JP2023059445A - Multiple-phase magnetic coupling reactor - Google Patents

Abstract

To suppress rapid change of magnetic characteristics of a core without increasing the size of the reactor.SOLUTION: A multiple-phase magnetic coupling reactor 12 includes: a core having a plurality of first leg parts 32, 34 corresponding to a plurality of phase currents, respectively, and at least one second leg part 36 connected to the first leg parts 32, 34 and forming a magnetic path for the phase currents with the first leg parts; and coils 14, 16 wound around the first leg parts 32, 34, in which the phase currents flow, the coils are wound around the core so that the magnetic flux flowing in the first leg parts face each other, the first leg parts 32, 34 are formed of a first core material, the second leg part 36 is formed of a second core material, and the differential relative magnetic permeability associated with increase of a magnetic field drops less rapidly than the differential relative magnetic permeability of the second core material.SELECTED DRAWING: Figure 7

Description

The present technology relates to a multi-phase magnetically coupled reactor that can be used in power converters such as DC-DC converters.

A DC-DC converter, which is connected to a DC power supply and used to convert voltage, uses a reactor and a switch to store and release power as magnetic flux to step up or down the voltage. For example, Japanese Patent Application Laid-Open No. 2012-065453 discloses a converter that can be used as a power supply circuit for a hybrid vehicle. The converter of this publication is, in particular, a two-phase converter using a magnetic coupling reactor. A two-phase converter uses two coils (inductors), and a current containing an AC component out of phase by switching flows through each coil. A magnetically coupled reactor is obtained by magnetically coupling these two coils with a magnetic core having three legs. Each coil is wound on the outer leg with the magnetic flux directions facing each other. That is, the magnetic flux generated by each coil passes through the common central leg in the same direction.

JP 2012-065453 A

DC current flows through the coil of the reactor for the DC-DC converter as described above. Excessive direct current bias magnetism is likely to occur. When the core material reaches magnetic saturation due to this excessive DC bias magnetism, the inductance of the core material drops sharply. As a result, an overcurrent is generated in the windings, causing a problem that the controllability of the converter is deteriorated.

In order to deal with this problem, a normal magnetic coupling reactor has an air gap of a certain length in the outer leg of the core, and the cross-sectional area of the outer leg is increased, so that the core is in a region where the change in magnetic properties is small. I'm trying to use However, such countermeasures increase the size of the reactor by the amount of thickening of the outer leg. Therefore, it is desirable to suppress abrupt changes in the magnetic properties of the core without increasing the size of the reactor.

One aspect of the present technology is a multi-phase magnetically coupled reactor comprising a plurality of first legs corresponding to each phase current, and connected to these first legs to a core having at least one second leg forming a magnetic path corresponding to and windings in which each phase current flows wound around each first leg, and the magnetic flux passing through each first leg faces each other The winding is wound on the core such that the first leg is formed of a first core material and the second leg is formed of a second core material, and with increasing magnetic field The decrease in differential permeability of the first core material is slower than the decrease in differential permeability of the second core material. In some embodiments, the first core material is a ferrous powder material and the second core material is a ferritic material. In some embodiments, the second core material is a Mn--Zn based ferrite. In this way, even if an increase in the magnetic field occurs due to the imbalance of the phase currents, the increase in the magnetic field is offset by the increase in the magnetic resistance due to the magnetic permeability characteristics of the first core material as described above, and the amplitude of the ripple current is reduced. Sudden changes can be suppressed. In addition, since an increase in the amplitude of the ripple current is permitted to some extent, it is not necessary to increase the cross-sectional area of the first leg, and as a result the reactor can be made smaller.

In some embodiments, each first leg has no air gap. In this way, there is no need to increase the cross-sectional area of the first leg, and there is no need to consider the effect (fringing) of the leakage magnetic flux from the air gap acting on the windings and generating eddy currents. Therefore, the size of the reactor can be reduced as a result.

