Storage Inductors for Energy-Efficient Applications

This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.

Energy-efficient devices are crucial for conserving resources and protecting the environment. The more efficient the electronics, the longer the battery life for mobile devices, and the lower the energy demand in large industrial and server facilities. The power supply significantly influences the foundation of energy-efficient devices. While linear regulators were the most used voltage regulators in the past, modern power electronics circuits now use switching power supplies. The continuous reduction of processor voltages has contributed to this shift. A few years ago, switching frequencies up to 300 kHz was common, but today, modern switching regulators based on GaN and SiC transistors typically operate at frequencies in the MHz. Switching losses, on the one hand, and particularly the losses of the storage inductor in this high-frequency range, on the other hand, are critical aspects in the design of switching power supplies.

In addition to energy efficiency, increasing energy demand is becoming increasingly important. Computers are becoming more powerful, which, in turn, requires more powerful power supplies. This means switching power supplies must deliver higher currents, and consequently, power inductors must have significantly greater current-carrying capacities. Achieving this capability is further complicated by the additional trend of miniaturization. Switching power supplies must become smaller and more compact while delivering the same or even higher power in a reduced volume. This increases the demands on the power density of the inductor.

To meet these requirements, continuous research is conducted on new material mixtures of iron alloys to further reduce core material losses in high-current storage inductors. The WE-MXGI series has been developed based on this, combining the best possible power density and current-carrying capability with the lowest R_DC and minimal self-losses, thanks to smart material selection and manufacturing technology.

Power supply designers are supported by the REDEXPERT online design platform, which allows the determination of DC and AC losses of storage inductors with unprecedented accuracy. This is achieved through a measurement-supported process that enables significantly more accurate core loss calculations than would be possible with the Steinmetz formulas.

Image used courtesy of Bodo’s Power Systems [PDF]

WE-MXGI Storage Inductor Overview

The WE-MXGI storage inductor is Würth Elektronik’s latest coil series in the molded storage inductor group. In conventional ferrite chokes, the copper wire is typically wound around the core and soldered or welded to the terminal. The outer shielding ring is assembled and bonded with the inner core and winding. Unlike a ferrite choke, the core powder consists of an innovative iron alloy that is molded around the winding, giving the WE-MXGI high inductance values in a small form factor. The unique core construction provides a self-shielding effect.

The core material is temperature-stable, showing no signs of thermal aging, with soft saturation behavior and minimal saturation drift over a wide temperature range. It also has high dielectric strength, enabling an operating voltage specification of 80 V. An explanation of how Würth defines the operating voltage can be found in Application Note ANP126. An additional protective layer is applied to the surface to make the core resistant to environmental influences and rust formation.

Most of the molded inductors in the market still contain a clip to which the winding is welded. In contrast, the WE-MXGI uses a direct contact method, eliminating soldering and welding processes by directly connecting the winding to the component’s connection pads. By eliminating the clip, the space within the core material is optimized, allowing for a larger coil diameter and the use of thicker copper wire. This results in a significantly reduced DC resistance (R_DC) of the winding (Figure 1).

Figure 1. The direct contact method of the WE-MXGI enables low RDC values. Image used courtesy of Bodo’s Power Systems [PDF]

In applications, the start of the coil winding is usually connected to the switching node of the switching regulator, and the component is marked accordingly. This reduces coupling effects and disturbances from the switching node, which is shielded by the winding. Due to the optimized wire geometry of the WE-MXGI, based on round wire, this shielding effect is made possible. Products based on flat wire, commonly found in the market, do not have this effect (Figure 2).

Figure 2. A self-shielding winding and core construction ensures improved EMC performance. Image used courtesy of Bodo’s Power Systems [PDF]

The WE-MXGI series is available in sizes 4 x 4 x 2 mm³ and 5 x 5 x 3 mm³, with continuous expansion planned (Figure 3).

Figure 3. Overview of the available sizes and products of the WE-MXGI inductor series. Image used courtesy of Bodo’s Power Systems [PDF]

Storage Inductor Losses

The losses in a storage inductor consist of core material losses and winding losses. The loss mechanisms are detailed in Application Note ANP031. A summary is provided below. Winding losses can be divided into DC losses, primarily influenced by the DC resistance RDC of the winding (Equation 1), and AC winding losses, resulting from the skin and proximity effects.

