Choke and choke core

The present invention relates to a choke with two coils and a core for interleaved applications in step-up or step-down circuits or power factor compensation circuits. The core comprises several core sections with several lateral legs and a middle leg, whereby the core is designed such, that a coupling factor k of the two coils is smaller than 3%-5%. Furthermore, the core is designed such, that the core section form two loops with the middle leg as a common section, whereby each of the two coils lies on different loops outside of the common section. The lateral legs have a cross section A1 and the middle leg for the common section has a cross section A2<2×A1.

FIELD OF THE INVENTION

The present invention relates to a choke with two coils and one core, optimized to be used in step-up or step-down circuits or power factor compensation (PFC) filters in an interleaved configuration. Furthermore, the present invention relates to an optimized double coil core for interleaved applications in step-up and step-down or power factor compensation (PFC) circuits.

BACKGROUND OF THE INVENTION

In the following, the term “choke” relates to a configuration from one or several coils placed on a common core.

A step-up or step-down circuit refers to a circuit, which can increase or decrease a direct-current voltage. Step-up and Step-down circuits operate according to similar principles like power factor compensation filters and partially use the same components.

A power factor correction is imposed in Germany for electric loads over 75 watt since 1 Jan. 2001 by the electromagnetic compatibility norm (EMC). Power factor describes the rate between the value of the effective power and the apparent power. A value less than 1 means that the apparent power, which is drawn from the power grid, is larger than the effective power, so that the power grid is additionally loaded by the apparent power, which has to be provided and transported and which partially has to flow back through the power grids. Hereby greater losses occur in the grid and the grid has to be dimensioned larger than actually necessary. Power factor correction filters make sure that the power factor is as close as possible to 1, i.e. only pure effective power is drawn from the power grid. In an active power factor correction (PFC) the drawn current is readjusted to the time dependent sinus shape voltage of the power grid.

A central component of step-up, step-down circuits and of PFC is a choke, which is in principle used to temporarily store Energy and release it on requirement. The following explanations confine on the use of the choke in PFC filters. However, similar reasoning is also true for step-up and step-down circuits.

A switch connected downstream of the choke which can adjust the coil output to a reference potential, is opened and closed by a controlling device so as on the one hand to deliver sufficient power to an electric load, but on the other hand so that the current of the grid voltage curve drawn from the grid is in-phase.

In a further development the input power voltage is divided between two coils which can be operated independently from one another. In general the switches are operated inverse to one another, i.e. if one switch is opened, the other switch is closed. In such an “interleaved” operational mode a choke branch (master) is directly controlled by the regulation circuit, i.e. the switching times for the choke are directly controlled by the regulation. The second choke branch (slave) generally follows the master with a phase shift of 180 degrees. Such an interleaved working arrangement has the advantage, that a more efficient power factor correction can be achieved. Since each choke has to cope with only half of the output power, smaller components can be dimensioned, so as to improve the power loss and heat generation and allow for smaller PFC-circuits. It is to be noted, that a correct functioning is possible also at other phase shifts <180°. That is, in general, the phasing can be variable. However, the majority of applications operate with a phase shift of 180°.

Active PFC circuits usually consist of a rectifier with a step-up convertor directly attached downstream with a coil and a switch, which charges a large capacitor to a voltage above the peak voltage of the grid network alternating current.FIG. 1Aschematically shows the principles of a step-up circuit in interleaved technology. At the input the input voltage VINis applied to the two choke coils L1and L2and the input current IINis divided between the two chokes. At the output of each coil or choke L1or L2a switch S1or S2, respectively, can set the output L1or L2controlled by a regulation circuit (not shown) to a reference potential. The outputs of coils L1and L2are connected through a diode to the capacitor COUT, which, in interaction with the coils L1and L2increases the voltage (step-up circuit) and smoothes the voltage, so that it can be delivered to the load resistance RLOAD.

The opening and closing times of switch S1are set by a controller (master), which ensures that on the one hand the load RLOADis provided with sufficient current IOUTand on the other hand the input voltage IINis following in-phase the input voltage VIN. The switch S1follows the switch S1phase shifted by 180 degrees (slave). This causes in principle a pulse width modulation of the input current, in which the pulse width is controlled by a controller.FIG. 1Bshows the switching characteristics of the switches S1and S2. The time in which switch S1is closed is denoted as Tonand is variable according to the controller. In the time in which switch S1is closed, switch S2is opened (180 degree phase shifted). The overall time, which consists from the sum of the time Ton, in which the switch S1is closed and the time Toff, in which the switch is opened, is denoted as period T and is constant. The duty cycle D=Ton/T is variable and dependent on the controller. InFIG. 1ba constant duty cycle D of 0.5 is shown.