1 is an electrical circuit diagram of a DC-DC step-down converter with a magnetic coupling reactor as one embodiment; FIG. FIG. 2 is a diagram representing magnetic flux generated in a core and magnetic circuits. 4 is a graph showing changes in coil winding current and core magnetic flux with respect to switching timing; FIG. 4 is a diagram showing an extra circulating magnetic flux component generated in the outer peripheral portion of the core in addition to the normal average magnetic flux due to the imbalance of the average currents flowing through the coils of each phase. FIG. 7 is a magnetic circuit diagram showing only extra circulating magnetic flux components of FIG. 6; FIG. 4 is a diagram showing overcurrent generated in windings due to excessive DC bias magnetism; 1 is a diagram showing a reactor using a core whose outer peripheral portion is made of an iron-based compacted powder material as one embodiment. FIG. 4 is a graph showing a schematic change in magnetic flux density of an iron-based compacted powder material and ferrite. 4 is a graph showing schematic changes in relative differential magnetic permeability of an iron-based compacted powder material and ferrite. FIG. 4 shows a core with uniform length air gaps in its outer legs and illustrates the parameters for calculating the reluctance of its outer legs. FIG. 11 illustrates the change in magnetic properties of the outer leg of the core of FIG. 10 when current imbalance occurs; FIG. 10 is a diagram showing changes in the magnetic properties of the outer leg of the core when the air gap length and cross-sectional area are increased. Figure 8 shows the change in magnetic properties of the outer leg of the core of Figure 7 when a current imbalance occurs; Fig. 10 shows a core with a different overall shape as another embodiment; FIG. 10 shows cores with different overall shapes as yet another embodiment.

Various embodiments of the present technology will be described below with reference to the drawings. In the following embodiments, portions with no substantial difference are denoted by similar reference numerals to avoid repetition of description.

〔converter〕
FIG. 1 is a circuit diagram of a DC-DC step-down converter 10 having a reactor 12 as one embodiment. Although the converter 10 will be described below as a two-phase step-down converter, the features described in the present application can be applied to various power converters having similar configurations, such as a boost converter, a buck-boost converter, and a multi-phase converter of three or more phases. can also be applied to The converter 10 is connected on the input side to a DC power supply (not shown) to constitute a switching power supply, and converts an input voltage Vi into an output voltage Vo. The DC power source can be, for example, a rechargeable secondary battery or a large capacity capacitor. A load is connected to the output side of the converter 10 . For example, converter 10 can be used as a power supply circuit for an AC motor by combining it with an inverter. AC motors can be mounted on electric vehicles such as hybrid vehicles and electric vehicles, and used to drive wheels.

Converter 10 includes switches 18 and 20 and diodes 22 and 24 in addition to reactor 12 . Each switch 18, 20 is periodically switched on and off such that it is 180 degrees out of phase with each other. The reactor 12 has two coils 14, 16 connected in series to the output sides of the switches 18, 20, respectively. . The switch is, for example, a switching element such as a field effect transistor (FET) or an insulated gate bipolar transistor (IGBT). Each diode 22,24 is connected between the corresponding switch 18,20 and the coil 14,16 to allow current from the load to flow back to the coil 14,16 when the switch 18,20 is off. Diodes 22 and 24 may be replaced by other switches (switching elements) that operate complementary to switches 18 and 20. FIG.

As shown in FIG. 2, the reactor 12 has a core 30 with a central leg 36 and two outer legs 32,34, the two coils 14,16 being magnetically coupled at the core 30. As shown in FIG. By magnetically coupling and integrating the coils 14 and 16, the number of parts of the reactor 12 can be reduced, and the device including the converter can be miniaturized. The windings of each coil 14, 16 are wound on the outer legs 32, 34 of the core 30 in such a direction that the magnetic fluxes are directed in opposite directions when DC current is applied. The magnetic properties of the two inductors formed by the coils 14, 16 and the outer legs 32, 34 should be the same. Although not shown, in the case of a multi-phase converter with three or more phases as another embodiment, a core with the same number of outer legs as the number of phases can be used in a similar manner.

