Patent Application
20100060404
1. Field of the Invention
This invention relates generally to an inductor including multiple center core pieces providing multiple gaps and, more particularly, to an inductor including end core pieces and multiple center core pieces providing multiple gaps where the end core pieces are made of an amorphous alloy to provide good magnetic properties and the center core pieces are a stamped plate laminate structure to provide ease of manufacturability.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source or rechargeable electrical energy storage device (RESS) in addition to the fuel cell stack, such as a high voltage DC battery, a super-capacitor or an ultracapacitor. The RESS provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to an electric traction motor through a DC high voltage bus line for vehicle operation. The RESS provides supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge RESS at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the RESS.
In the hybrid vehicle discussed above, a boost DC/DC converter is sometimes employed to match the lower voltage fuel cell stack to the higher voltage RESS. DC/DC converters often employ a multi-phase array of inductors that provide the task of increasing the DC voltage. A typical inductor in the inductor array includes a magnetic core, such as an iron core, where a gap is provided between core pieces. Isolated metal windings are wrapped around the core as a coil and a current propagating through the windings generates a magnetic flux in the core and the gap. Suitable switching is provided to switch the voltage applied to the windings so that the magnetic flux change increases the voltage potential at the output of the converter.
There has been an effort in the industry to reduce the size, weight and cost of DC/DC converters in fuel cell systems for vehicles, and increase their reliability and efficiency.
In accordance with the teachings of the present invention, an inductor is disclosed that includes end core pieces and a plurality of center block core pieces between the end core pieces that define a plurality of gaps. The end core pieces are made of, for example, an amorphous alloy to provide good magnetic properties and the center core pieces are, for example, stamped electrical steel sheets forming a laminate structure to provide ease of manufacturability. In one non-limiting embodiment, the end core pieces are an amorphous iron alloy and the center core pieces are stamped silicon-iron.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to an inductor including end core pieces made of an amorphous alloy and center core pieces providing multiple gaps formed of stamped metal sheets is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the inductor of the invention has particular application for an inductor in an inductor array for a DC/DC boost converter in a fuel cell system. However, as will be appreciated by those skilled in the art, the inductor of the invention may have application for other systems.
A simplified DC/DC converter 26 is provided to boost the fuel cell stack voltage to the higher voltage of the bus lines 16a and 16b. The DC/DC converter 26 includes an inductor array 28 and a controlled switching network 30, where the switching network 30 switches inductors 38 in the inductor array 28 from high voltage ground to high voltage output in order to boost the fuel cell stack voltage in a manner that is well understood to those skilled in the art. This conversion method is commonly known as boost conversion. Boost converters can be built using a single inductor or by using a plurality of time shifted switching inductors in parallel boost stages to smooth the switching ripple effects. The DC/DC converter 26 provides a 3-phase boost converter design for this purpose.
Because the gap 46 defines the amount of magnetic energy that will be stored in the inductor, it is often desirable to make the gap 46 wide. However, as the width of the gap 46 increases, a fringing flux at the corners of the U-shaped pieces 42 and 44 increases where the magnetic field lines extend away from the core 40. The fringing flux creates eddy current losses in the core material and windings and adds an efficiency loss to the system.
In order to overcome the drawback of fringing flux, and still provide significant gap space for high power applications, it is known in the art to provide more than one gap in an inductor core.
Traditional cores for inductors of this type are typically laminated pieces of iron or other ferromagnetic material that provide good magnetic field properties. In order to reduce the losses in the core material, it is known in the art to use an amorphous alloy material that has superior magnetic properties to regular iron. However, the amorphous alloy cannot economically be manufactured into as small of a size as the pieces 56, 58, 60 and 62 would need to be for a fuel cell system boost converter application using standard low cost manufacturing and segmenting cutting processes because of the materials brittleness.
Various amorphous alloys are known in the art for magnetic cores in an inductor. These amorphous alloys include alloys made from iron, nickel, cobalt, etc. The core material needs to be manufactured in a highly controlled manner to produce an amorphous structure, i.e., non-crystalline, that provides the desired magnetic properties. In one known technique, a molten alloy is cast through a nozzle onto a rotating cooled copper wheel, which causes a thin layer of the alloy to immediately solidify and peel off of the wheel in a thin endless ribbon. The solidification process is so fast that the alloy cannot form crystals. These ribbons have excellent magnetic properties, but because they are thin and have low electrical conductivity due to their amorphous structure, eddy current losses in the material is extremely small. Therefore, those materials can be used in high power density, high frequency applications where regular iron core laminates would overheat from the losses. The ribbons are wound on a mandrel with traces of glue in between to get a solid O-shaped core. The O-shaped core is typically cut in the middle to obtain the two U-shaped end pieces discussed herein.
The present invention proposes a hybrid core for the core 50 that includes both the amorphous alloy for the U-shaped pieces 52 and 54, and the traditional stamped metal sheet cores for the center pieces 56, 58, 60 and 62. Thus, the center pieces 56, 58, 60 and 62 are stamped pieces that have been laminated together by known manufacturing processes suitable for the size of the sections 56, 58, 60 and 62, and combined with the amorphous alloy pieces 52 and 54 that complete the magnetic flux path. Various materials are known in the art that are suitable for the core material of an inductor and are suitable for the known stamping processes of sheet materials that are formed together into laminates. For example, silicon-iron materials can be used, where the iron provides the desired magnetic properties and the silicon improves the performance, generally by reducing eddy current losses.
The losses of the core 50 are determined by the majority of the material, which will still be the amorphous metal, while the smaller gap forming pieces have relatively high specific losses, but in a very small volume so that they do not contribute significantly to the overall losses. The magnetic properties, especially saturation flux density, of amorphous iron and regular stamped sheet iron are sufficiently close so that the core performance is not limited by either of the materials. Combining the advantages of the amorphous metal with the advantages of the regular stamped sheet cores allows smaller power conversion inductors to be built with lower cost, size, weight and magnetic losses.
As can be seen, magnetic flux lines 90 from the inductors 106 and 108 travel in opposite directions in the core piece 124, and magnetic flux lines 90 of the inductors 108 and 110 travel in opposite directions in the core piece 126. Thus, as the strength of the flux lines 90 increases and decreases in response to the switching phase relationship between the inductors, there is at least a partial cancellation of the flux lines 90 propagating in opposite directions in the common flux part segment.
Because the inductors 106 and 108 share a core piece and the inductors 108 and 110 share a core piece, the amount of core material used in the array 100 can be reduced over the amount of core material used in the array 70 for the same size inductor array. Therefore, the size and weight of the inductor array 100 can be reduced over the inductor array 70. For example, the length of the array 100 can be about 215 mm for one particular boost converter application, which is a 30% improvement over the length of the array 70. Further, by combining the core pieces as described herein, the electrical losses of the inductor array 100 can be reduced from the electrical losses of the inductor array 70.
The core pieces 124 and 126 have an indented portion 138 at their centers halfway between the gaps in the inductors, such as would be formed by two back-to-back U-shaped members. As the flux cancels at least partly, the common core pieces can even be made such that they are smaller in cross-section than would be required for an individual core piece. In an alternate embodiment, those indented portions could be significantly smaller to further reduce the size of the core pieces 124 and 126, and thus the cost and weight of the array 100.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