FIG. 1Cshows the currents I1and I2through the coils L1and L2. The current I1through coil L1consists from a direct current component Idc1and a ripple component Iac1, generated by the switching processes. Accordingly, the current I2through the coil L2consists from a direct current component Idc2and a ripple component Iac2(alternating current component caused by switching processes). Since the switches are connected with a phase shift by 180 degrees, the phase shift between Iac1and Iac2is 180 degrees. On the capacitor COUTthe currents I1and I2are added. I.e. the complete direct current component results in Idc=Idc1+Idc2. From Idc1=Idc2=IIN/2 follows for Idc that Idc=IIN. For the complete ripple current component (alternating current component) follows Iac=Iac1−Iac2, since Iac1and Iac2are phase shifted by 180 degrees. This, however, is only true for a duty cycle of D=0.5, i.e. for ton=toff. I.e. for a duty cycle of D=0.5 the ripple current components mutually compensate. At different duty cycles the ripple current components do not precisely compensate each other. In any case, on the whole, in the interleaved design the ripple current component is reduced giving a smoother current curve.

It should be noted, that at a phase shift of 180° the ripple current maximum in the middle leg is reached at a duty cycle of D=0.5. The interleaved choke, however, also functions at other phasings <180°. Hereby only the duty cycle D, at which the maximum of the alternating current ripple occurs, is shifted. I.e. that in general the phasing can be variable. However, the majority of applications operates at a phase shift of 180°.

Chokes for use in interleaved step-up circuits and PFC steps are known from state of the art. In the simplest case two coils are wound on a common core, like for example shown in U.S. Pat. No. 6,362,986 B1 of the Volterra company.FIG. 2schematically shows the coil arrangement of this patent with switches40for the interleaved operation mode. The two coils20and30are arranged on a common ring-shaped core10, i.e. the pair of coils20,30works with a strong magnetic coupling, similar to a transformer. Since the magnetic flows from the coils sum up, the core geometries are correspondingly large, so as to reach a high magnetic conductivity and at the same time not to stress the core up to the saturation magnetization.

U.S. Pat. No. 8,217,746 B2 describes a further development of a choke coil for interleaved PVC circuits, in which the coil core for the two coils is designed such that the two coils are only weakly magnetically coupled.FIG. 3shows a schematic view of the coil- and core configuration of U.S. Pat. No. 8,217,746 B2. The core consists from two E-shaped parts110and120, which are separated from one another by an I-shaped part130. The coils20and30are wound on the middle legs of the E-shaped parts110and120. Since the magnetic flows Φ1and Φ2in the coils20and30from the middle legs of the E-shaped parts divides between the lateral legs of the E-shaped parts, the cross section A2of the lateral legs can be half the size of the cross section A1of the middle legs. Since the coils20and30are wound or connected in phase opposition, the direct current components of the magnetic flows Φ1or. Φ2of the coils20and30in the I-shaped portion of part130extent compensate one another to a large, so that the cross section of the I-shaped part130can be designed smaller than the cross section A1of the middle legs of the E-shaped parts110and120. By connecting the two E-shaped parts110and120and of the I-shaped part130air gaps140are formed at the joints.

SUMMARY OF THE INVENTION

In view of new power safe technologies, such as in automotive engineering in the domain of hybrid and electro vehicles there is a growing demand for chokes for interleaved PFC circuits with low weight and high efficiency so as to safe energy on the one hand (weight) and at the other hand to efficiently transport energy, for instance, if motion energy in electric or hybrid vehicles is retrieved with a generator and supplied into the on-board electrical grid. It is therefore a task of the present invention to provide a choke with an optimized core geometry for a choke coil pair for use in interleaved PFC-application, which is compact and has small losses and a low weight.

The task is solved with a choke with two coils and one core according to the present invention.

In particular this task is solved by a choke with two coils and one core, wherein the core contains several core section with several lateral legs and one middle leg, wherein the core is designed such, that the core section form two loops with the middle leg as a common section, wherein each of the two coils lies on different loops outside of the common section, so that the lateral legs have a cross section A1, and that the middle leg for the common section has a cross section A2<2×A1.