[Changes in coil current and magnetic flux]
As shown in FIG. 3, switches 18, 20 are typically controlled by pulse width modulated (PWM) signals generated by a controller (not shown). When the first phase switch 18 is turned on, a current I _L1 from the DC power supply flows into the windings of the coil 14 . Due to the inductance L1 of the inductor formed by the coil 14 and the outer leg 32, as the current I _L1 flowing through the windings of the coil 14 increases, its energy is stored as magnetic flux Φo1 in the outer leg 32 around which the coil 14 is wound. be done. When the switch 18 is turned off, the energy stored in the coil 14 is discharged as current to the load side. Similarly for the second phase, a magnetic flux Φo2 is induced in the other outer leg 34 according to the current I _L2 flowing through the coil 16 due to the inductance L2. Since both of the magnetic fluxes generated by the two coils 14 and 16 pass through the central leg 36, the magnetic flux .PHI.c is nearly twice the value of each magnetic flux. A graph of half the value of the magnetic flux Φc of the central leg 36 is superimposed on FIG. 3 for reference. Also, since the magnetic flux Φc of the central leg 36 is a superposition of two magnetic flux components that are 180 degrees out of phase, the frequency of fluctuation is twice the switching frequency of the switches 18 and 20 .

The currents I _L1 and I _L2 flowing through the windings of the coils 14 and 16 have waveforms containing AC (ripple) components. The average values of the currents I _L1 and I _L2 depend on the connected load, but the ripple amplitude decreases as the inductances L1 and L2 of the coils 14 and 16 increase. The output voltage Vo supplied to the load side becomes smaller than the input voltage Vi from the power supply side according to the duty ratio of the PWM signal.

The converter 10 may have capacitors 26 and 28 for smoothing on the input side and the output side, respectively. Although not shown, current sensors for measuring the currents I _L1 and I _L2 flowing through the coils 14 and 16 of each phase and voltage sensors for detecting the input voltage and the output voltage of the converter 10 are provided, and switching is performed based on the measured values. can also be controlled.

[DC biased magnetism due to unbalanced phase currents]
As shown in the lower part of FIG. 2, the characteristics of the reactor 12 can be expressed using a magnetic circuit. At this time, the currents _IL1 and _IL2 flowing through the coils 14 and 16 can be replaced with the magnetomotive force in the magnetic circuit, and the inductance (inverse of) of the core 30 can be replaced with the magnetic resistances Rmc and Rmo. Here, it is assumed that the number of turns N of the two coils and the magnetic resistance Rmo of the two outer legs 32, 34 are the same.

Although the currents I _L1 and I _L2 flowing through the coils 14 and 16 contain ripple components, ideally they should have the same value when averaged over time. However, if there are variations in the characteristics of the circuit elements arranged in each phase, such as the switches 18 and 20 and sensors, an imbalance can occur in the two average phase currents _IL1 and _IL2 as shown in FIG. Here, it is assumed that the average current _IL1 flowing through the first-phase coil 14 is larger than the reference value _IL by _δIL , and the average current _IL2 of the second-phase coil 16 is smaller than the reference value _IL by _δIL . FIG. 5 is a magnetic circuit diagram that expresses only the difference from the equilibrium state, and the magnetomotive forces NI _L1 and NI _L2 corresponding to the two coils 14 and 16 are opposite to each other. Therefore, the core 30 always has a DC magnetic flux component δΦ circulating through the outer peripheral portion 38 formed by the two outer legs 32, 34 as shown in FIGS. In particular, since the DC magnetic flux component δΦ in the same direction as the normal average DC magnetic flux is added to the outer leg portion 32 corresponding to the first phase, excessive DC bias magnetism is generated. Under such circumstances, when the core material of the outer leg 32 reaches magnetic saturation, the magnetic resistance rises (the inductance decreases), and the outer leg 32 loses its performance as a core. Therefore, as shown in FIG. 6, an overcurrent may occur in the current _IL1 , causing problems such as damage to circuit elements.

[Core material]
As shown in FIG. 7, in consideration of the phenomenon described above, the core 30 is composed of a first core material and a second core material (highly part drawn). Specifically, the outer legs 32, 34 of the core 30 are formed of a first core material that exhibits a relatively gradual decrease in differential permeability than the second core material, and the central leg 36 is formed of the second core material. form with More precisely, the outer peripheral portion 38 of the core 30 excluding the central leg 36 (that is, the outer legs 32, 34 and the T-junction) is formed of the first core material. As a specific embodiment, the first core material can be a ferrite-based material, and the second core material can be an iron-based dust material (iron-based dust material). The iron-based dust material can be Sendust (registered trademark), for example.

