In recent years, the performance of switched-mode DC-DC converters—sometimes called regulators—has improved dramatically. Efficiency has increased, often reaching 90 percent and higher. Quiescent current has decreased, while rail-output noise and radiated EMI have decreased, and transient response has sped up. Much of this improvement comes from advances in DC-DC controller ICs, which now address multiple limitations seen in earlier designs. Meanwhile, the availability of enhanced power devices with lower static and dynamic losses has been met with improvements in power-device drivers.
However, these innovative ICs are only part of the story. There’s a less obvious component that contributes to improved performance: the discrete inductor, which is mandatory in every switching-converter design. This two-terminal passive component—defined by its inductance value—appears simple, at least at first. However, the reality is that there are many considerations associated with inductor selection.
Inductors are critical to overall converter efficiency, EMI performance, and thermal stability, with the first two of those factors increasingly defined by mandates and regulations. As is usually the case, there is no single “best” choice of inductor, since the selection must be balanced with tradeoffs among operating frequency, core losses, saturation behavior, and physical size constraints, to name a few.
The chosen inductor must provide the required performance, but it must also minimize design-in obstacles such as unexpected EMI issues, thermal instability, or production-related risks. The challenge to strike this balance can be overcome with Würth Elektronik WE-MAPI inductors, which mitigate these hurdles by offering stable performance, long-term availability, and design support through modeling tools.
This blog discusses how intelligent inductor selection and modeling tools boost DC-DC converter performance and reliability.
Modeling Steers the Selection
Inductor selection begins with key parameters such as nominal inductance value and operating frequency, as an inductor that is well-suited for a 500kHz switching frequency may not be a good fit for a 2MHz design. Other top-tier parameters that characterize the inductor are current-handling capabilities, efficiency targets, thermal drift, operating temperature, EMI considerations, possible shielding, and physical size.
Balancing the “must have” and “nice to have” objectives can be frustrating due to the number of variables and their often non-linear interactions. That’s where Würth Elektronik’s REDEXPERT tool saves designers time while showing them viable options. Among other considerations, REDEXPERT facilitates AC- and DC-loss modeling and optimal inductor selection for the WE-MAPI family of inductors (Figure 1).
This powerful tool is much more than just a computer-based version of the standard inductor-loss model based on the Steinmetz equations and its extended variations, as those are inadequate for use in switching-converter design. Their shortcomings include the inability to properly capture effects factors, such as time-related changes in flux density, AC losses of the windings, losses in the component windings due to skin and proximity effects.
In contrast, REDEXPERT uses a family of empirical data models, each calibrated to the specific performance characteristics of Würth Elektronik’s inductors. It allows users to effectively select the inductor and optimize the system. To do this, REDEXPERT divides the total loss of the inductor into two separate losses. There is power dissipation due to DC current in the inductor windings (DC loss) and the additional power loss that is due to AC flux swings in the coil (AC loss). It also provides temperature estimates based on calculations of the complete AC losses.
REDEXPERT supports the three most common DC-DC converter topologies: buck, boost, and single-ended primary-inductor converter (SEPIC). The model is accurate over a wide frequency range (10kHz to 10MHz) with the constant of the power equation derived over a wide range with respect to the flux swing. It accommodates even small changes in core material and winding structure and is valid for components with more than one material. Furthermore, REDEXPERT accurately estimates losses of iron powder and metal alloy materials, is valid for any core shape and winding structure, and includes AC winding losses.
Why Engineers Rely on WE-MAPI Inductors
In addition to the WE-MAPI family’s superior performance, it also reduces risk and associated headaches for designers. To keep a project on track, engineers typically avoid new components with limited field history, opting instead for well-understood, widely adopted solutions. For example, with active components, many designers still choose to use op-amps, which are ten years old (or older), even though there are “better” options on the market. The reason for this is simple: these older components are fully understood and characterized, their subtleties are known, and any manufacturing problems have been resolved.
In the case of a new switching-converter design, designers can select a newer controller IC that offers advanced features and performance while still relying on proven passive components. The reality is that every component has its idiosyncrasies, which are well-known for components that have been “in the field” for many years. In contrast, recently released components can introduce unexpected issues that are not called out on datasheets or application notes.
Established components also bring predictability in sourcing and manufacturing, which helps teams avoid supply chain surprises and stay on schedule.
Technology Enables Inductor Innovation
Due to their design and multiple innovations, the WE-MAPI inductors from Würth Elektronik are well suited for telecom and server power supplies. This suitability is due to their efficiency under high loads. They are also a good fit for FPGA and AI processor power as a result of their low-voltage, high-current regulation performance.
These inductors are much more than just wire coil wound on a suitable core. Advanced iron-alloy group material compositions are an improvement over ferrite materials and have significantly reduced the core-material losses for high-current power inductors.
The WE-MAPI core consists of an innovative metal alloy pressed around the winding. As a result, the series offers high inductance values in a small, shielded package. These components are available in package sizes from 1.6 × 1.6 × 1.0 millimeters (mm) up to 5.4 × 5.4 × 3.1 mm, with inductors ranging from 33 nanohenries (nH) to 47 microhenries (uH) and saturation current up to approximately 40 amps, depending on the specific model.
The special construction of the core provides the desirable magnetic self-shielding effect. The core material is temperature stable with only a slight drift and “soft” saturation behavior. A protective coating layer is also applied around the core, making its surface resistant to detrimental environmental influences.
Physical construction and the winding attachment are other critical factors. Conventional coils generally use enameled copper wire that is wound around the core and soldered or welded to the terminal with a clip. The outer shielding ring is then mounted and bonded with the inner core and winding.
However, in the WE-MAPI series, the winding makes direct contact with the component’s connection pad without soldering and welding (Figure 2). Eliminating the clip means that the effective diameter is increased, so fewer windings are needed for the same inductance values. This greatly reduces DC resistance of the winding, thus lowering DC losses.
Many of today’s switching converters operate at higher frequencies to reduce overall size, among other advantages. While earlier switches operated at 300kHz to 500kHz, frequencies of one megahertz are now in widespread use, and some converters even go to several megahertz. The WE-MAPI series is optimized for use in designs operating at 800kHz and higher, making them a good fit for these higher-frequency converters.
Further, these inductors can handle the high ripple current associated with these high-efficiency DC-DC converters. They are available for a standard operating temperature range of -40°C to +125°C, with some models also available for the wider -55°C to +150°C range for increased reliability in harsh environments.
Conclusion
Among a growing range of options, WE-MAPI inductors from Würth Elektronik stand out for their field-proven reliability, efficient performance, and construction tailored to the demands of high-frequency power designs. Their performance has been validated across real-world applications, making them a dependable choice for engineers looking to reduce design risk while meeting rigorous efficiency, thermal, and EMI requirements.
Author
Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.
