How to Avoid Inductor Saturation in Power Supply Designs

Inductors are an important component in DC/DC power supplies. Selecting an inductor requires consideration of many factors such as inductance value, DCR, size, and saturation current. The saturation characteristics of inductors are often misunderstood and cause trouble. This article will discuss how inductors saturate, how saturation affects circuits, and methods for detecting inductor saturation.

Causes of Inductor Saturation

See Figure 1 and the inductor saturation steps below to see how an inductor saturates:

• When current passes through the coil in Figure 1, the coil will generate a magnetic field;

• The core is magnetized by a magnetic field, and its internal magnetic domains rotate slowly.

• When the core is fully magnetized, the orientation of the magnetic domains is consistent with the magnetic field. At this time, even if an external magnetic field is added, there are no rotating magnetic domains in the core. At this point, the inductance reaches saturation.

Figure 1: Schematic diagram of inductor saturation

Figure 2 expresses the saturation of the inductor from another angle, and shows how the magnetic flux density (B) and magnetic field strength (H) of the system affect the inductance through the equation.

When the magnetic flux density reaches BM, the magnetic flux density no longer increases with the increase of the magnetic field strength, and the inductance reaches saturation at this time.

Look at the relationship between inductance and permeability (μ). When the inductor is saturated, the magnetic permeability will be greatly reduced, resulting in a large decrease in inductance, and the loss of the ability to suppress the current.

Figure 2: Magnetization curve and formula

Tips for judging inductor saturation:

The methods for judging inductance saturation in practical applications are divided into two categories: theoretical calculation and experimental testing. Figure 3 summarizes these methods.

Figure 3: Method for judging inductor saturation

Theoretical calculation needs to calculate the maximum magnetic flux density or the maximum inductor current, while the experimental test mainly focuses on observing the inductor current waveform and other preliminary judgment methods. Please see the detailed description below. 

Method 1: Calculating Magnetic Flux Density

This method is suitable for scenarios where magnetic cores are used to design inductors. Core parameters include magnetic path length (lE) and effective area (AE). The core type also determines the corresponding magnetic grade. Magnetic materials must also be specified accordingly for core loss and saturation flux density (see Figure 4).

Figure 4: Inductor parameters and characteristics

With these data, we can calculate the maximum magnetic flux density according to the actual design scheme. Figure 5 shows the formula for calculating the maximum magnetic flux density.

Figure 5: Calculation formula for magnetic flux density

The actual calculation can be simplified by replacing µR with µI. When compared with the saturation magnetic flux density of the magnetic material, it can be judged whether there is a risk of saturation in the designed inductance.

Method 2: Calculating the Maximum Inductor Current

This approach can be used when designing circuits with existing inductors. Under different circuit topologies, the calculation formula of the inductor current is also different.

 Taking the switch-mode converter MP2145 as an example, the inductor current can be calculated according to the following formula, and the calculation result can be compared with the inductor specification to determine whether the inductor will reach saturation (see Figure 6).

Figure 6: Taking MP2145 as an example to calculate the maximum inductor current

Method 3: Judging whether the inductor is saturated by the inductor current waveform

This method is the most common and practical method available to engineers. 

We use the MPSmart simulation tool and take MP2145 as an example to illustrate. It can be seen from the simulation waveform that when the inductor is not saturated, the inductor current is a triangular wave with a certain slope. When an inductor saturates, the inductor current waveform is distorted significantly, which is caused by the reduction in inductance after saturation (see Figure 7).

Figure 7: Simulated inductor current waveform of MP2145

We can observe the distortion in the inductor current waveform to determine when the inductor reaches saturation.

Figure 8 shows the waveforms measured on the MP2145 evaluation board. It can be seen that there is obvious distortion when the inductor is saturated, which is consistent with the simulation results.

Figure 8: Actual inductor current waveform for the MP2145 evaluation board

Method 4: Measure the abnormal temperature rise of the inductor and judge it through the audible noise

Determining the saturation current of an inductor can be difficult without knowing the core model of the system. Sometimes, it is not very convenient to measure the inductor current, because the inductor may need to be partially lifted from the PCB to measure its current. So, we can use another trick, which is to measure the inductor temperature with a thermal imager. If the temperature significantly exceeds design expectations, it may indicate that the inductor is saturated (see Figure 9). Also, put your ear close to the inductor, if it makes a sound, that may also indicate that it is saturated.

Figure 9: Measuring inductor temperature with a thermal imager

When designing power supplies with inductors, it is important to avoid inductor saturation. This article describes some of the physical properties that lead to magnetic saturation, gives a formula for choosing an appropriate inductor value for a circuit, and a graph of the current waveform when an inductor saturates. It also provides some other tips for observing whether an inductor is saturated in an application.