Here, the difference in magnetic properties between ferrite-based materials and iron-based compacted powder materials (iron-based dust materials) will be mentioned. As shown in FIG. 8, the magnetic flux density B of the iron-based dust material gently increases as the applied magnetic field H increases. On the other hand, when the magnetic field H is small, the magnetic flux density B of the ferrite material also increases sharply as the magnetic field increases. FIG. 9 illustrates these characteristics in terms of the differential permeability μr corresponding to the slope dB/dH at each point in the graph of FIG. The differential magnetic permeability μr of the iron-based dust material gently decreases as the magnetic field H (or magnetic flux density B) increases. In contrast, when the magnetic field is small, the differential permeability μr of the ferrite-based material maintains a high value even though it decreases slightly. That is, compared with iron-based compacted powder materials, ferrite-based materials have characteristic magnetic properties in which the differential magnetic permeability of ferrite-based materials has distinct gradual change regions and sharp change regions on the low magnetic field side and high magnetic field side, respectively.

In one embodiment, the ferritic material used for the center leg 36 can be a Mn--Zn based ferrite with low core loss per unit volume. The core loss of Mn--Zn ferrite is about 1/20 of that of Ni--Zn ferrite. Alternatively, as another embodiment, the first core material can be Ni--Zn ferrite whose magnetic permeability is less likely to decrease on the high frequency side. The relative permeability of Mn--Zn ferrite drops sharply as the frequency of the alternating magnetic field increases, whereas the relative permeability of Ni--Zn ferrite drops relatively slowly on the high frequency side. As described above, the frequency of fluctuation of the magnetic flux Φc of the central leg 36 is twice the switching frequency. It is possible to suppress the performance deterioration as. This effect increases as the number of phases in the multiphase converter increases.

式中のlcoreoはエアギャップ３３Ａ、３５Ａを除いた外側脚部３２Ａ、３４Ａのコア材料を通る磁路長（lcoreo1+lcoreo2）、Acoreoは外側脚部３２Ａ、３４Ａの断面積、μrは外側脚部３２Ａ、３４Ａのコア材料の微分比透磁率である。例えばコア３０Ａが全てフェライト系材料からなる場合、上で述べたフェライト系材料の特徴的な透磁率特性がそのままコアの特性に反映される。このため、図１１に示すように、電流アンバランス率（例えばδI_L/I_Lで定義される）が高まると外側脚部３２Ａ、３４Ａの磁気抵抗Rmoが急激に増加する。特に、フェライトが磁気飽和に達すると相電流のリップル成分の振幅も過大になる。これに対し、外側脚部のエアギャップ３３Ａ、３５Ａをより長くし、それに応じて外側脚部３２Ａ、３４Ａの断面積Acoreoも大きくすることも考えられる。この場合、図１２に示すように、電流アンバランス率が高まっても磁気飽和が生じにくくなる。これは、図１１に示した左端に近い領域（使用可能な領域）だけを使用するような設計に相当する。しかし、外側脚部３２Ａ、３４Ａの断面積の増加によってリアクトルのサイズが大きくなってしまう。 [Magnetic Properties of Outer Legs of Core]
In the following, we will compare the characteristics of cores with various structures when there is an imbalance in the phase currents. As shown in FIG. 10, when air gaps 33A and 35A of a uniform length lg are provided in the outer legs 32A and 34A of the core 30A as is conventionally practiced, the magnetic field formed by one outer leg is The path reluctance Rmo is expressed as the sum of the reluctances connected in series as follows:

In the formula, lcoreo is the magnetic path length through the core material of the outer legs 32A and 34A excluding the air gaps 33A and 35A (lcoreo1+lcoreo2), Acoreo is the cross-sectional area of the outer legs 32A and 34A, and μr is the outer leg. 32A, 34A is the differential permeability of the core material. For example, when the core 30A is entirely made of a ferrite material, the magnetic permeability characteristics characteristic of the ferrite material described above are directly reflected in the characteristics of the core. Therefore, as shown in FIG. 11, the magnetic resistance Rmo of the outer legs 32A and 34A sharply increases when the current imbalance ratio (eg, defined by δI _L /I _L ) increases. In particular, when the ferrite reaches magnetic saturation, the amplitude of the ripple component of the phase current also becomes excessive. On the other hand, it is conceivable to lengthen the air gaps 33A, 35A of the outer legs and correspondingly increase the cross-sectional area Acoreo of the outer legs 32A, 34A. In this case, as shown in FIG. 12, magnetic saturation is less likely to occur even if the current imbalance rate increases. This corresponds to the design shown in FIG. 11 where only the area near the left edge (usable area) is used. However, the increase in the cross-sectional area of the outer leg portions 32A, 34A increases the size of the reactor.

