EP2453450A1

EP2453450A1 - Hybrid core for power inductor

Info

Publication number: EP2453450A1
Application number: EP10014552A
Authority: EP
Inventors: Javier Duràn; Mauricio Esguerra; Umberto Gibellini
Original assignee: Falco Electronics Ltd
Current assignee: Falco Electronics Ltd
Priority date: 2010-11-12
Filing date: 2010-11-12
Publication date: 2012-05-16

Abstract

A hybrid core for a power inductor is made of at least two different soft magnetic materials which differ from each other in at least two magnetic parameters. Inductors for both single-phase and three-phase applications have superior performance with respect to targeted properties such as inductance, dc bias capability, core losses, winding losses and/or dimensions as compared to equivalent inductors made of a single magnetic material.

Description

The present invention belongs to the field of power magnetics, more specifically to the inductors used as part of the circuit of a power device used to convert voltage and current.
Power inductors must satisfy two main conditions: they must provide enough inductance at high DC or low frequency AC bias currents and have the lowest possible losses in order to allow high power conversion efficiencies.
Power inductors are used in applications such as inverters for uninterrupted power supplies (UPS) and photovoltaics (PV) for rated powers between 2 to 2000 kW for one or three phases. The nominal voltage of said applications ranges from 100 to 480 V resulting in very large currents up to 4 kA. DC to DC converter stages with voltages as low as 12 V are often found in these applications as well, where the inductor filters a higher switching frequency while passing an often larger DC current component. The switching frequency of said devices spans from 1 to 100 kHz and is mainly chosen so as to minimize the power losses from both power semiconductors such as diodes and IGBTs as well as inductors. The required inductance values lie usually in the range 100 µH to 10 mH.
Inductors are designed using a plurality of soft magnetic materials with either high permeability such as steel, amorphous alloys, nanocrystalline alloys and ferrites or low permeability such as iron powder or iron alloy powder. These materials cover in more or less degree a wide range of magnetic properties relevant for the application such as saturation, power losses and magnetrostriction; last property is responsible for the audible noise at switching frequencies below approximately 20 kHz.
In conventional inductor design, the material choice is made based on considerations of space availability, performance and cost. The adaption to the specific requirements is done by selecting suitable core geometry with air gap(s) if a high permeable material is used. Since the fringing flux at the air gap can cause substantial additional winding losses, one way to mitigate this effect is to use low permeable spacers instead. One disadvantage of this technique is that the physical size of a low-permeability spacer is given by the air gap size times its permeability causing a considerable size increase of the component.
The present invention relates to power inductors designed with a combination of soft magnetic materials with different properties in a serial magnetic circuit building up a hybrid core. The materials must significantly differ in at least two magnetic parameters such as permeability, saturation, power losses or magnetostriction at the intended operating temperature and frequency. Due to the high DC bias current requirements, usually one of the materials will have a low permeability. Unlike the above mentioned magnetic spacer solution its use is not only related to help minimize the fringing flux close to the windings but is an integral part of the magnetic circuit.
The main feature of a hybrid magnetic circuit is to obtain additional degrees of freedom so as to optimize the inductor with respect to its functional parameters under constraints such as space, losses, temperature, weight and cost. This can be achieved in one of the following ways:

1) Improvement of a design using a single material by replacing one or more sections of the magnetic circuit by other materials in the same or a similar shape. This can be done by analyzing the contribution of the different sectors to the overall inductor specification and by identifying shortcomings rooted in the magnetic material properties of the original material.
2) Sectional design by assigning to each section of the magnetic circuit its main contribution to a certain region of the target specification so as to use the material which has the best performance in this region.

