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With the growing demand for servers to address ever-increasing data processing requirements, reduced energy consumption provides a major driving force to the key performance issues confronting server power supply designers. Energy efficiency is extremely important but by no means the only design consideration that needs to be addressed to select the right high current inductor for a particular application.
This white paper discusses the key performance targets of high current inductors for use in Multi-phase and Vcore regulators, Voltage Regulator Modules (VRMs), Point-of-Load, and memory power applications. It also highlights the key electrical and mechanical parameters that engineers should be addressing to select the optimum product. Finally, the paper summarizes the Eaton high current product line capabilities and provide access to additional specifications and sample information.
With efficiency requirements dictated by regulations from governments around the world as well as the need to reduce the carbon footprint of their products, server power supply designers are under extreme pressure to maximize the highest efficiency possible. Improving efficiency and reducing power losses in server power systems are also required for strictly competitive reasons.
The high current inductor is among the components that can deliver improved efficiency once the design is optimized. In addition to the higher current, higher operating temperature and higher frequency requirements needed to obtain higher efficiency other important selection criteria for power inductors include:
Eaton’s Electronics Division is a leading supplier of high current and other inductors for DC-DC power conversion and switch-mode power supply applications requiring high frequency operation. Eaton designs and develops inductor products to achieve high efficiency using high frequency ferrite material to satisfy light load and full load efficiency requirements for VR13HC and VR14 applications. Eaton’s products designed specifically for these applications are widely used in several IC reference designs.
Key parameters for assessing an inductor’s performance include:
It should be clear from the foregoing, that optimizing the selection of high-current inductors requires evaluation of a wide assortment of physical and electrical variables. While all customers desire higher efficiency and lower losses, all customer applications are not the same. Thermal management, space constraints and other aspects make the optimal choice of a power inductor a nontrivial task. At the root of this process for Eaton’s inductor designers is the selection of the core material.
For high current, the two most common inductor core materials are ferrite and powder. Selection of the best core material occurs based on inductance, tolerance of the inductance, current rating, saturation current, shielded or non-shielded, DCR, Q at certain frequencies, self-resonant frequency, operating temperature, mounting type and package dimensions required for the application. It should be noted that both electrical and physical attributes are important in this selection process.
Understanding application trade-offs is key to device selection and hence material selection for inductor designers. Powder core devices exhibit better temperature stability and Bsat. So, in some cases for higher power density, a powder core may be preferred but this choice sacrifices efficiency. Ferrite is the most efficient material used in high power inductors. High efficiency inductors are made by developing inductor core materials that produce high saturation flux density with low core loss for operation from 300 kHz to 1.0 MHz and higher. However, higher current operation requires higher saturation flux density (Bsat) materials. Higher ripple current and frequencies result in higher core losses. Eaton’s inductor designers understand these materials and performance trade-offs well to maximize overall device performance.
Ferrite advantages in Server Power applications, compared with powder core, are: 1) Higher operating frequency (3MHz now, up to 10MHz coming); 2) Much lower core loss for a given frequency and AC flux density resulting in higher efficiency; and 3) Higher initial permeability (Ui), up to 3300 (10-20 times powder material), to achieve higher OCL.
An important comparison that system designers find on a specific data sheet is the change in core loss versus increasing Bp-p. In Figure 1, core losses increase with increasing Bp-p and with increasing frequencies. As demonstrated in this example, as an inductor’s total losses increase the devices temperature will also increase. Increasing temperature also increases the DCR value.
Eaton’s magnetics product line offers a wide selection of inductors with different inductance and current values and advanced packaging to meet today’s demand for high current power supplies for the server and memory industry. The success of these products is based on the continuous investment and resulting outstanding capabilities of design, manufacturing and test facilities within Eaton’s global group. The facilities have a combination of certifications and recognitions which include:
In addition to those products that have established Eaton’s product line as an industry leader, our research and development efforts with leading IC manufacturers and OEM system companies have created new and exciting solutions to address industry trends and the need for ever-increasing efficiency.
The electronics industry’s need for high efficiency and high power density in a smaller area is driving inductor suppliers to reduce the high current inductor’s form factor overall size. To meet this need, factors that impact the inductor’s design include:
Eaton addresses the latest design criteria with several new inductors to allow designers achieve an optimum solution.
