Only by fully understanding the role of the inductor in the DC/DC circuit can the DC/DC circuit be better designed. This article also includes an explanation of the concepts of synchronous DC/DC and asynchronous DC/DC.
The design of inductors in switching power supply design presents many challenges for engineers. Engineers must not only choose the inductance value, but also the current that the inductor can withstand, winding resistance, mechanical dimensions, and so on. This article focuses on explaining the effect of DC current on the inductor. This will also provide the necessary information for selecting the right inductor.
Understand the function of the inductor
Inductance is often understood as the L in the LC filter circuit in the output of the switching power supply (C is the output capacitor therein). Although this understanding is correct, in order to understand the design of the inductor, it is necessary to understand the behavior of the inductor more deeply.
In step-down conversion, one end of the inductor is connected to the DC output voltage. The other end is switched to the input voltage or GND by the switching frequency.
During state 1, the inductor is connected to the input voltage via a (high-side "high-side") MOSFET. During state 2, the inductor is connected to GND. Due to the use of this type of controller, inductive grounding can be achieved in two ways: through diode grounding or through (low-side "low-side") MOSFET grounding. If it is the latter way, the converter is called "synchronus" mode.
Now consider the current flowing through the inductor in these two states if it changes. In State 1, one end of the inductor is connected to the input voltage and the other end is connected to the output voltage. For a buck converter, the input voltage must be higher than the output voltage, and therefore a positive voltage drop across the inductor. In contrast, during state 2, one end of the inductor originally connected to the input voltage is connected to ground. For a buck converter, the output voltage must be positive, and a negative voltage drop can be created across the inductor.
We use the voltage calculation formula on the inductor:
V=L(dI/dt)
Therefore, when the voltage on the inductor is positive (state 1), the current on the inductor increases; when the voltage on the inductor is negative (state 2), the current on the inductor decreases. The current through the inductor is shown in Figure 2:
From the above figure, we can see that the maximum current flowing through the inductor is half of the DC current plus the switching peak-to-peak current. The above figure is also called ripple current. According to the above formula, we can calculate the peak current:
Among them, ton is the time of the state 1, T is the switching cycle (reciprocal of the switching frequency), DC is the duty cycle of the state 1.
Warning: The above calculation assumes that the voltage drop across the various components (on-voltage drop across the MOSFET, the turn-on voltage drop across the inductor, or the forward voltage drop across the Schottky diode in the asynchronous circuit) is comparable to the input and output voltages. Ignored.
If the drop of the device is not negligible, the following formula should be used for accurate calculation:
Synchronous conversion circuit:
Asynchronous conversion circuit:
s is the impedance of the sense resistor plus the inductor winding resistance. Vf is the forward voltage drop of the Schottky diode. R is Rs plus MOSFET on resistance, R=Rs+Rm.
Inductor core saturation
From the calculated peak inductor current, we can see what is happening on the inductor. It is easy to know that as the current through the inductor increases, its inductance decreases. This is due to the physical properties of the core material. It is important to reduce the amount of inductance: If the inductance is reduced a lot, the converter will not work properly. When the current through the inductor is as large as the inductor is effective, the current at this time is called the "saturation current." This is also the basic parameter of the inductor. In fact, the switching power inductor in the converter circuit always has a "soft" saturation. To understand this concept you can observe the actual measured inductance Vs DC current curve:
When the current increases to a certain extent, the inductance will not drastically drop. This is called "soft" saturation. If the current increases again, the inductor will be damaged. Note: The drop in inductance will exist in many types of inductors. For example: toroids, gapped E-cores, etc. However, rod core inductance will not have this change. With this soft-saturation feature, we can know why in all converters, we will specify the minimum inductance at the DC output current; and because the change in ripple current will not seriously affect the inductance. In all applications, it is desirable that the ripple current be as small as possible because it will affect the output voltage ripple. This is why everyone is always concerned about the inductance at the DC output current and will ignore the inductance at the ripple current in Spec.
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