wire length required to cover the surface.

But more importantly, it provides screening to reduce the core’s magnetic field from producing electromagnetic interference.

Advantages of Toroidal Transformers

These advantages and disadvantages of toroidal transformers give an inverse reflection of the benefits and drawbacks of the standard type.

Easy to Mount

You can easily mount a toroidal transformer using an epoxy resin center potting. Alternatively, you can use a screw to center the washer. Other ways include:

  • Through-hole PCB mounting
  • Encapsulation in metal or plastic housings
  • Pressure-less mounting plates
  • DIN rail mounting

But standard transformers need at least four bolts or screws.

Compact and Lightweight Design

Toroidal transformers are smaller and lighter than the standard type because they require fewer materials.

In E-I transformers, the cores have three limbs, and only one carries the conductor coil. So the core only utilizes a third of its structure.

 U-I cores are more space efficient than E-I units because they have two limbs. 

And both bear the coil. But they have a larger yoke, which increases their overall size and weight.

A laminated-core transformer (note the wasted limb space)

A laminated-core transformer (note the wasted limb space)

On the other hand, toroidal transformers utilize their entire doughnut core and have no additional return paths due to the looped design. 

So their construction is more compact and lighter.

A compact toroidal transformer on a PCB

A compact toroidal transformer on a PCB

Additionally, the mounting nuts, center washer, and bolts help further reduce the mounting weight.

High Efficiency

We measure a transformer’s efficiency by comparing its secondary winding output power versus the primary winding power supply.

 Toroidal transformers have an efficiency rating of 95-99%, while the standard laminated type offers less than 90%.

This high efficiency is due to the component’s near-ideal design that contains all the magnetic flux. 

The transformer has zero flux leakage because it concentrates the magnetic field in the windings distributed across the toroid surface.

So the device uses the flux efficiently to couple the primary and secondary windings, resulting in high percent efficiency ratings.

In electronic circuits, high efficiency implies a lower voltage drop from the expected value.

Low Mechanical Humming

Transformer noise or vibration occurs due to magnetostriction, a core vibrating phenomenon that impacts adjacent parts. 

The phenomenon is more evident in transformer cores with loose components, such as the laminated cores in E-I transformers.

A toroidal transformer with a primary and secondary coil

A toroidal transformer with a primary and secondary coil

These cores can become loose over time, vibrating even more and creating more noise. 

Additionally, they have yokes and air gaps between limbs that can contact each other when vibrating.

But toroidal units feature continuous structures with zero air gaps.

 So the surfaces that vibrate and contact each other are minimal, resulting in reduced mechanical humming.

Reduced Heat Generation

Energy losses in transformers occur as heat and vibrations. 

Usually, Eddy currents, copper resistance, and hysteresis generate heat. 

Toroidal transformers have lower energy losses than their standard counterparts because they are more efficient. 

So they produce less heat and don’t need advanced cooling systems.

No Air Gap (Low Stray Magnetic Fields)

The air gap in laminated transformers creates flux leakage spreading. 

Also known as fringing, this phenomenon causes magnetic flux leaks to flow to adjacent external components.

The air gap in laminated-core transformers

The air gap in laminated-core transformers

But Toroidal transformers don’t have air gaps, which minimizes the flux leakage spreading.

Reduced Signal Distortion

Stray magnetic fields or leakage flux can induce an electrical current in nearby conductors if the magnetic field lines get across. 

These stray electrical currents cause noise or interference on sensitive signal lines, such as in low-power circuits.

But since toroidal transformers have low leakage flux, they induce minimal leakage currents in other components, minimizing interference. 

This effective flux containment creates some form of electromagnetic shielding.

Low Off-Load Losses

These losses occur when you magnetize the core with no load connected to the secondary windings. 

So transformers still consume electricity even when in standby mode, and this power disappears into thin air as losses.

But since toroidal transformers have an efficient construction, their standby-mode losses are lower than those in conventional units.

Dimensional Flexibility

Toroid transformers are flexible heightwise and diameter-wise to suit different applications. But the cross-section surface must remain constant. 

On the other hand, it is not easy to adjust the size of a standard transformer, which limits their applications.

Disadvantages of Toroidal Transformers

Toroidal core transformers also have their disadvantages, which include the following.

Expensive to Build

Toroid transformers might be smaller and require fewer materials, but manufacturing them costs more. 

The challenge comes about in the coil winding process. 

Accessing the slot inside the doughnut is challenging, so the winding takes place one coil at a time.

But the case is different with E-I or U-I cores. These components get disassembled, making the limbs easily accessible for copper winding in a mass production setting.

A non-assembled E-I transformer with only the E ferromagnetic metal cores

A non-assembled E-I transformer with only the E ferromagnetic metal cores

Not Ideal for Transforming Multiple Phases

These devices usually only work in single-phase applications. 

It is possible to make a three-phase toroidal transformer, but it consists of three separate single-phase toroidal units. 

This design makes the devices expensive, so they are not popular.

But the laminated core or shell-type transformers can have their secondary and primary coils wound on adjacent limbs to accommodate three phases.

 Laminated core transformers have three limbs, while the shell type has five. 

And all limbs share the upper and lower yoke. This configuration makes them better for transforming three-phase power.

High Inrush Current

When you switch on power to a transformer, the large initial current drawn by the primary circuit spikes.

 This high inrush current occurs when the secondary coil has a load connected.

And its magnitude matches the transformer’s efficiency. 

The higher the efficiency, the higher the inrush current. 

So the toroid transformer type experiences a higher spike than conventional units.

This electric current can damage the transformer or components connected to it, such as fuses or trip circuit breakers.

 But it is possible to use passive or active devices to protect the circuit from these peak currents. 

The latter are costlier and can expand the system’s size but are more sensitive.

Circuit breakers to protect against high inrush current

Circuit breakers to protect against high inrush current

Applications of Toroidal Transformers

  • Power analyzers
  • Medical equipment (isolation transformers)
  • Measuring instruments
  • Audio systems
  • Chargers and inverters
  • Computers
  • Amplifiers
  • TVs
  • Welding machines
  • Three-phase industrial power
  • Locomotive HVAC systems
A toroidal core transformer on an LCD TV motherboard

A toroidal core transformer on an LCD TV motherboard

Wrap Up

Toroidal transformers are the clear winners in this comparison. 

Their efficiency, low noise, reduced interference, compact design, and low heat generation advantages make them ideal for modern electrical circuits. 

So they are the best choice to use in PCBs. But you must arrest their high inrush current using passive or active components for safe use. 

We hope you found this article insightful. Comment below to let us know what you think. Cheers!