P = I² · RDC (Equation 1)

There are several methods to determine the AC losses of the winding; for example, the Dowell, Ferreira, or Nan/Sullivan methods.

The significance of losses in modern switching regulators can be determined with a simple setup and measurement of the corresponding losses. For example, a buck converter with an input voltage of 24 V is used. The output provides a voltage of 6 V at a current of 8 A. The switching frequency is 1 MHz. In the comparison shown in Figure 4, a 2.2 µH inductor from the WE-MXGI 5030 series was measured and compared with a similarly sized inductor. It is evident that both the ACDC losses of the WE-MXGI are lower than those of the competing products.

Figure 4. AC and DC loss components of a 2.2 µH coil (WE-MXGI) in a buck converter with 24 V input, 6 V output, 8 A output current, and a switching frequency of 1 MHz, compared to another coil. Image used courtesy of Bodo’s Power Systems [PDF]

In switching regulators, the coil is one of the most important components. Therefore, accurately determining losses and temperature rise is critical in selecting the right component. To predict temperature rise, AC losses must first be accurately determined.

One approach is the Steinmetz models, which offer an acceptable approximation, particularly for sinusoidal excitations and a 50% duty cycle. 

The AC loss calculator in REDEXPERT includes a model to determine the total AC losses in inductors precisely. This model is based on empirical data obtained from a real-time application setup where the total losses of the inductor are divided into AC and DC losses.

The empirical data is collected using a DC/DC converter. A pulsating voltage is applied to the inductor, with input power P_in and output power P_out measured. Based on this, P_loss = P_in - P_out is determined, and the system losses, the DC losses, and the AC losses of the inductor P_AC are separated. This process is measured for various parameter settings – such as variations in magnetic flux, switching frequency, ripple current, etc. – with all the data recorded. Using the empirical data, a model for calculating AC losses is created as a function of the test conditions (Equation 2).

P_AC = f(∆I, freq, DC, k1, k2) (Equation 2)

AC Loss Model Advantages

The AC loss model has been extensively validated and compared with existing models and measured data. AC losses for various materials, such as WE-Superflux, iron powder, NiZn, MnZn, etc., have been measured over large duty cycles and frequency ranges and compared with theoretical models (Figure 5).

• Empirical data is based on a DC/DC converter
• Accurate determination of losses for any given duty cycle
• Accurate over a wide frequency range (10 kHz to 10 MHz)
• Considers even the smallest changes in the core material and winding structure
• Applicable to components with more than one material
• Accurate determination of losses in components with iron powder and metal alloys
• Valid for any core shape and winding structure
• Includes AC winding losses

The diagrams show the core losses determined by the Steinmetz power equation (P_st), Modified Steinmetz Equation (P_mse), and Generalized Steinmetz Equation (P_gse). In REDEXPERT, the AC loss is marked after calculation with the Würth AC loss model. “Real” represents the measured AC loss.

Image used courtesy of Bodo’s Power Systems [PDF]

Figure 5. AC losses in MnZn and iron powder core materials at a 33% duty cycle, as calculated by various Steinmetz models, simulated with REDEXPERT, and measured in reality. Image used courtesy of Bodo’s Power Systems [PDF

Selecting WE-MXGI with REDEXPERT

The WE-MXGI storage inductors, with their innovative core material and thoughtful design, are optimized for maximum power and efficiency in the smallest possible space, making them ideal for modern switching converters. For energy-efficient switching regulators, the appropriate WE-MXGI storage inductor is best selected using REDEXPERT (Figure 6). It integrates the world’s most accurate AC loss model, achieving high accuracy over various parameters such as frequency, ripple current, and duty cycle. Moreover, REDEXPERT suggests suitable products once the required parameters of a customer application have been entered.

Figure 6. Simulation of a DC/DC buck converter in REDEXPERT using WE-MXGI components. Image used courtesy of Bodo’s Power Systems [PDF]

Würth Elektronik’s current rating calculator helps select the appropriate product, including a thermal model of each inductor based on measurement data, to determine the rated current depending on PCB dimensions. An explanation of the thermal behavior of power chokes can be found in Application Note ANP096.

This article originally appeared in Bodo’s Power Systems [PDF] magazine.