The quality factor (Q value) of the inductor is one of the important indicators to measure the performance of the inductor. It is generally defined as the ratio of the inductance's inductive reactance to its equivalent series resistance. The higher the Q value, the better the performance of the inductor and the smaller the parasitic parameters of the inductor.
Calculation of inductance Q:
Thereinto:
- XL is the inductive reactance of the inductor,
- R 是电感的直流电阻（DCR）。
Why is Q value an important indicator of inductor performance?
Let's take a look at the ideal model of inductor, and the actual model:
It can be seen that in the actual model, with more parasitic capacitance and equivalent series resistance, then the ideal inductance impedance is:
The inductor impedance in the actual model is:
The impedance is affected by frequency, by calculating the corresponding impedance at different frequencies, we can draw the impedance frequency curve with the help of computer-aided calculation software, here is the calculation and drawing of Excel, as follows, where the DCR of Zreal_1 is less than the DCR of Zreal_2, that is Zreal_1 the corresponding inductance Q value is higher, and it is less affected by DCR.
Since the calculation of the Q value is affected by the inductive reactance, that is, by the frequency, in practice, the inductive reactance at the resonant frequency point is commonly used to calculate the Q value, and the resonant frequency is:
Then the corresponding Q value is:
The following is an electrical parameter and impedance frequency curve on an inductor specification, which indicates the amount of inductance, DC series resistance, resonant frequency, and the estimation of parasitic capacitance can also be indirectly deduced from the impedance characteristic curve.
In the RLC series circuit, let's look at the output impedance vs. the total impedance of the circuit as a function of frequency:
The RLC series circuit is shown in the following diagram:
Then the ratio of the output resistance to the total impedance of the circuit is as follows:
When at the resonant frequency point, XL = Xc, the ratio is 1, and the total impedance of the resistor is R, the gain characteristic curve is plotted as shown below, where Q1 > Q2.
On the left side of the resonant frequency, the circuit as a whole is capacitive, on the right side of the resonant frequency, the circuit as a whole is inductive, and at the resonant frequency point, the circuit is purely resistive.
When comparing two inductors, if the inductance is the same, we need to pay special attention to the quality factor, from the quality factor can see the impact of the inductor manufacturing on the overall performance, and can also see the advantages and disadvantages of the inductor.