As an energy storage element, inductors are widely used in various modules of hardware circuits. The more common ones are DCDC circuit, filter circuit and oscillation circuit. For the selection of inductors, most people tend to only care about the inductance value. The larger the inductance value, the stronger the energy storage and the smaller the ripple. However, in addition to the inductance value, some other parameters of the inductor are also very important.
The following editor will talk about the understanding of other important parameters besides the inductance value when selecting an inductor.
1 Saturation current of the inductor
Inductors have magnetic saturation characteristics. Due to the characteristics of the magnetic material itself, the magnetic flux passing through it cannot increase infinitely. The magnetic flux passing through a certain volume of magnetically permeable material will not increase until it reaches a certain amount. At this time, no matter how much you increase the current or the number of turns, the magnetic flux will not increase, or the increase will be very small. When the current flowing through the inductor is greater than this current value, the inductance will drop significantly, and this current value at this time is called the saturation current of the inductor. In general practice, we call the current corresponding to a 30% drop in inductance the saturation current of the inductor. At the same time, it can also be seen from Figure 1 that the inductance of the same packaging material, as the inductance value increases, its saturation current decreases .
The current value corresponding to a 30% drop in the inductance value of a 0.47nH inductor is about 6A
2 The temperature rise current of the inductor
It mainly refers to the effective value of the current flowing through the inductor, which mainly causes the temperature of the inductor to rise and the inductance to drop significantly. The temperature rise is mainly caused by the DCR of the inductor. Generally, we define the current for the inductor to produce a 30-degree temperature rise at room temperature.
The current value corresponding to the 50-degree 0.47nH inductor is roughly 5.4A
The temperature rise current is often lower than the saturation current. At the same time, it can also be seen from Figure 1 that the inductance of the same packaging material, as the inductance value increases, the temperature rise current decreases.
As the inductance increases, the curve shifts to the left, and the temperature rise current decreases.
3 DC resistance
Generally, the greater the inductance value, the greater its DC resistance. Inductors used for low frequency signals have lower DC resistance than those used for high frequency signals and power supplies. DCR is generally milliohm level, but also a few ohms. The size of DCR will affect the heating power caused by the inductor current.
4 Q value
Also called the quality factor, it refers to the ratio of the inductive reactance presented by an inductor to its equivalent impedance when it operates at a certain frequency. The higher the Q value of the inductor, the smaller the loss and the higher the efficiency. The quality factor of the inductance is related to the DC resistance of the coil, the dielectric loss of the coil bobbin and the loss caused by the battery core.
5 Self-resonant frequency F0
The inductor itself has parasitic capacitance. The actual inductor can be equivalent to an ideal inductor connected in parallel with a capacitor. It is inductive at a certain frequency and capacitive above a certain frequency. The turning point of this frequency is the self-resonant frequency.
When the L is larger, the package is often larger, the occupied PCB area is larger, the DCR is larger, the heat generation is serious, and the efficiency will be low. The advantage is that the energy storage capacity mentioned at the beginning of the article becomes stronger, and the ripple will be smaller.
In the inductor specification book, we often wonder why the maximum value of the inductor saturation and temperature rise current is less than the typical value?
In fact, this doubt accompanied me for a long time when I first entered the mobile phone industry after graduation. At that time, it was discovered that the power inductors were selected on the project to sort out the inductor parameter values of each model.
Take the temperature rise current as an example, it can be understood as follows: the inductance usually has a tolerance, the range is ±20% , and the typical value refers to the temperature rise current value of the inductance at the nominal value . The maximum value refers to the temperature rise current value of the inductor when the inductance value is +20% . Because the larger the inductance value, the smaller the temperature rise current, so there is a maximum value that is smaller than the typical value. The maximum typical value here refers to the inductance value rather than the current value we are accustomed to think of, because the inductance value cannot be guaranteed to be 100% the value described in the specification when the manufacturer produces it .
It can be seen from the figure that when the inductance value is 22nH+20%, that is, the inductance value is the largest, the temperature rise current curve is shifted to the left, and the temperature rise current will also decrease at the same time. So MAX:22nH+20% current is less than Typ:22nH