How to choose power inductors for DC-DC converter ?

2024-12-15

First, let us learn about the Power inductors characteristics

Power inductors, or power chokes, are essential components in various electronic circuits, particularly in power supply units (PSUs) and other power management systems. They are designed to store energy in the form of a magnetic field and release it when needed, thereby smoothing out fluctuations in the power supply and ensuring stable current flow. Below is an overview of power inductors:

Basic Structure and Working Principle

Structure: Power inductors typically consist of a coil of wire wound around a ferromagnetic core, which may be made of materials like iron, nickel, or cobalt. The core enhances the inductance by providing a path for the magnetic field generated by the current flowing through the coil.

Working Principle: When a current flows through the coil, it generates a magnetic field around the core. This magnetic field stores energy and opposes changes in the current. When the current decreases, the magnetic field collapses, releasing the stored energy back into the circuit, thereby maintaining a stable current flow.

Types

There are several types of power inductors, each designed for specific applications:

Wirewound Inductors: These are the most common type and are constructed by winding a wire around a ferromagnetic core. They offer high inductance and are suitable for applications requiring high current handling capabilities.

Multilayer Inductors: These are constructed by stacking thin layers of ferromagnetic material and conductive traces. They offer better precision and stability and are suitable for applications requiring high frequencies and small sizes.

SMD (Surface Mount Device) Inductors: These are designed for surface mounting on printed circuit boards (PCBs) and offer high inductance and low profile. They are suitable for applications requiring high density and small size.

Applications

Power inductors are widely used in various electronic circuits and systems, including:

Power Supply Units (PSUs): They are used to smooth out fluctuations in the input power and ensure stable output voltage and current.

DC-DC Converters: They help in converting one DC voltage level to another by storing and releasing energy.

Motor Control Circuits: They are used to control the speed and torque of motors by regulating the current flow.

Communication Circuits: They are used to filter out noise and interference in communication signals.

Key Parameters

When selecting a power inductor, several key parameters need to be considered:

Inductance: This determines the ability of the inductor to store energy in the form of a magnetic field.

Saturation Current: This is the maximum current that can flow through the inductor without causing the core to saturate and lose its inductance.

DC Resistance (DCR): This is the resistance of the coil to the flow of DC current and affects the efficiency of the circuit.

Frequency Response: This determines how well the inductor performs at different frequencies.

The professional knowledge of power inductors

A coil is a generic name for an electrode in the shape of a spiral. Among the different types of coils, there are coils called “inductors” which are used for electrical applications. Inductors can be further categorized into RF inductors used for signal processing, and power inductors for power supply lines. The power inductors discussed in this section form part of the voltage conversion circuit in a DC-DC converter or other device.

Here we will explain the operation of a power inductor in a DC-DC converter. A power inductor is used in a step-up, step-down, or step-up/step-down circuit to convert a certain voltage to the required voltage. Among those different circuits, it is primarily used in a type of circuit called a “switching regulator.”

It uses an IC, power inductor, and capacitor to convert a DC input voltage to the required output voltage. The power inductor works with the capacitor to play the role of rectifying the rectangular wave output from the IC to a direct current.

If either one of these components is missing, the output cannot be properly rectified.

Basic characteristics of power inductors

Now what parameters should you pay attention to when selecting a power inductor? The power inductor catalogs of each manufacturer list the following principal specifications.

.

Inductance

Rated current

DC superposition rated current Isat

Temperature increase rated current Itemp

DC resistance Rdc

Operating temperature range

Power Inductor Basic Course

However, we don’t know the right specification for which power inductor to choose with only this information. For example, questions such as whether a high or low inductance is better, or what rated current is required, must be considered to appropriately choose an inductor according to the operating conditions of the DC-DC converter.  we will cover how to read power inductor specifications.

What is an Inductance?

The inductance value is an extremely important parameter which affects the ripple current and load response characteristics. As shown in Figure 1-3, an exponential current flows through a power inductor used in a DC-DC converter. Generally speaking, it is considered to be a good idea to set the ripple current ⊿IL to about 30% of the load current Iout. Therefore, once the DC-DC converter conditions are decided, you can calculate the proper power inductor inductance with the following equation.

However, most DC-DC converters list the proper inductance value as a reference. Therefore, you can also follow the manufacturer reference to select the power inductor without calculating the formula above.

Rated current

The rated current stipulates the current value at which the quality can no longer be guaranteed once the DC current exceeds it. The rated current of a power inductor stipulates a value for the DC superposition rated current (Saturation) and a value for the temperature increase rated current (Temperature). The meaning of each value is important, so in many cases they are listed as separate specifications.

DC superposition rated current Isat

One characteristic of an inductor is the DC superposition characteristic. To obtain a high inductance, ferrite and other magnetic materials are used in the core (magnetic core) of the inductor. When a current flows through the inductor, the phenomenon called the magnetic saturation of the magnetic substance occurs, and the inductance decreases. This characteristic is called “DC superposition.” The DC superposition rated current stipulates the current value when this inductance decreases at a constant rate with regard to the initial characteristic that the current is not superimposed on.

Temperature increase rated current Itemp

This is the rated current requirement which uses component heat generation as an index and use beyond this range will lead to component and set damage. Typically, it is defined as the current value where the temperature increase is ⊿40°C.

Now let’s look at how to determine these rated currents when used as a power inductor. As shown in Figure 1-3, the maximum current which flows through the inductor is Iout + ⊿IL/2. When this current value exceeds Isat, the decrease in inductance becomes quite large; the shape of the exponential current becomes abnormal as shown by the red line in Figure 1-4, and the ripple current increases. Because the ripple current causes the output voltage to fluctuate, it induces abnormal system behavior on the load side when the ripple current increases. Therefore, you must choose a power inductor with an Isat that is greater than the maximum current.

At the same time, with regard to the temperature increase rated current, the inductor is not immediately damaged even if the rated value is exceeded. Accordingly, you should select an Itemp value which is greater than Iout as a general rule.

Power Inductor Basic Course

DC resistance Rdc

The DC resistance is the resistance value when the direct current flows. Because power loss occurs through heat generation due to this resistance value, a lower DC resistance can reduce loss. However, reducing the Rdc involves a trade-off between the DC superposition characteristic and size miniaturization. Therefore, it is probably better to select an inductor with a smaller Rdc from among those inductors which satisfy the necessary characteristics such as inductance and the rated currents discussed above.

Operating temperature range

The operating temperature range stipulates the allowable range of ambient temperature when using an inductor. The effect of temperature can change even based on the circuit operating environment, so please select an inductor based on the expected practical use environment.

Now let’s take a look at an example of selecting a power inductor in an actual DC-DC converter. This example is based on the step-down DC-DC converter shown in Figure 1-5. It is expected to operate under the following conditions:

With the following equation, we see that the magnitude of the proper inductance is about 1.0 μH.

Moreover, because Iout = 1.5 A and ⊿IL is 0.45 A at about 30% of Iout, the maximum current is as follows:

Iout+⊿IL/2＝1.725A

Therefore, based on this result, we need an inductor with an Itemp of 1.5 A or more and an Isat of 1.8 A or more.

In conclusion, power inductors are crucial components in electronic circuits and systems, offering stable current flow and energy storage capabilities. They come in various types and sizes, each designed for specific applications and requirements. When selecting a power inductor, it is important to consider its key parameters and performance characteristics to ensure optimal performance in the desired application.

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