Circuit inductor DC / DC selection
Only fully understand the role played by the inductor DC / DC circuit in order to better design of DC / DC circuit. Also included herein explanation of synchronous DC / DC asynchronous DC / DC and the concepts.
Many challenges inductor design engineers to bring in the design of switching power supply. Engineers not only to choose the inductance value, but also consider the inductance can withstand current, winding resistance, mechanical dimensions and the like. This article focuses on interpretation: DC current effect on the inductance. This will provide the necessary information to select an appropriate inductor.
Understand the function of inductance
An inductor is often understood as a switching power supply output terminal of the LC filter circuit L (C in which the output capacitance). While this understanding is correct, but in order to understand the inductor design must be a better understanding of the behavior of the inductor.
In a buck converter, the end of the inductor is connected to the DC output voltage. The other end is connected to the switching frequency of the switched input voltage or GND.
In the process of state 1, the input voltage is connected to the inductance through (high-side "high-side") MOSFET. In state 2 the process, the inductor connected to GND. Due to the use of such a controller can be implemented in two ways inductance ground: The ground through the diode (low-side "low-side") MOSFET or ground. In the latter embodiment, the converter is called "synchronous (the SYNCHRONUS)" mode.
Now think again in these two states current flowing through the inductor is that if change. In the process of state 1, one end of the inductor is connected to an input voltage, and the other end connected to the output voltage. For a buck converter, the input voltage must be higher than the output voltage, the forward voltage drop formed in the inductor. In contrast, during the state 2, the original end of the inductor is connected to the input voltage is connected to ground. For a buck converter, the output voltage of the positive terminal is bound, thus forming a negative voltage drop across the inductor.
We calculate the voltage across the inductor formula:
V = L (dI / dt)
Thus, when the voltage across the inductor is positive (state 1), the inductor current will increase; when the voltage across the inductor is negative (state 2), the inductor current will be reduced. Current through the inductor shown in Figure 2:
By the figure we can see, the maximum current flowing through the inductor DC current is applied to half peak current of the switch. The figure also called ripple current. According to the above equation, we can calculate the peak current:
Wherein, ton is the time of the state 1, T is the switching period (reciprocal of the switching frequency), DC 1 state duty cycle.
Warning: The above calculation assumes that the various components (voltage drop on the MOSFET, the inductor voltage drop or asynchronous circuit Schottky diode forward voltage drop) Comparative drop the input and output voltage is Ignored.
If the device is not negligible decrease, it is necessary for accurate calculation of the following equations:
Asynchronous conversion circuit:
Asynchronous conversion circuit:
Wherein, Rs is the sense resistor plus the impedance of the inductor winding resistance barrier. Vf Schottky diode forward voltage drop. R is Rs plus MOSFET on-resistance, R = Rs + Rm.
The inductor core saturation
The peak current through the inductor has been calculated, we can discover what produced the inductor. It is easy to know, with the increase in current through the inductor, its inductance will be reduced. This is due to the physical properties of the core material determined. Inductance reduces the number is very important: if the inductance is reduced a lot, the converter will not work normal. When the current through the inductor to a large extent the effectiveness of the inductor, the current at this time is referred to as "saturation current." This is also the basic parameters of the inductor.
In fact, there is always inductive switching power conversion circuit has a "soft" saturation. To understand this concept can be seen Vs DC inductor current curve actually measured:
When the current is increased to a certain extent, the inductance would not have fallen sharply, which is called "soft" saturation characteristics. If the current is further increased, the inductor can be damaged.
Note: inductance drop will exist in many classes of the inductor. For example: toroids, gapped E-cores and the like. However, rod core inductors there would be no change.
With this soft saturation characteristics, we can know the minimum inductance Why are all of the provisions under DC converter output current; and because of the ripple current changes will not seriously affect the inductance. In all applications want ripple current as small as possible, because it will affect the output voltage ripple. This is why we are always very concerned about the amount of inductance in the DC output current, and ignores the lower inductance ripple current in the Spec.
