Power inductor selection is a trick

In the actual power supply design, the choice of power inductor is particularly critical. In DC-DC converters, inductors are the core components second only to ics. Higher conversion efficiency can be obtained by selecting the appropriate inductor. The main parameters used in the selection of inductors are inductance value, rated current, DC resistance, etc. When choosing the rated current of the power inductor, according to the specification of the inductor, it will be found that there are two types of rated current of the inductor, what is the definition of these two rated currents, and how we should choose the appropriate current parameters when selecting the inductor?
Explanation of two rated currents
1. Heating Current
In a DC-DC circuit, when the current passes through, a certain amount of power (Copperloss+ magnetic loss Coreloss) will be consumed, which will cause the temperature of the inductor to rise. Generally, the operating temperature of the inductor has a certain range, such as the allowable operating temperature of our inductor: -40°C - + 125°C, ambient temperature: -40°C - + 85°C. In order for the inductor to work normally within a certain temperature range, the inductor manufacturer will provide a "rated Current based on the rise of the inductor temperature", that is, Heating Current. The limited value of this parameter is defined according to the thermal effect of the current on the inductor. In the inductor manuals of most companies, the current is represented by Idc(DC current).
Heating Current is defined as the value of the current that passes through the inductor when the temperature on the inductor rises to 40°C under natural convection cooling conditions.
The definition of Heating Current of different manufacturers may be slightly different. When selecting the device, please refer to the note below the data table in detail.
Note: The Heating Current evaluation is generally DC current, because DC current does not cause iron loss in the inductor, if the AC current evaluation is much more complicated.
2. Saturation Current
When the DC current in the inductor increases, the value of the inductance permeability will gradually decrease, so the inductance value will slowly decrease. The rated Current of the inductor manufacturer is based on the degree of decline of the inductor value. "Rated current based on the rate of change of the inductor value", that is, the Saturation Current, most inductor manufacturers use the DC current through the inductor when the inductor value drops by 20% as the rated current. The following diagram shows the inductance value vs current of an inductor from Wurth. The inductance specification sets the saturation current as the current value when the inductance value drops by 10%, which is 16.6A. If the saturation current is set to the current value when the inductance drops by 20%, it is about 18.2A.
How to choose rated current in the design
Heating Current is the thermal effect of the inductor evaluated. If used beyond this parameter, the inductor will overheat.
Saturation Current is the rated current with the decrease of the inductance value as the index. In switching power supply, if used outside this range, the IC control may be unstable due to the increase of ripple current.
Therefore, in actual use, in simple terms, the smaller of the two current values is generally selected as the design limit. For example, the Irated of the four PLUSE inductors in the following figure, the relatively small one of Heating Current and Saturation Current is taken as the rated current. (Note the PA1513.321 device, why is the Irated not 45A? Note that Pulse Saturation Current is defined as a 20% drop in inductance, 41A is only a 10% drop in inductance.)
In order to make the system more reliable, in the actual design, it is generally used for more than 20% derating.