How to selecte a Ferrite coil inductor
Selection of the proper ferrite coil inductor is very important for circuit performance. First of all, the value of inductance should be defined. In order to provide stable working of circuit, it is important to select an inductor with appropriate inductance value.
The next thing to consider is the maximum current that the inductor could handle. The saturation of inductor would cause significant reduction of its performance. Also, the core material is important because there are different ferrite materials for specific frequency range.
Frequency response is the next parameter to take into account, especially in case of high-frequency applications when it is necessary to use low core loss ferrite materials.
The temperature stability is also important especially for automotive and industrial electronics.
The size and packaging are important parameters in compact electronic devices. Small inductors should satisfy the electrical requirements of the circuit.
Also, low DC resistance is required for ferrite inductors to prevent energy losses and overheating.
In order to ensure proper functioning of the circuit, especially in case of sensitive communication or medical devices, it is important to pay attention to EMI performance. Shielded ferrite inductors could be used in noisy environment.
Moreover, cost is an important consideration but should not influence performance or reliability.
Working with trusted manufacturer allows to ensure consistent quality and stable supply.
Finally, the testing and prototyping are also important steps in the selection process. They help to ensure that selected inductor performs properly in real-life operating conditions.
TIPS FOR AVOID COMMON DESIGN MISTAKES
One of the common mistakes in ferrite coil inductor design is incorrect selection of inductance value, which causes unstable working of the circuit. Another mistake is lack of attention to the current saturation of the inductor, which would cause performance degradation under high loads.
Poor thermal design could cause overheating and malfunction of the inductor. The engineers also forget about EMI performance of inductors, which causes noise in finished product. Low quality ferrite material decreases efficiency and increases energy losses. The wrong winding could also influence the inductance value stability. The proper simulation, testing and material selection could help to avoid these mistakes.
How to Calculation of a Ferrite Core Inductor Inductance
It seems to be the main point to consider during the designing of the power electronics – calculating the inductance of the ferrite core inductor. The point is to know all the parameters of the core and use a proper model for your application. The ferrite cores are considered to be a preferable choice in the high frequency applications due to their low core losses. There are many approaches that may be used for the estimation of inductance of ferrite core inductor.
Basic Approach: The Inductance Factor (A_L)
A typical approach and the easiest one is using inductance factor (or A_L value). This is the most typical approach especially for standard core shapes. A_L (which is usually expressed in nH/N²) is the core constant. It is an inductance of a single turn of a wire. It depends on the geometry of the core and its permeability.
The inductance (L) can be easily calculated using the next formula:
L = A L N 2
Where:
L is the inductance in nanohenries (nH).
A_L is the inductance factor of the core (nH/N2).
N is the number of turns of the winding .
It is a practical approach in order to make a first order design. The A_L value can be found by the manufacturer in the core datasheet and is usually measured at low signal level (e.g., 1 kHz, B < 10 gauss). The standard A_L values for many cores are defined by the international standard like IEC 62358.
If you have a core with A_L = 250 nH/N² and you wound 10 turns, then the inductance will be L = 250 nH * 10² = 2500 nH or 25 uH.
Calculation of Inductance Based on Principles of Magnetic Circuit
You may also calculate inductance according to the principles of the magnetic circuit. This approach is more flexible and can be used when you want to take into consideration some additional aspects such as air gaps.
Inductance is determined by:
L = N ^ 2 / \mathcal{R}_{total}
where $\mathcal{R}_{\text{total}}$ is the total reluctance of the magnetic circuit. Reluctance is the magnetic equivalent of electrical resistance and is calculated for each part of the magnetic circuit (the core and air gaps) using the next formula:
ℛ = l μ 0 μ r A
Location:
l is the average length of the magnetic path of the section.
μ 0 is the permeability of free space (4π × 10 −7 H/m).
μᵣ is the material’s relative permeability.
A is the cross-section area of section.
The total reluctance of a core with uniform cross-section and length is the sum of the reluctance of the core and the reluctance of the air gap. ℛtotal = ℛcore + ℛgap. This approach is useful . It shows how adding an air gap (with a low μᵣ = 1) increases the total reluctance and, therefore, decreases the inductance. This is a design approach that is used to prevent the core saturation under the influence of high DC bias currents.
Additional Considerations
These approaches give us a good start, but calculating the “true” inductance of the ferrite core inductor might be a bit complicated. The inductance is not constant. It changes depending on the operating conditions. Therefore, while making the design, you may take into account these aspects:
DC bias: The effective inductance reduces and the core is saturated more when the DC current flows through the winding. That is why it is preferable for the designers to use A_L value from the chart which considers the required LI^2 energy, rather than nominal un-biased value .
Frequency: The permeability of ferrite materials is frequency dependent. That is why inductance and core losses (ESR) should be taken into account as frequency-dependent for the high frequency applications.
Gap Modeling: The simple reluctance formula is less effective for the cores with big air gaps due to the presence of fringing flux. The more complex model is required for the accurate calculation of the air gap reluctance.
Conclusion
As it was mentioned above, the A_L approach, based on the formula L = A_L * N², is the fast and accepted way of determining inductance, but the more complex design may require the usage of the magnetic circuit approach or even frequency-dependent models.