Since the objective of the design is to maximize the overall performance the properties of the winding such as its material, winding window and electrical properties are also affected by the choice of magnetic materials and geometrical shapes of the sections of the magnetic circuit.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying figures.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a front view of an embodiment of a single phase power inductor constructed with two different materials according to the present invention.
Figure 2 is a side view of an embodiment of a single phase power inductor constructed with two different materials according to the present invention.
Figure 3 is a front view of an embodiment of a three-phase power inductor using a three- leg core according to the present invention.
Figure 4 is a front view of an embodiment of a three-phase power inductor using a five-leg core according to the present invention.
Figure 5 is a graph of inductance versus DC current for a conventional single-phase inductor constructed with a single core material, a single-phase inductor constructed with a high permeability/low loss and a low permeability/high loss material according to the present invention, and a three-phase inductor constructed with a high permeability/low loss and a low permeability/high loss material according to the present invention.
Figure 6a is a front view of an embodiment of a power inductor using U cores in two different materials.
Figure 6b is a detail of Fig. 6a showing the magneto-mechanical behaviour for the case of a core using materials with positive magnetostriction for both core halves when the flux density B > 0.
Figure 6c is a detail of Fig. 6a showing the magneto-mechanical behaviour for the case of a core using materials with opposite magnetostriction for both core halves to minimize acoustic noise according to the present invention when the flux density B > 0.

Detailed description of preferential embodiments

For the purpose of illustration, the present invention will be described with reference to an output single-phase and to an output three-phase inductor used in inverter applications. The output inductor is used in a low-pass filter to allow the fundamental waveform (typically sinusoidal at 50 or 60 Hz) to pass while blocking the ripple current at the switching frequency (typically in the kHz range). To achieve this task, the inductor needs to have a high enough inductance up to the peak current of the fundamental waveform so as to achieve a low total harmonic distortion (THD) of the signal. Also, since the amplitude of the ripple current is inversely proportional to the inductance of the output inductor, the power losses at the ripple frequency in both the winding and the core are lower the higher the inductance for similar winding and core design. The inductance versus DC current characteristic (Fig. 5) shows the current up to which the expected function is fulfilled.
Figure 1 depicts an example of a single-phase inductor 10 made with an U-core consisting of two different magnetic materials for the vertical legs 11 (material MA) and for the horizontal legs 12 (material MB). The materials differ from each other in the following two parameters:

The saturation flux density saturation B_s of material MB is larger than material MA by a factor of approximately 1.6
The core loss per volume p_v of material MA is lower by approximately a factor of 2 than material MB for the same flux density.

Both horizontal legs are wound with a round wire and connected in series to form the coil (alternatively foil or rectangular or litz wire could be used and coils could be connected in parallel if advantageous for higher current levels). The cross sectional area of the wound horizontal legs is chosen to be smaller than the vertical legs in material 11 in proportion to the saturation flux densities; this is achieved as shown in figure 2 by choosing the width of the vertical leg broader than the horizontal leg (or alternately, the thickness greater than the horizontal leg). Hence, the mean turn path of the winding is shorter than if it would be wound around the vertical legs of material 11 and so the DC electrical resistance of the coil and the associated losses are lower. For a width to height aspect ratio of 2 for the horizontal legs, the ratio in winding losses is given by the ratio of the perimeter of the legs $\frac{P_{w i n d i n g, 12}}{P_{w i n d i n g, 11}} = \frac{2 (h_{12} + w_{12})}{2 (h_{11} + w_{11})} = \frac{1 + \frac{w_{12}}{h_{12}}}{1 + \frac{w_{11}}{h_{11}}} = \frac{1 + 2}{1 + 2 \cdot 1.6} = 0.71$
The winding losses are so reduced by 29%.
Since the cross sectional area of the vertical legs is larger than the horizontal legs, the flux density is smaller than in the horizontal legs. The core losses depend on the flux density by an exponential law with an exponent of 2.1, so as a result the core losses in the vertical legs 11 is significantly smaller than the losses in the horizontal 12 legs. If the vertical and horizontal legs have the same length, the ratio of the core losses for the vertical legs to the horizontal legs is $\frac{P_{core, 11}}{P_{core, 12}} = \frac{p_{v, 11}}{p_{v, 12}} {(\frac{B_{11}}{B_{12}})}^{2.1} \frac{A_{11} l_{11}}{A_{12} l_{12}} = 0.5 \cdot {(\frac{1}{1.6})}^{2.1} 1.6 = 0.30$
The core losses for all legs are so reduced by $1 - \frac{2 (1 + 0.3)}{2 (1 + 1)} = 65 %$
as compared to a magnetic circuit made with all legs in material MB.
Figure 3 depicts one way of making a three-phase inductor 30 with a common, three-leg core made of two different magnetic materials. Material MC is used for the legs of the outer frame 31 (vertical) and material MD for the wound legs (horizontal) 32 and 33 with three coils 35.
Figure 4 shows another way to make a three-phase inductor 40 with a common, five-leg core. The arrangement is similar to DE3305708 , which however uses the same material for all legs with multiple non-magnetic gaps in the wound legs. Material MC is used for the legs of the outer frame 41 (vertical) and 44 (horizontal), while material MD is used for the wound legs (horizontal) 42 and 43 with three coils 45. The materials for this example differ from each other in the following two parameters:

