Selecting the Optimal Inductor for Power Converter Applications

The basic operation of an inductor, which can be described simply as a component that stores energy in the form of magnetic flux. Faraday’s Law of Induction states that change in magnetic flux induces electromotive force which opposes the driving current. This relationship describes the inductor: 

                                                        e = -N dφ/dt = -L dI/dt

Current through an inductor is transformed to produce a magnetic field. It charges and the current rises when the circuit is on. When off, the inductor acts as a current source, discharging current that flows through the circuit as the magnetic flux collapses.

The voltage and current components of the inductor are out of phase such that the voltage leads by 90 ˚.

 

Selecting the Shielding Environment

One of the first design considerations before selecting an inductor is its placement on the board and the sensitivity of components that will be in its immediate vicinity. From this either a shielded or non-shielded configuration can be chosen, each of which has distinct advantages and disadvantages. The main advantage of a shielded inductor is its low radiation which can be characterized from its lowest coupling factor among other type of inductors, whereas the magnetic flux of its non-shielded counterpart is not confined to a given vicinity. These inductors experience the highest coupling factor.

Inductor Rated Currents

When current flows through an inductor the temperature of the component rises, AC ripple current causes core loss, and DC current causes inductance to drop. The amount of steady state DC current, referred to as Irms, which causes the temperature to rise is typically in the 20-40 ˚C range and effectively provides a benchmark of power dissipation for the component. Another way to classify Irms is the output or average current of a switching regulator. 

Core Material and Loss

Core loss is related to the material chosen and the core cross sectional area as well as the ripple current, switching frequency, and inductance associated with the circuit. An inductor dissipates power both in the material of the core and in the windings. Core loss is calculated by taking the difference between the energy that goes into the core during the on portion of the cycle and the energy that goes out of the core during the off cycle. The following table provides data on the core loss and related cost of four different core types. 

Temperature and Saturation

When examining the temperature range for the inductor, a designer should be aware that the operating temperature and the ambient temperature are not one and the same, but rather the operating temperature is the difference of the ambient temperature and self-rising temperature. For example, a given component with an operating temperature in the range of -40 to +125 ˚C with a self-rise temperature of +40 ˚C means the ambient temperature is actually -40 to +85 ˚C where the difference is taken only on the maximal temperature. Since the operating temperature cutoff is -40 ˚C, the range is not able to extend beneath that value. Furthermore, temperature is a function of power dissipation and surface area given by the equation:

ΔT = f(power dissipation) / f(surface area)

Saturation current denoted as Isat, is defined as the amount of DC current to cause inductance drop. This is typically 5-35 % of its initial value. Isat is a benchmark of current handling capability for an inductor, characterizing the amount of energy that can be stored based on the relationship of energy that is stored in the inductor, which is ½ LI2 and essentially infinite. The term saturation current does not imply that the inductor core has reached the saturation stage, rather the component is at the magnetized stage. Isat can be considered as the peak current of a switching regulator to account for worst case saturation. Since a high saturation current does not drive the component to zero inductance, it becomes an air inductor remaining above zero. If the current goes high enough during operation then the core material and wire insulation can be damaged at high temperature. In most cases, the operation of an inductor is limited by its temperature rise.

 Choosing the right part is straightforward when the following sequence is employed:

 1. Choose the configuration that corresponds to the design’s current based on the maximal output current. 

2. Find the components which have the necessary inductance. 

3. Choose the desired DC resistance. 

4. Narrow the choices to the type of construction or coil type desired. .

5. Select the specific part number. 

Some more points need to consider: 

• Product    

1. Automotive Inductors
2. General Purpose
3. Power Inductors
4. RF Inductors
         
• Type     
1. AC Line Filter
2. Choke
3. DC Filter Choke
4. Data Line Choke
5. EMC Choke
6. Established Reliability
7. Filter Choke
8. Ground Wire Choke
9. HF Choke
10. Hash Choke
11. Line Choke
12. Linear Choke
13. Molded
14. Multilayer
15. Multilayer Organic (MLO)
16. NFC Inductor

• Termination Style   

1. Axial
2. Radial
3. SMD/SMT
4. Screw
5. Solder Tab
6. Wire Leads       

• Package/Case   
• Shielding     
1. Semi-Shielded
2. Shielded
3. Unshielded        
• Inductance        
• Tolerance           
• Maximum DC Current   
• Maximum DC Resistance             
• Saturation Current         
• Minimum Operating Temperature          
• Maximum Operating Temperature         
• Q Minimum       
• Self Resonant Frequency            
• Termination      
1. Flexible (Soft)
2. Standard
• Mounting Style
1. Chassis Mount
2. PCB Mount
• Length 
• Width   
• Height  
• Diameter            
• Core Material   
• Qualification  
1. AEC-Q200
2. M39010/X     
• Series   
• Packaging

Reference :  

1. https://www.mouser.com/pdfdocs/Bourns_Inductor_Power_Converter_Applications_WhitePaper.pdf
2. https://www.mouser.com/pdfDocs/Coilcraft_Baiscs_Inductor_Selection.pdf
3. https://mou.sr/3z8n7hY