With this arrangement a coupling factor k of the two coils smaller than 5%, preferably smaller than 3%, and more preferably smaller than 1% is realizable, so that the core cross section can be kept small in the lateral legs, since the magnetic fields of the coils no longer overlap in the lateral legs. Furthermore, the magnetic flux, which corresponds to the direct current component, compensates in the common section, so that the cross section of the common section can be designed small so as to save material. Since the coils are not arranged coaxially like in U.S. Pat. No. 8,217,746 B2, but rather are placed on the lateral legs, less material is required for the core, which saves weight. This is for instance reached by arranging the two coils on two opposing lateral legs.

In another embodiment the cross section A1lies in a range between 0.5 A1and 0.2 A1, so that more weight can be saved.

In order to reach a coupling factor of less than 5%, 3% or 1%, the core is designed such, that the magnetic resistance in at least one of the lateral legs RMAis larger than the magnetic resistance of the middle leg RMI, wherein RMA>20 RMI(5%), RMA>33 RMI(3%) or RMA>100 RMI(1%).

In another embodiment, for the core section in the middle section a material with a high permeability is used to keep the coupling between the two windings or their flows low. In the lateral legs a material with a high saturation flow density is preferably used so as to keep the magnetic cross section of the lateral legs low.

This embodiment with different materials for lateral legs and middle legs is only advantageous in specific cases, in which a too strong coupling between the two windings and high losses should be avoided. A high permeability is not necessary in general, since the core is sheared. Common power ferrites which can be used generally have an initial permeability μi between 1000 and 3000. A high permeability in the middle leg is advantageous since it reduces the coupling. The influence of the permeability of the lateral legs on the coupling is negligible, since the air gap dominates the magnetic resistance. For this reason rather a highly permeable magnetic material is used in the middle leg. Since losses dominate due to the increased exchange flow duty cycle caused by the reduction of the cross section, the cross section of the middle leg cannot be reduced up to the saturation limit, so that in this case preferably a material with lower losses having a slightly lower saturation flow density is used. In the lateral leg a material with a high duty cycle is used—like in a regular choke. In most applications the entire core can consist of one material. Only in specific cases (too high coupling, high losses in the middle leg) one will use different materials for the lateral legs and the middle leg.

In order to reach a 100 fold increased magnetic resistance in one of the lateral legs as compared to the middle leg, in one embodiment the lateral leg can feature an air gap, which is preferably arranged in the areas of the coils.

In order to reach the core geometry according to this invention, different embodiments are possible.

In one embodiment the core section are formed by two E-shaped parts, which are combined, such that their free ends meet, so that the connected middle legs of the two E-shaped parts form the common section. With this configuration a shorter and thus more compact design than for instance shown in U.S. Pat. No. 8,217,746 is possible.

In another embodiment the lateral legs are formed by two U-shaped parts, which are connected such, that their free ends meet, creating a magnetic circuit, whereby the middle leg for the common section has a T-shape and is inserted such between the two coils into the magnetic circuit, that the magnetic circuit is short-circuited, so that the magnetic circuit is divided into the two magnetically weakly coupled loops. Besides the compact design this embodiment has the advantage, that when connecting the two formed components, only two surfaces meet, contrarily to the E-shaped forming component, in which three surfaces meet. If three surfaces meet the formed components have to be manufactured with a very high precision so as to avoid uncontrolled air gaps. Due to manufacturing tolerances these air gaps are virtually unavoidable. If two surfaces meet like for the U-shaped components and the T-shaped component, this effect does not occur, so that with this embodiment chokes with lower tolerances can be manufactured.

In one of its embodiments the height H2of a vertical part of the T-shaped middle leg corresponds to a height H1of the lateral leg. Furthermore, a width B2of the vertical part of the T-shaped middle leg corresponds to a clear distance between the two coils and a depth T2of the vertical part of the T-shaped core section corresponds to an inner distance T1of the opposite lateral legs of the magnetic circuit. This contributes to a compact design, since the space between the coils within the magnetic circuit is filled free of clearance and is, thus, completely usable for the magnetic flow. In a further embodiment, a horizontal part of the T-shaped middle leg is supported by a lateral leg. Overall, the T-shaped design of the middle leg allows a simple and precise positioning of the magnetic short-circuit between the two coils. By the supporting surfaces, formed by the horizontal parts of the T-shaped middle leg, the middle leg is precisely inserted up to the correct depth into the magnetic circuit.

