How to judge saturation of an inductor? After reading these few tips, you will understand in seconds

2023-10-06Archive

1

Inductor Saturation Causes

Intuitively first understand what saturation of an inductor is, as shown in Figure 1. 1:

Picture 1

• We know that when current passes through coil in fig. 1, coil generates a magnetic field;

• The magnetic core will be magnetized by magnetic field, and its internal magnetic domains will slowly rotate;

• When magnetic core is fully magnetized, all directions of magnetic domains are consistent with magnetic field. Even if external magnetic field is increased, magnetic core has no magnetic domains to rotate, and inductance enters saturation at this time.

• From another point of view, magnetization curve shown in Figure 2, relationship between magnetic flux density B and magnetic field strength H, satisfies formula on right side of Figure 2:

• When magnetic flux density reaches Bm, magnetic flux density no longer increases significantly with increasing magnetic field strength, at which time inductance reaches saturation.

• From relation between inductance and permeability µ we can learn:

• When an inductor saturates, µ will greatly decrease, eventually resulting in a significant reduction in inductance and loss of current suppression capability.

• Picture 2

2

Inductor Saturation Tips

Is there a way to estimate saturation of an inductor in practice?

Can be divided into two categories: theoretical calculations and experimental tests.

• Theoretical calculations can be started with maximum magnetic flux density and maximum inductor current;

• Experimental tests are mainly focused on shape of current curve of inductor and some other preliminary evaluation methods.

• These methods are presented one by one below.

【Calculation of magnetic flux density】

This method is suitable for a scenario where a magnetic core is used to design an inductor. The parameters of magnetic circuit include length of magnetic circuit le, effective area Ae, etc. The type of magnetic circuit also determines appropriate brand of magnetic material, and magnetic material has appropriate norms for losses of magnetic circuit and saturation magnetic flux density.

For these materials, we can calculate maximum magnetic flux density according to actual design situation using following formula:

In practice, calculation can be simplified, and ur can be replaced by ui; finally, compared with saturation flux density of magnetic material, it can be judged whether calculated inductance has a risk of saturation.

【Calculate maximum inductor current】

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

Different circuit topologies use different formulas to calculate inductor current.

Using Buck MP2145 as an example, it can be calculated using following formula, and result of calculation can be compared with characteristic value of inductor to determine if it will saturate.

【From waveform of inductor current】

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

Let's take MP2145 as an example and use MPSmart simulator. From simulation waveform, we can learn that when inductor is not saturated, inductor current is a triangular wave with a certain slope. When inductor is saturated, inductor current waveform will have a pronounced distortion due to decrease in degree of aftersaturation.

In engineering practice, we can observe whether current waveform of an inductor is distorted based on this to judge saturation of inductor.

Below is waveform measured on MP2145 demo board. It can be seen that obvious distortions are observed after saturation, which is consistent with simulation result.

【Measure if inductor is overheating and listen for abnormal howling】

There are still many situations in engineering practice. We cannot know exactly type of magnetic core, and it is difficult to know saturation current of an inductor. Sometimes it is not convenient to check inductor current; at this time, we can also measure whether inductor has an abnormal temperature rise, or listening to abnormal howling and other means to preliminarily judge whether saturation has occurred.

So far, several tips have been presented for assessing saturation of an inductor. I hope to be helpful.