Inductance is an important part of DC/DC power supply. There are many factors to consider when choosing an inductor, such as inductance value, DCR, size, and saturation current. The saturation characteristics of inductors are often misunderstood and cause trouble. This paper will discuss how the inductance reaches saturation, how saturation affects the circuit, and the method of detecting inductance saturation.
Inductance saturation causes
First, intuitively understand what inductance saturation is, as shown in Figure 1:
Figure 1
We know that when a current is passed through the coil in Figure 1, the coil will generate a magnetic field;
The magnetic core will be magnetized under the action of the magnetic field, and the internal magnetic domains will slowly rotate.
When the magnetic core is completely magnetized, the direction of the magnetic domain is all the same as the magnetic field, even if the external magnetic field is increased, the magnetic core has no magnetic domain that can rotate, and the inductance enters a saturated state.
From another point of view, in the magnetization curve shown in Figure 2, the relationship between magnetic flux density B and magnetic field strength H meets the formula on the right in Figure 2:
When the magnetic flux density reaches Bm, the magnetic flux density no longer increases significantly with the increase of the magnetic field intensity, and the inductance reaches saturation.
From the relationship between inductance and permeability µ, we can see:
When the inductance is saturated, the µm will be greatly reduced, and eventually the inductance will be greatly reduced and the ability to suppress the current will be lost.
Figure 2
Tips for determining inductance saturation
Are there any tips for judging inductance saturation in practical applications?
It can be summarized into two main categories: theoretical calculation and experimental testing.
☆The theoretical calculation can start from the maximum magnetic flux density and the maximum inductance current.
☆The experimental test mainly focuses on the inductance current waveform and some other preliminary judgment methods.
These methods are described below.
Calculate the magnetic flux density
This method is suitable for designing inductance using magnetic core. Core parameters include magnetic circuit length le, effective area Ae and so on. The type of magnetic core also determines the corresponding magnetic material grade, and the magnetic material makes corresponding provisions on the loss of the magnetic core and the saturation magnetic flux density.
With these materials, we can calculate the maximum magnetic flux density according to the actual design situation, as follows:
In practice, the calculation can be simplified, using ui instead of ur; Finally, compared with the saturation flux density of the magnetic material, we can judge whether the designed inductance has the risk of saturation.
Calculate the maximum inductance current
This method is suitable for designing circuit directly by using finished inductors.
Different circuit topologies have different formulas for calculating inductance current.
Take Buck chip MP2145 as an example, it can be calculated according to the following formula, and the calculated result can be compared with the inductance specification value to determine whether the inductance will be saturated.
Judging by inductive current waveform
This method is also the most common and practical method in engineering practice.
Taking MP2145 as an example, MPSmart simulation tool is used for simulation. From the simulation waveform, it can be seen that when the inductor is not saturated, the inductor current is a triangular wave with a certain slope. When the inductor is saturated, the inductor current waveform will have an obvious distortion, which is caused by the decrease of inductance after saturation.
In engineering practice, we can observe whether there is distortion of inductance current waveform based on this to judge whether the inductance is saturated.
Below is the measured waveform on the MP2145 Demo board. It can be seen that there is obvious distortion after saturation, which is consistent with the simulation results.
Measure whether the inductance is abnormally heated and listen for abnormal whistling
There are many situations in engineering practice, we may not know the exact core type, it is difficult to know the inductance saturation current size, and sometimes it is not convenient to test the inductance current; At this time, we can also preliminarily determine whether saturation has occurred by measuring whether the inductance has an abnormal temperature rise, or listening to whether there is an abnormal scream.
A few tips for determining inductance saturation have been introduced here. I hope it was helpful.
Post time: Jul-07-2023