In a further embodiment the lateral legs are formed by two U-shaped parts, whose free ends oppose each other and are separated without play from one another by a straight elongated core section, which serves as a middle leg, so that the two legs are formed with the middle leg as a common section, which form two jointly weakly coupled magnetic circuits. A straight elongated core section, which serves as middle leg, has in comparison to a T-shaped core section the advantage, that micro air gaps created between the T-part and the lateral legs are avoided. Hereby the coupling between the magnetic circuits is reduced. At the same time this arrangement can compensate for the tolerance in the lateral air gaps or lateral legs, respectively, since the middle leg can now be flexibly glued to the lateral legs or side plates. Small excess ends of the middle leg are of no problem, but also a slightly shorter middle leg only insignificantly influences the current flow. In one embodiment each U-shaped part or E-shaped part is composed from several straight parts. These straight parts can be, for example, glued, so that tolerances due to uncontrolled micro air gaps are reduced.

In an embodiment the core sections are manufactured from a plate stack from a magnetically soft material. With this technique arbitrary core shapes can be realized with little technical effort.

The above-mentioned task can also be solved with a choke core, which comprises several core sections consisting from several lateral legs and a middle leg. Hereby the core sections are arranged such, that the core section form two loops with the middle leg as a common section, so as to form two weakly coupled magnetic circuits, whereby the lateral legs have a cross section A1and whereby the middle leg for the common section has a cross section A2smaller than 2×A1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention was made to provide chokes with an optimized compact core geometry for PFC-devices with interleaved-topology. Especially in growing electromobility, technology with electric vehicles (EV) and hybrid electric vehicles (HEV), new, compact, i.e. having low weight, chokes and choke cores are needed, which can be used at frequencies above 100 kHz.

FIG. 4shows the principle of the present invention. Two coils20and30are placed on an optimized compact core200, which consists from several lateral legs230and240with a middle leg250. The lateral legs consist from two lateral legs230with coils and two lateral legs240, which serve as connection elements for the lateral legs230carrying the coils that form a magnetic circuit. The middle leg250, which runs in parallel to the coils carrying lateral legs230, and which connects approximately the middles of the lateral legs240, causes a magnetic short-circuit between the connection elements240and divides the magnetic circuit in two loops200-A and200-B. The lateral legs have a cross section A1. The middle leg has a cross section A2which is smaller than 2×A1. In the PFC-application the coils20and30are connected such that the direct current component of the magnetic flux in the middle leg250runs in opposite direction and thus compensates itself. Thanks to the compensated DC-flow (direct current) the cross section of the middle leg can be significantly reduced. However, the alternating current of the coils20and30in general adds in the middle leg, since the alternating current amplitudes sum up in the middle leg due to the inversed poling of the coils20and30. At a duty cycle of D=0.5, i.e. Ton=Toff, the maximal alternating current is FiAC max=Φac1(alternating current through coil20)+Φac2(alternating current through the coil30). At other duty cycles the maximal alternating current amplitude through the middle leg250is reduced. If the lateral legs230and240are operated up to a saturation current density Bsattof common ferrite materials of 350-400 mT, the relation between the ripple current Iac and the total current Iac+Idc is adjusted to a value between 0.1 and 0.5, the minimal cross section A2of the middle leg240can become 0.2 to 1 fold of the cross section A1of the lateral legs. Preferably, the PFC-steps are adjusted such, that A1is in the range between 1×A1to 0.2×A1.

In order to reach a coupling between the coils20and30, the magnetic resistance RMAin the lateral legs should be 100 times the magnetic resistance RMIin the middle leg. The coupling factor results from k=RMI/RMA, wherein RMIis the magnetic resistance in in the middle leg and RMAis the magnetic resistance in the lateral legs. Despite a small cross section A2of the middle leg250this is reached through the air gaps L, which can be, for example, incorporated into the lateral legs230in the area of the coils20and30, so as to avoid that a relatively large direct current portion through the coils makes the core in the lateral legs reach saturation. Through the small magnetic coupling the magnetic fields of the coil20do not penetrate into the core section of the lateral legs of the coil30and inversely, as it would be the case for a strong coupling. For a strong magnetic coupling the magnetic fields of the coils would at least partially enhance each other, so that the saturation magnetization in the lateral legs would be reached faster, i.e. for a strong magnetic coupling of the coils20and30the cross section of the lateral legs would have to be dimensioned larger. However, in general it is advantageous to use materials with a low magnetic resistance (high permeability) in the middle leg and a high saturation magnetization in the lateral leg.

FIG. 5shows an implementation of the present invention according to a first embodiment with two E-shaped formed components210and220, which are connected such that their free ends meet. To illustrate the E-shape inFIG. 5larger gaps L1, L2and L3between the free ends of the two E-shaped parts210and220are shown. When implementing, as far as possible, no gaps L1, L2and L3should occur in order to avoid undefined air gaps, i.e. the ending surfaces on the free ends of the E-shaped parts have to be manufactured so precisely, that they lie in one plane, so that no air gaps occur. Air gaps are exemplarily selectively introduced in the lateral legs230in the area of the coils20and30. The cross section A2of the middle leg250is, as explained in detail above, smaller than the cross section A1of the lateral legs240and230.

