Analysis of inductor current| GETWELL

The design of inductor brings many challenges to engineers in the design of switching power supply. Engineers should not only choose the inductance value, but also consider the current that the inductor can bear, winding resistance, mechanical size and so on. The DC current effect on the inductor, which will also provide the necessary information for selecting the appropriate inductor.

Understand the function of inductor

The inductor is often understood as the L in the LC filter circuit in the output of the switching power supply (C is the output capacitor). Although this understanding is correct, it is necessary to have a deeper understanding of the behavior of inductors in order to understand the design of inductors.

In the step-down conversion, one end of the inductor is connected to the DC output voltage. The other end is connected to the input voltage or GND through switching frequency switching.

The inductor is connected to the input voltage through the MOSFET, and the inductor is connected to the GND. Due to the use of this type of controller, the inductor can be grounded in two ways: by diode grounding or by MOSFET grounding. If it is the latter way, the converter is called "synchronus" mode.

Now consider again if the current flowing through the inductor in these two states changes. One end of the inductor is connected to the input voltage and the other end is connected to the output voltage. For a step-down converter, the input voltage must be higher than the output voltage, so a positive voltage drop will be formed on the inductor. On the contrary, during state 2, one end of the inductor originally connected to the input voltage is connected to the ground. For a step-down converter, the output voltage must be positive, so a negative voltage drop will be formed on the inductor.

Therefore, when the voltage on the inductor is positive, the current on the inductor will increase; when the voltage on the inductor is negative, the current on the inductor will decrease.

The on-voltage drop of the inductor or the forward voltage drop of the Schottky diode in the asynchronous circuit can be ignored compared with the input and output voltage.

Saturation of inductor core

Through the peak current of the inductor that has been calculated, we can find out what is produced on the inductor. It is easy to know that as the current through the inductor increases, its inductance decreases. This is determined by the physical properties of the magnetic core material. How much inductance will be reduced is important: if the inductance is reduced a lot, the converter will not work properly. When the current passing through the inductor is so large that the inductor is effective, the current is called "saturation current". This is also the basic parameter of inductor.

In fact, the switching power inductor in the conversion circuit always has a "soft" saturation. When the current increases to a certain extent, the inductance will not decrease sharply, which is called "soft" saturation characteristic. If the current increases again, the inductor will be damaged. The decline of inductance exists in many types of inductors.

With this soft saturation feature, we can know why the minimum inductance under the DC output current is specified in all converters, and the change of ripple current will not seriously affect the inductance. In all applications, the ripple current is expected to be as small as possible, because it will affect the ripple of the output voltage. This is why people are always concerned about the inductance under the output current of the DC and ignore the inductance under the ripple current in the Spec.

The above is the introduction of inductor current analysis, if you want to know more about inductors, please feel free to contact us.