Core parameters of inductors

1. Inductance L
The inductance L represents the inherent characteristics of the coil itself, regardless of the magnitude of the current. In addition to the special inductance coil (color code inductance), the inductance is generally not specifically marked on the coil, but with a specific name. Inductance, also known as self-inductance coefficient, is a physical quantity that indicates the ability of an inductor to produce self-induction.
The inductance of the inductor mainly depends on the number of turns of the coil (the number of turns), the winding method, the presence or absence of the magnetic core and the material of the magnetic core. Generally, the more the number of coil circles, the more dense the coil, the greater the inductance. A coil with a magnetic core has a larger inductance than a coil without a magnetic core. A coil with a larger magnetic permeability has a larger inductance.

The basic unit of inductance is Henry (Hen for short), which is represented by the letter H.

2. Inductive reactance XL
The size of the blocking effect of the inductance coil on AC current is called inductive reactance XL, and the unit is ohm. Its relationship with inductance L and AC frequency f is XL = 2πfL.
3. Quality factor Q
The quality factor Q is a physical quantity representing the quality of the coil, and Q is the ratio of the inductive reactance XL to its equivalent resistance, I .e., Q = XL/R. The higher the Q value of the coil, the smaller the loss of the loop. The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shield or the iron core, and the influence of the high frequency skin effect. The Q value of the coil is usually several tens to several hundreds. The use of magnetic core coils, multi-strand thick coils can improve the Q value of the coil.
4. DC resistance DCR
The resistance of an inductor coil measured under non-alternating current. In inductor design, the smaller the DC resistance, the better. The measurement unit is ohm, usually marked with its maximum value.
5. Distributed capacitance
The capacitance between the turns of the coil, between the coil and the shield, and between the coil and the bottom plate is called the distributed capacitance. The existence of the distributed capacitance reduces the Q value of the coil and deteriorates the stability. Therefore, the smaller the distributed capacitance of the coil, the better. The use of segmented winding method can reduce the distributed capacitance.
6. Self-resonant frequency (Self-Resonance Frequency)

Due to the existence of Cp, together with L, a resonant circuit is formed, and its resonant frequency is the self-resonant frequency of the inductor. Before the self-resonant frequency, the impedance of the inductor becomes larger as the frequency increases; after the self-resonant frequency, the impedance of the inductor becomes smaller as the frequency increases, showing capacitance.
7. Allowable error
The percentage of the difference between the actual value and the nominal value of the inductance divided by the nominal value.
The allowable deviation is the allowable error between the nominal inductance of the inductor and the actual inductance.

Generally used in the oscillation or filter circuit of the inductor requirements of high precision, allowable deviation of ± 0.2%~ ± 0.5% and used for coupling, high frequency blocking coil precision requirements are not high, allowable deviation of ± 10% ~ ± 15%.

8. Nominal current
Also called rated current, it refers to the current allowed to pass through the coil, usually expressed by letters A, B, C, D and E respectively. The nominal current values are 50mA, 150mA, 300mA, 700mA and 1600mA. The rated current is the continuous DC current intensity allowed to pass through an inductor. The DC current intensity is based on the maximum temperature rise of the inductor at the maximum rated ambient temperature. The rated current is related to the ability of an inductor to reduce the winding loss by low DC resistance, and also related to the ability of the inductor to dissipate the winding energy loss. Therefore, the rated current can be increased by reducing the DC resistance or increasing the size of the inductor, for low-frequency current waveforms, the root-mean-square current value can be used to replace the DC rated current, which is independent of the magnetism of the inductor.
9. Saturation current Isat
A certain amount of DC bias current is applied to the inductor to reduce the inductance value of the inductor by 10% (ferrite core) or 20% (iron powder core) relative to the inductance value when no current is applied. This DC bias current is called the saturation current of the inductor. Air core, ceramic core inductance is no saturation current.
10. DC impedance Rdc
The impedance value of the inductor refers to the sum (complex) of all the impedances under current, including the AC and DC parts. The impedance value of the DC part is only the DC resistance (real part) of the winding, and the impedance value of the AC part includes the reactance (imaginary part) of the inductor. In this sense, the inductor can also be considered an "AC resistor". The resistance value of the inductance when direct current is passed. The most direct influence of this parameter is the heating loss, so the smaller the DC impedance, the less the loss. Reducing Rdc is slightly in conflict with conditions such as size miniaturization. A product having a smaller Rdc may be selected from among the inductors satisfying necessary characteristics such as inductance and rated current described above.
11. Impedance frequency characteristics
The impedance of the ideal inductance increases with the increase of frequency, but the actual inductance is inductive at a certain frequency due to the existence of parasitic capacitance and parasitic resistance, and the impedance decreases with the increase of frequency, which is the turning frequency.