In order to avoid a situation, as it can occur with E-shaped parts with the three gaps L1, L2and L3, two U-shaped core parts260and270can be used, which are connected such, that their free ends meet.

FIG. 6Ashows a schematic top view of a core from two U-shaped parts260and270with a middle leg250according to a second and third embodiment of the present invention. InFIG. 6Athe meeting edges of the free ends of the U-parts260or270are not visible. The coils20and30are positioned on the coils carrying lateral legs230of the core. The middle leg250fills the gap between the coils20and30and also forms a magnetic short-circuit between the lateral legs240, so that two magnetic loops are formed. The air gap L in the lateral legs230in the area of the coils20and30leads to an operation outside of the magnetic saturation in the lateral legs and, at the same time, to a lower coupling of less than one percent between the two loops, respectively loops20and30.

FIG. 6Bshows a perspective view of the scheme ofFIG. 6Aaccording to a second embodiment with an extracted middle leg350.FIG. 6Bshows only the core arrangement without the coil windings20and30according to the second embodiment. The core is composed of two U-shaped parts260and270, which meet at the face surfaces of the free ends of the U-shaped parts, as shown by line280inFIG. 6B. Since there are only two face surfaces280, it is easier to avoid uncontrolled air gaps in the lateral legs. The lateral legs have a cross section of A1=B1×H1. The distance of the lateral legs240, which connect the coils-wearing lateral legs230is T1. The middle leg350, which is extracted in the illustration ofFIG. 6Bfrom the ring structure, is designed in a T-shape with a vertical part350-1and a horizontal part350-2. The vertical part350-1has a height H2, a length T2and a width B2. In order to optimally fill the air space between coils20and30(seeFIG. 6A), the width B2of the vertical part of the T-shaped middle leg350corresponds to the clear distance of the space in-between the coils. The length T2of the T-shaped middle leg350corresponds to the distance T1between the lateral legs240and the height H2of the vertical part of the T-shaped middle leg350corresponds to the height H1of the lateral legs230and240. The excess ends of the horizontal part350-2of the T-shaped middle leg350have a length L1and are supported by the lateral legs240. The maximal length L2of the horizontal part350-2of the T-shaped middle leg350is maximally T1+2×B1, or the length L1of the excess ends of the excess part350-2, supported by the lateral legs240, are about B1. The air gaps L in the U-shaped parts260or270can be realized through filling materials like CEM1 or FR4. The thickness B2of the T-shaped middle leg350is smaller than the width B1of the lateral legs230and240.

FIG. 6Cshows a perspective view of the core according toFIG. 6Bin composite form. The reference signs inFIG. 6C, which are identical to the reference signs inFIG. 6B, denote the same technical features so that the explanations are not repeated at this place. The air gaps between the vertical part350-1of the middle leg350and the coil-carrying lateral legs230(the coils are not shown inFIG. 6C) are almost completely filled by the windings of the coils. InFIG. 6Cthe horizontal part350-2of the T-shaped middle leg350is flush with the lateral edges of the lateral legs240. However, small deviations, i.e. excess ends and shorter ends do not affect the magnetic behavior of the whole core.

FIG. 6Dshows a perspective view of a core according to a third embodiment of the present invention. The core is composed like inFIG. 6Dfrom two U-shaped parts260and270. Between the U-shaped parts260and270a middle leg450is positioned, so that the free surfaces280-1and280-2of the opened ends of the U-shaped parts260and270meet with the opposite sides of the middle leg450, which is rectangular-shaped with a height H3, width B3and a length of T1+2 B1. The size indications T1, B1, and H1correspond to the size indications inFIG. 6B. Otherwise, the U-shaped parts inFIG. 6Bcan be identical with the U-shaped part inFIG. 6D. InFIG. 6Dreference signs, which are identical to those in the previous figures, denote the same technical features so that a repetition is waived here.FIG. 6Dshows a schematic three-dimensional arrangement in which the single elements260,270and450are shown pulled apart. In the assembled state the middle leg450is flexibly glued to the lateral legs240-A,240-B,240-C and240-D, which can compensate for the tolerances in the lateral air gaps L. Small excess ends of the middle leg or slightly shorter middle legs only insignificantly influence the current flow from the lateral legs in the middle legs.