The flux density saturation of material MD is larger than material MC by a factor of approximately 4.5
The initial permeability of material MC is higher than material MD by a factor of approximately 46

Figure 5 shows curves of the inductance over the DC bias current 52 for one of the coils 45 of the five-leg three-phase inductor from Fig. 4 and 51 for the one phase inductor from Fig. 1. The inductance for I_DC=0 of the three-phase inductor is higher and the curves coincide at I_DC=30 A, which is the nominal current for the inductor of this example.
One advantage using a three-phase inductor is to minimize the magnetic volume as compared to three separate one-phase inductors. The main advantage for applications such as inverters is the fact that the excitation of the magnetic circuit under symmetrical 120° phase shifted currents allows to minimizing the low frequency or DC flux and its effect on the inductance drop of each coil as current level increases. Since the ripple current amplitude is inversely proportional to the inductance this yields lower core and high frequency winding losses at higher current levels. Considering the above mentioned lower magnetic volume, the overall losses are lower as compared to three individual single-phase inductors.
Figures 6a depicts one power inductor 60 made of two U core halves consisting of two different magnetic materials for the left core half 62 (material ME) and for the right core half 61 (material MF).
In one case (Fig. 6b) both materials have positive magnetostriction and both core halves expand when a magnetic field is applied yielding a flux density B >0. The two core halves collide with each other. If the field is varied periodically at frequencies in the audible range, this will result in disturbing acoustic noise, which is a well known effect in conventional inductor design.
In another case (Fig. 6c) and according to the present invention the materials differ from each other in the following two parameters:

The initial permeability µ _i of material ME is larger than material MF by a factor of approximately 4
The magnetostriction of material ME is positive while the magnetostriction for material MF is negative

In this case core half 62 expands upon application of a magnetic field yielding a flux density B >0, while core half 61 contracts thus avoiding the collision described above. This prevents the generation of acoustic noise if the magnetic field varies periodically at frequencies in the audible range.

Claims

A soft magnetic core, for use in an inductor for power applications, comprising a plurality of legs;
said plurality of legs being made of at least two soft magnetic materials having at least two different magnetic parameters.
The soft magnetic core according to claim 1 wherein said inductor is a single-phase power inductor.
The soft magnetic core according to claim 1 wherein said inductor is a three-phase power inductor.
The power inductor according to any preceding claim wherein said inductor has lower core and winding losses than an inductor with the same inductance and current rating made of a single soft magnetic material.
The power inductor according to any preceding claim wherein said inductor has a higher inductance vs. DC bias characteristic compared to an inductor with the same dimensions made of a single soft magnetic material.
The three-phase inductor according to claim 3 wherein said inductor has a higher inductance vs. DC bias characteristic compared to three individual single-phase inductors with the same inductance at nominal DC current.
The three-phase inductor according to claim 3 wherein said inductor has lower winding and core losses as compared to three individual single-phase inductors with the same inductance at nominal DC current.
The power inductor according to any preceding claim wherein said inductor does not generate acoustic noise caused by the plurality of legs colliding with each other upon application of a periodically varying magnetic field at frequencies in the audible range.