12. Curie temperature

The Curie temperature is an important parameter of the core, beyond which the ferrite core will lose its magnetism. Therefore, it should be noted that the operating temperature of the inductor cannot exceed the Curie temperature of the core. The magnetic permeability of the iron core generally rises rapidly when it is close to the Curie temperature, so the inductance value also rises, and the permeability of the Curie temperature drops to a very low level, thus causing the inductance value to drop rapidly. When the permeability drops to 10% at room temperature, its temperature is called the Curie temperature.

13. Test frequency

The test frequency is used to measure the inductance value or Q value of the inductor. The test frequencies commonly used in the industry include: 1KHz, 79.6KHz, 252KHz, 796KHz, 2.52MHz, 7.96MHz, 25.2MHz, 50MHz. The current trend is to use the frequency as the test frequency according to the customer.

14. Core loss

Core loss, referred to as iron loss, is mainly caused by eddy current loss and hysteresis loss. The size of the eddy current loss is mainly to see whether the core material is easy to "conductive"; if the conductivity is high, that is, the resistivity is low, the eddy current loss is high, such as the resistivity of the ferrite is high, the eddy current loss is relatively low. Eddy current loss is also related to frequency, the higher the frequency, the greater the eddy current loss, so the core material will determine the appropriate operating frequency of the core. Generally speaking, the working frequency of iron powder core can be up to 1MHz, while the working frequency of ferrite can be up to 10MHz. If the operating frequency exceeds this frequency, the eddy current loss will increase rapidly and the core temperature will also increase. However, with the rapid development of core materials, higher operating frequency cores should be just around the corner.

Another iron loss is hysteresis loss, which is proportional to the area surrounded by the hysteresis curve, that is, it is related to the swing (swing) amplitude of the current AC component; the larger the AC swing, the larger the hysteresis loss.

In the equivalent circuit of an inductor, a resistor in parallel with the inductor is often used to represent the iron loss. When the frequency is equal to SRF, the inductive reactance and the capacitive reactance cancel out, and the equivalent reactance is zero. At this time, the impedance of the inductor is equivalent to this iron loss resistance series winding resistance, and the iron loss resistance is much larger than the winding resistance, so the impedance at SRF is approximately equal to the iron loss resistance. Taking a low-voltage inductor as an example, its iron loss resistance is about 20kΩ. If the effective value voltage of 5V at both ends of the inductor is estimated, its iron loss is about 1.25mW, which also shows that the larger the iron loss resistance, the better.

15. Package structure (shield structure)

The packaging structure of ferrite inductors is non-shielded, semi-shielded with magnetic glue, and shielded, and there is a considerable air gap in either of them. Obviously, this air gap will have magnetic leakage, and in the worst case, it will interfere with the surrounding small signal circuit, or, if there is a magnetic material nearby, its inductance value will be changed accordingly. Another packaging structure is a stamping type iron powder inductor. Since there is no gap inside the inductor and the winding structure is solid, the magnetic field dissipation problem is relatively small. Fig. 10 uses the FFT function of RTO 1004 oscilloscope to measure the leakage magnetic field 3mm above and on the side of stamping inductor. Table 4 lists the leakage magnetic field size comparison of inductors with different package structures. It can be seen that the leakage magnetic field of non-shielded inductors is the most serious; the leakage magnetic field of molded inductors is the smallest, which shows that the magnetic shielding effect is the best. The leakage magnetic field of the inductors of the two structures differs by about 14dB, which is nearly 5 times.

16. Coupling (coupling)

In some applications, sometimes there are multiple groups of DC converters on the PCB, which are usually arranged adjacent to each other, and their corresponding inductors are also arranged adjacent to each other. If non-shielded or semi-shielded inductors with magnetic glue are used, they may be coupled to each other to form EMI interference. Therefore, when placing the inductor, it is recommended to mark the polarity of the inductor first, connect the winding point of the innermost layer of the inductor to the switching voltage of the converter, such as VSW of the buck converter, I .e. the moving point, and terminate the outgoing line of the outer layer of the inductor to the output capacitor, I .e. the static point; Therefore, the winding resistance of the copper wire is like forming a certain degree of electric field shielding. In the wiring arrangement of the multiplexer, the polarity of the fixed inductor helps to fix the size of the mutual inductance and avoid some unexpected EMI problems.