Flux walking is an issue in full-bridge, half-bridge and push-pull transformers….Flux walking (flux stair casing) is...

# Avoiding Core Saturation

Lines of magnetic flux are created when a current flows through a conductor. They can transport energy from one winding to another usually though a permeable core of some kind. The amount of flux per area is known as flux density. Transformer and inductor cores have finite flux densities. That is, they can only handle so much flux over the core area before they saturate.

Flux loves permeable materials such as iron and ferrite cores. This is where is wants to be (the reasons for which can be found from using quantum physics). Lines of flux will “go out” of their way to “get inside” iron or ferrite. They don’t like each other though but are willing to tolerate a certain number of nearby neighbors if they can be within iron or ferrite. Eventually too many are trying to occupy the same volume and it is then no more will go in. Even though they love iron and ferrite so much their dislike for their own kind becomes too great when they “feel” too many of their own kind are within the iron or ferrite. This point at which no more lines of flux will enter iron or ferrite is known as core saturation. Different core materials act as different hosts for flux. Some are more inviting and will allow more lines of flux in than others and so have a greater MAX flux density level.

There are ways to design both transformers and inductors to ensure the core will not saturate. First, using the well-known Faraday’s Law of induction (ignoring the sign):

This is known as the EMF Equation of a Transformer

It can be arranged to show the flux density for the given parameters; this equation needs to be satisfied.

Now if you know the core you are using has a particular MAX flux density with a particular core area, you options are the number of turns, the applied voltage and/or how long that voltage is applied.

Clearly the more turns you have the lower the flux density will be within the core.

Likewise the lower the applied voltage the lower the flux density in the core and lastly, the less time you apply the voltage the lower the flux density in the core.

If you have no choice over the voltage applied or how long it is applied and you cannot fit the required number of turns into the winding window then your options are limited. You can either find a core with a higher MAX flux density and/or use a core with a larger magnetic area.

For a transformer you will want to be sure the following is always obeyed:

This states that the applied volt-seconds are always less than the allowable volt-seconds before your transformer saturates.

For an inductor it is often more helpful to use another equation based on the same principles. Using the fundamental inductor equation:

It can be arranged to show the flux density for the given parameters; this equation needs to be satisfied.

What this says is that the product of the inductance and current through the inductor, need be reduced by using enough turns and/or a large enough core area to ensure the inductor will not saturate.

You can often change parameter values as needed for both the inductor and transformer but in order to ensure non-saturation of the core these equations must always be satisfied.