Although the inductor is rarely used in a current sense application, tight DCR tolerance remains a desired product feature. Tighter DCR and OCL tolerances provide designers with important benefits in assuring circuit stability and consistent performance from lot-to-lot in volume manufacturing. Achieving this level of precision is the result of the combination of several factors:
All of these aspects are addressed in Eaton’s industry-leading approach to high-power inductor design and manufacturing.
For Eaton, an application-specific approach allows the selection of the right inductor for each application. This can involve modifying an existing design to fit a customer’s needs. While this is not uncommon in the inductor business, the differentiation occurs with the experience and expertise of the inductor supplier.
For new product design, or a simple derivative of an existing product, with the customer’s input regarding the application specifics (including AC current, DC current, operating frequency, operating temperature and more) and an interactive dialog, our design experts select the appropriate product or core material to meet the application operating requirements. The interactive process with proposed prototypes and testing in the customer’s circuit and modification, if necessary, occurs until the highest efficiency/optimized solution is achieved.
Knowing whether the core material and other design factors will properly address a specific application is part of the expertise derived from Eaton’s long history and breadth of products for highfrequency power supply applications.
Reducing the size of the inductor (item 1, above) to extremely small (a few millimeters on each side) dimension is desired, even as the saturation current per phase is increasing. This requirement demands high flux density material with lower core loss at high frequencies.
To satisfy the industry’s requirement for smaller size, Eaton has developed a ferrite-based series of 4 mm by 4 mm and 5 mm by 5 mm inductors with varying height. The small size allows designers to include more content/new features on the PCB and the inductor’s low fringing flux enables a high-density layout. The height ranges from 3 mm to 6.6 mm. See Figure 4 for other product capabilities.
Folding the leads under the inductor (Figure 5 (a)) increases the gap from 0.2 mm to 0.3 mm. This initially allowed mounting the inductor over traces on the PCB. The industry followed Eaton’s lead and adopted this design approach. The deep mold channel (Figure 5 (b)) is another innovative Eaton solution that achieves a gap of as much as 0.8 mm.
In addition to mounting the power stage under the inductor, which is increasingly popular with point of load (POL) applications, other options include a chip resistor or chip capacitor placed under the inductor.
The actual space required for the cutout depends on the customer application and Eaton has been able to modify this design for this purpose. Figure 6 shows the capabilities of the industry’s newest package.
To address the increasing need for greater system efficiency, higher power density and other current trends, Eaton’s inductor experts have developed and released the aforementioned new products. These new products are being rapidly adopted and becoming the next generation of standard products in the marketplace.
Additional products that designers can expect to see in the near future include: swing choke, hybrid ferrite with composite magnetic materials, ultra-low DCR inductors, and more. While each addresses a different design issue and many are covered by patents, these inductors all target greater system efficiency in high-current power applications.
A major task for a system design engineer is system design validation. Device selection is an important part of this step and sometimes system engineers will compare inductors from different manufacturers to determine which part to choose. Although comparison testing is good engineering practice, it can lead to misleading results. This is because each product family is designed and optimized for a targeted application, i.e. operating frequency range, ripple current, and/or DC current.
Each product family incorporates voice of the customer requirements such as inductance versus DC current at different temperatures, core loss at application conditions, DC resistance requirements, temperature rise versus total loss and estimated total loss at application conditions. Typically there are performance trade-offs amongst these requirements resulting in different products utilizing different materials and/or processes, each optimized for its application. Therefore, comparing 150 μH parts of similar size from two different manufacturers, for example, may not result in the correct conclusion. At Eaton, we strongly recommend discussing your application with our Field Application Engineers and Internal Application and Design teams to ensure you are selecting the best product for your application.
Satisfying the server power requirements for increased efficiency, higher power and reduced footprint and other critical design parameters requires exceptional design and manufacturing expertise for power inductor suppliers. In addition to the obvious design criteria of AC current, DC current, frequency and temperature that drive core material and other inductor design aspects, there are subtle applications issues that must be addressed. Many times system designers are not fully aware of these issues and are not asked the right questions.
Based on the numerous times that Eaton has worked with unusual design requirements, the iterative, interactive process used by Eaton’s inductor experts brings out these design aspects to ensure the proper, optimized application-specific inductor solution.
