（１）Isat

As the current flowing through the inductor increases, the L value decreases due to magnetic saturation. The current value at which the L value drops to a certain level is defined as the rated current. This is Isat. Isat refers to the current value at which the L value drops to 20% to 30%. (Specific values vary depending on the manufacturer and type of inductor, so please check the data sheet of each inductor.)

Isat must be greater than or equal to the maximum current (inductor peak current: ILpeak hereafter).
The maximum current ILpeak is the value obtained by adding 1/2 of the ripple current (ΔIL) to the load current (hereinafter Iout). ILpeak can be calculated from the following formula.

Ripple current (ΔIL) is expressed by the following equation.

If there is no margin in the rated current and the current in the load increases, the following chain will occur:

Current increase → Magnetic saturation → L value decrease → Peak current further increases

In this case, the overcurrent protection function (*) of the power supply IC operates, causing problems such as a drop in the output voltage Vout.

(Note)

• At high temperatures, saturation begins with a small current value.
• Even if the Isat is exceeded, the inductor will not be damaged, but the ripple current (ΔIL) will increase due to the drop in the L value. As a result, the operation of the power supply IC may become unstable, or the rated current of elements other than the inductor may be exceeded.

(*) The overcurrent protection function is a function that stops the output of the power supply IC when the output current becomes abnormally large due to some accident such as an output short circuit. The method of detecting overcurrent and the method of stopping the output differ depending on the product, but the overcurrent protection function of the power supply IC this time is called the frequency foldback type, which lowers the oscillation frequency and reduces the ON duty. This method limits the output current by

（２）Itemp

When current flows through an inductor, it heats up due to the resistance of the windings. The rated current value based on the self-temperature rise is specified as Itemp.
In some cases, parts may fail or be damaged if used beyond this specified value.

Itemp is defined by the current value at which the temperature rise is ⊿40°C. (Specific values vary depending on the manufacturer and type of inductor, so please check the data sheet of each inductor.)

Select the value of Itemp based on Iout or more as a guideline.

(3) Simulation

In the simulation, let's compare inductor L1 with a high rated current of 3.7A​ ​(HFL4020-222) and a low rated current of 1.05A​ ​(EPL2010-222) for a load current of 2.5A. The circuit to be simulated is shown in Figure 1.
Here, instead of a normal inductor model (static model), we use a saturation model that allows us to check the saturation state through simulation, and check the state of the inductor current when the rated current is exceeded. The saturation model is available from CoileCraft 's website at:
https://www.coilcraft.com/ja-jp/models/howto/model-libraries-for-ltspice/

(a) Simulation results for an inductor with sufficient rated current margin

Even at a load current of 2.5A, the inductor does not saturate and the inductor current and output voltage are stable.

(b) Simulation results for an inductor with insufficient rated current margin

When the load current is 2.5A, magnetic saturation occurs and the inductance value decreases, resulting in a further current increase. And you can confirm that the overcurrent protection function of the power supply IC has been activated and the output voltage has decreased.

"point"

1. Inductor rated currents include "DC superimposition rated current (Isat)" and "Temperature rise rated current (Itemp)".
2. Isat is the current value at which the L value drops by 20% to 30%.
3. Select Isat that is greater than the maximum current (inductor peak current).
4. When the temperature rises, saturation starts with a small current value.
5. Itemp is the current value at which the temperature rise is ⊿40℃.
6. The value of Itemp should be selected so that it is greater than or equal to the load current Iout.
7. If there is no margin for the rated current and the load current increases, the overcurrent protection function of the power supply IC will activate, causing problems such as a drop in the output voltage.

"point"

• A smaller Rdc can reduce power loss due to heat generation.
• There is a trade-off relationship between the reduction of Rdc, the DC superimposition characteristics, and the miniaturization of the size.
• Select an inductor with a smaller Rdc from among the inductors that satisfy the required characteristics such as L value and rated current.

"point"

1. In actual inductors, self-resonance occurs due to stray capacitance.
2. It exhibits inductive characteristics up to the self-resonant frequency, and exhibits capacitive characteristics beyond the self-resonant frequency.
3. Since the inductor does not function at frequencies higher than the self-resonant frequency, select an inductor with a higher self-resonant frequency than the switching frequency of the power supply IC.
4. The smaller the L value, the higher the self-resonant frequency.

(1) Classify by magnetic material

There are two types of magnetic (core) materials for inductors: ferrite and metal composite. Until now, ferrite has been popular, but in recent years, the metal composite type using a metal magnetic material has been attracting attention.
The metal composite type has better magnetic saturation characteristics and thermal stability than the ferrite type. On the contrary, the ferrite type has excellent DC resistance. A summary is given in the following table.

(2) Classify by the state of magnetic flux

There are two types of power system inductors: the open magnetic circuit type and the closed magnetic circuit (shield) type.
When a magnetic toroidal core (doughnut-shaped core) is wound and a current is passed through it, the magnetic flux circulates inside the core. Such a magnetic circuit is called a closed magnetic circuit. When a rod-shaped or drum-shaped core is used, the magnetic flux exits from the inside of the core, becomes leakage magnetic flux, and forms a loop that returns to the core again. This is called an open magnetic path.

If leakage magnetic flux is magnetically coupled with other coils or wiring patterns, it will cause noise. Therefore, the open magnetic circuit type has larger radiation noise. However, even with the closed magnetic circuit type, there are some products that have a gap and cause magnetic leakage, so you need to be careful.

Due to the difference in magnetic structure, there is a difference in saturation characteristics between a closed magnetic circuit and an open magnetic circuit. Therefore, there will be differences in the DC superimposition characteristics. In the closed magnetic circuit, the L value gradually decreases as the DC superimposed current increases. On the other hand, in the case of an open magnetic circuit, the L value is relatively flat up to a predetermined current value against an increase in DC superimposed current, but the L value tends to decrease sharply after the predetermined current value. I have. However, the tendency of the curve changes depending on the properties of the magnetic material used.

A summary is given in the following table.