When only the iron-based powder material is used for the outer legs 32 and 34 (see FIG. 7), as shown in FIG. 13, the characteristics of the iron-based powder material (arrow 50 ), the reluctance Rmo of the outer legs 32, 34 gradually increases as the magnetic field strength increases due to the current imbalance (arrow 54). This increase in magnetic resistance cancels out the increase in the magnetic field, and as a result, increases in the magnetic flux density and the ripple component of the phase current are suppressed (arrows 52 and 56). Therefore, since the magnetic properties of the core gradually change, the controllability of the converter is improved. It should be noted that the cross-sectional area and length of the outer legs 32, 34 can be determined based on the allowable increase (arrow 58) in the ripple current amplitude. In this way, allowing a certain increase in the amplitude of the ripple current does not lead to abrupt changes in the core characteristics, so there is no need to provide an air gap or increase the cross-sectional area of the outer legs 32, 34, resulting in The reactor can be substantially miniaturized.

The central leg 36 can be provided with an air gap 37 if desired. As another embodiment, not shown, the outer legs 32, 34 made of iron-based compacted powder material can also be provided with air gaps as needed.

[Modified example of core shape]
Alternatively, the shape of the core can be varied as desired. As in the core 30C shown in FIG. 14, by dividing the portion corresponding to the central leg portion 36 in the above embodiment into two leg portions 36C, the magnetic flux corresponding to each phase current is directed to two magnetic paths. It can also be split. As still another embodiment, as shown in FIG. 15, it is also possible to divide the windings 14 and 16 corresponding to each phase current into two coils and wind them. Each of the cores 30C, 30D has two first legs 32C, 34C, 32D, 34D (corresponding to the outer legs 32, 34 in the above-described embodiment) corresponding to each phase current, and these first legs and at least one second leg portion 36C, 36D (corresponding to the central leg portion 36) which forms a magnetic path corresponding to each phase current together with each first leg portion. 14 and 15, the first leg is drawn dark. Therefore, the first leg portion (including the connection portion with the second leg portion) can be made of, for example, an iron-based compacted powder material, and the second leg portion can be made of a ferrite-based material. Moreover, those skilled in the art will easily understand that the various features described above can also be applied to magnetic coupling reactors of three or more phases.

As described above, the present technology has been described with specific embodiments, but the present technology is not limited to these embodiments, and a person skilled in the art can make various substitutions, improvements, and improvements without departing from the purpose of the present technology. Changes can be made.

10 Converter 12 Reactor 14, 16 Coil 18, 20 Switch 22, 24 Diode 26, 28 Capacitor 30 Core 32, 34 Core Outer Leg 36 Core Central Leg 37 Central Leg Air Gap 38 Core Periphery

Claims (4)

A multi-phase magnetic coupling reactor,
It has a plurality of first legs corresponding to each phase current and at least one second leg connected to the first legs and forming a magnetic path corresponding to each phase current together with each first leg. a core;
a winding through which each phase current is wound around each first leg, wherein the winding is wound around the core so that the magnetic flux passing through each first leg faces each other;
the first leg is formed of a first core material and the second leg is formed of a second core material;
A multi-phase magnetic coupling reactor, wherein the differential permeability of the first core material decreases more slowly with increasing magnetic field than the differential permeability of the second core material. 2. The multi-phase magnetic coupling reactor of claim 1, wherein the first core material is iron-based compacted powder material and the second core material is ferrite-based material. 3. The multi-phase magnetic coupling reactor of claim 2, wherein said second core material is Mn--Zn based ferrite. 3. The multi-phase magnetic coupling reactor of claim 1 or 2, wherein each first leg has no air gap.

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