Useful Tips | How to determine inductor saturation? Understand it immediately after reading this

Latest update time：2021-08-30

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Causes of Inductor Saturation

First, let’s have an intuitive understanding of what inductor saturation is, as shown in Figure 1:

figure 1

We know that when current passes through the coil in Figure 1, the coil will generate a magnetic field;
The core will be magnetized under the influence of the magnetic field, and its internal magnetic domains will slowly rotate;
When the core is fully magnetized, the directions of the magnetic domains are all consistent with the magnetic field. Even if the external magnetic field is increased, the core has no magnetic domains that can rotate. At this time, the inductance enters a saturation state.

From another perspective, as shown in the magnetization curve in Figure 2, the magnetic flux density B and the magnetic field intensity H satisfy the formula on the right side of Figure 2:

When the magnetic flux density reaches Bm, the magnetic flux density no longer increases significantly with the increase of magnetic field strength, and the inductance reaches saturation.

From the relationship between inductance and magnetic permeability µ, we can know that:

When the inductor is saturated, µ will be greatly reduced, which will eventually lead to a significant decrease in inductance and the loss of the ability to suppress current.

figure 2

Tips for judging inductor saturation

Is there any trick to determine inductor saturation in practical applications?

It can be summarized into two categories: theoretical calculation and experimental testing.

Theoretical calculations can start with the maximum magnetic flux density and the maximum inductor current;
The experimental test mainly focuses on the inductor current waveform and some other preliminary judgment methods.

These methods are introduced one by one below.

Calculating the magnetic flux density

This method is suitable for the scenario of using magnetic core to design inductor. The magnetic core parameters include magnetic path length le, effective area Ae, etc. The model of the magnetic core also determines the corresponding magnetic material grade, and the magnetic material has corresponding provisions for core loss, saturation flux density, etc.

With these materials, we can calculate the maximum magnetic flux density based on the actual design situation, the formula is as follows:

In practice, the calculation can be simplified by using ui instead of ur; finally, by comparing it with the saturation flux density of the magnetic material, it can be determined whether the designed inductor has the risk of saturation.

Calculating the Maximum Inductor Current

This method is suitable for designing circuits directly using finished inductors.

Different circuit topologies have different formulas for calculating inductor current.

Taking the Buck chip MP2145 as an example, it can be calculated according to the following formula. By comparing the calculation result with the inductor specification value, it can be determined whether the inductor will be saturated.

Judging by the inductor current waveform

This method is also the most common and practical method in engineering practice.

Taking MP2145 as an example, we use the MPSmart simulation tool to simulate. From the simulation waveform, we can know that when the inductor is not saturated, the inductor current is a triangle wave with a certain slope. When the inductor is saturated, the inductor current waveform will have an obvious distortion, which is caused by the reduction of inductance after saturation.

In engineering practice, we can observe whether the inductor current waveform is distorted to determine whether the inductor is saturated.

The following is the waveform measured on the MP2145 Demo board. It can be seen that there is obvious distortion after saturation, which is consistent with the simulation result.

Measure whether the inductor heats up abnormally and listen for abnormal howling.

In actual engineering, there are many situations where we may not know the core model accurately, and it is difficult to know the size of the inductor saturation current. Sometimes it is not convenient to test the inductor current. At this time, we can also preliminarily judge whether saturation has occurred by measuring whether the inductor has an abnormal temperature rise, or listening for abnormal howling.

So far, several tips for judging inductor saturation have been introduced. I hope it will be helpful to you.

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