A Novel Power Inductor and Its Application for Compact, Large Current DC-DC Converters
Yunfan Zhang
Research and Development
POCO Holding Co., Ltd
Shenzhen, China
yunfan_zhang@pocomagnetic.com
Xiongzhi Guo
Research and Development
POCO Holding Co., Ltd
Shenzhen, China
tony@pocomagnetic.com
Guohua Wang
Research and Development
POCO Holding Co., Ltd
Shenzhen, China
guohua_wang@pocomagnetic.com
Qiang Xiao
Research and Development
POCO Holding Co., Ltd
Shenzhen, China
qiang_xiao@pocomagnetic.com
Abstract—This paper presents a novel power inductor forapplication in compact and large current DC-DC converters.This inductor uses a Fe-based alloy powder named NPA-F as themagnetic material and a straight, flat copper wire as the coil. Themagnetic material and fabrication procedure together enable theproposed inductor to exhibit excellent performance. The inductance at 100 kHz is approximately 173 nH with a DC resistance of 0.17 mΩ. The inductor’s performance is compared with a commercial ferrite power inductor in a 40-A buck converter. The volume of this metal inductor is only 43.9% of conventional ferrite inductor. Moreover, the maximum converter efficiency at 1.5 MHz when using the proposed inductor is 0.5% higher than using the ferrite inductor. In other words, this downsized power inductor is able to operate in high frequency applications and helps large current converters achieve higher efficiency. Keywords—power inductor, Fe-based alloy powder, large current, high efficiency, converter
I. INTRODUCTION
In order to achieve better performance than ever before,engineers are required to provide smaller power supply with higher efficiency for ICs, such as FPGA and GPU. [1][2] Reduced volume not only improves users’ experience, but also decreases costs resulted from manufacture and transportation. Pushing switching frequency to a higher level is a common practice to realize high power density. [3] It’s known that bulky passive components take up most space of a DC-DC converter and high frequency enables smaller inductance and capacitance [4][5]. However, operating at higher frequency leads to larger power consumption of power inductors and will in return affect the overall efficiency and thermal management. [6] Besides, FPGA and other ICs tend to operate at lower voltage and larger current. To meet all these requirements, power inductors should be able to work under large current with loss power loss.Generally, there are two main types of power inductors popular in the market: ferrite and metal composite. Possessing high permeability, it’s quite advantageous to select wire wound ferrite for high inductance applications. And most commercial ferrite inductors encapsulate the coil with magnetic powder and resin to reduce flux leakage. On the other hand, ferrite has small saturation flux density which limits size reduction of power inductors and also faces temperature reliability problems[7][8]. Metal composite type inductor is made of alloy powder and organic binders with coil molded inside and its saturation flux density is higher compared to ferrite. Unfortunately, the relatively large amount of organic resin lowers its permeability and organic resin may even degrade under high temperature, which leads to performance deterioration. Besides, its power loss performance is inferior to ferrite inductor due to larger DCR and higher core loss.
This paper presents a novel power inductor, featuring high saturation flux density, low loss, and high reliability, which can be used in compact and large current DC-DC converter serving for FPGAs and GPUs. It not only helps achieve high efficiency, but also reduce converter size significantly. In Section II inductor’s magnetic material and fabrication procedure are described. The characteristics of the inductor and its performance in a commercial converter are presented in Section III. Moreover, its comparison with ferrite inductor is also discussed. Section IV concludes the paper.
II. DEVICE DESCRIPTION
A. Magnetic Material
The proposed inductor adopts a Fe-based alloy powder named NPA-F from POCO as the magnetic material. This material plays an important role in optimizing magnetic components’ efficiency and volume due to its low loss and high saturation characteristics.
NPA-F powders are produced using the gas atomization method, featuring a spherical shape and less oxygen content. Good sphericity reduces the risk of insulation layer cracking upon compaction, ensuring low eddy current loss even working at high frequency region. High purity improves material’s
coercive force and hysteresis loss.
Shown in Fig.1 is the static magnetization curve of NPA-F measured by a vibrating sample magnetometer. It indicates a competitive saturation magnetization that is able to handle large current. Table 1 compares some typical soft magnetic materials. The core loss is tested using toroid cores by a B-H analyzer Iwatsu SY-8219. High Flux and Sendust are traditional metal powder core materials, while NPA-F’s power loss has been improved greatly compared with them. 3C95 and 3F46 are both MnZn ferrite from Ferroxcube. 3C95 is a low to medium frequency power material and is recommended to be used below 500 kHz. [9] Even though it has a similar power loss with NPA-F at 500 kHz, it gets much worse at 1 MHz. 3F46 is more suitable for high frequency application use of 1~3 MHz [10] and it has the best loss performance.
Thermal conductivity is an important factor indicating the material’s capability of heat removal and it will eventually affect inductor efficiency. According to [11], the thermal conductivity of alloy powder materials is more than ten times that of ferrite. Material with a low power loss intuitively may lose its superiority if the heat removal is less efficient.
B. Inductor Fabrication
Fig.2 shows the basic structure of this power inductor. It mainly consists of a metal alloy material NPA-F and an embedded single-turn copper winding. It is noted that the fabrication procedure is another key element to its excellent performance, which makes the best of the magnetic materials’ advantages. Fig.3 illustrates the fabrication process. First, 1 wt% insulation material is mixed with NPA-F powders acting as an electrical insulating layer as well as distributed air gaps. It is known that distributed gap cores hold better Bmax and DC bias at high temperatures. Then insulated powder particles are compacted together at 12 T/cm2 by a servo press with a 2.3 mm wide and 0.75 mm thick copper wire placed in the center. The copper wire is coated with ceramics and can withstand high temperatures. Third, the as-compacted inductor is annealed at 973K for one hour to relieve inner stress. The inductor’s dimensions are 11 mm×7 mm× 6.5 mm. The inductance at 100 kHz is approximately 173 nH with a DC resistance of 0.17 mΩ.
Even though the amount of insulating material is pretty small, it is enough to avoid inter-particle eddy current and minimize eddy current loss. Besides, fewer additives prevent the deterioration of thermal conductivity. Compared with commercial metal composite, this novel inductor adopts high pressure compaction and has no organic binders, which fully exhibits the superior characteristics of the magnetic powder material. Therefore, this inductor is able to achieve relatively high permeability, and only a single-turn winding is needed to realize the required inductance. Different from the traditional ferrite inductor, this integral structure leaves no space between copper and magnetic materials, contributing to high thermal conductivity as well as weakening the flux leakage problems.
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