In the first half of the last century, the range of litz wire usage was consistent with the technology level of the day. For example, in 1923 the first medium frequency radio broadcast was made possible by litz wires in the coils. In the 1940's litz wire was used in the first ultrasonic diagnostic systems and basic RFID systems. In the 1950's litz wire was used in USW chokes. With the explosive growth of new electronic components in the second half of the 20th century, litz wire usage also expanded rapidly.

Elektrisola began supply of high frequency litz wires in 1951 to meet increasing customer demand for innovative quality products. Elektrisola litz wire was quickly incorporated into new developments such as ferrite core chokes for electromagnetic lighting ballasts in the 1960's, as well as magnetic resonance imaging systems developed in the 1970's and 1980's, and high frequency switching power supplies in the 1990's.

Since the beginning, Elektrisola has demonstrated an active partnership with its customers in joint development of new and innovative litz wire solutions. This close customer support continues today with new litz wire applications in the fields of renewable energy, e-mobility, and medical technologies being developed for use in future products.

Elektrisola offers a wide range of high frequency litz wires. Due to the different demands resulting from the broad field of applications there are several types of litz wires available.

Overview Litz Wire Types

The following table shows an overview of the basic product variations and their technical reference values. For direct comparison of features of selected product types click the link below.

Basic Litz Wire

Basic litz wires are bunched in one or several steps. For more stringent requirements, it serves as the base for serving, extruding, or other functional coatings.

Taped Litz Wire

Additional insulation can be added by wrapping a tape around the basic litz wire. Increased dielectric strength, high breakdown voltage, thermal endurance as well as flexibility can be achieved by a suitable combination of taping material, number of tapes, and the degree of overlapping of the tapes.

EFOLIT^®

For safety relevant applications demanding explicit certified high dielectric strength our EFOLIT ^®- Litz Wire family provides reinforced insulation with at least three layers of tape. Working voltages up to 1414 Vpeak for both temperature classes F/155°C and H/180°C can be supplied. The VDE certification with manufacturing surveillance guarantees an approved and continous high performance  level.

Extruded Litz Wire

Coating litz wires with extruded thermoplastic materials offers additional options for insulating litz wire conductors electrically. Extruded coatings are highly flexible and can also provide additional protection against moisture and chemical exposure.

TIW Extruded Litz Wire

TIW is extruded with ETFE (Ethylene tetrafluoroethylene) to insulate the litz wire in high voltage environment. TIW extruded litz wires are very flexible and meet a variety of safety requirements where Reinforced levels of insulation are required.

Product and process technology for the special extrusion comes from Rubadue Wire, USA, and is located in the Chinese Elektrisola Hangzhou plant to serve select customers for the electromobility OBCs and chargers in Asia.

Profiled Litz Wire

Basic litz wires and some types of served or taped litz wires can be produced with a square or rectangular cross section by a profiling process. The compacted profile provides an optimized copper filling factor for more efficient electrical properties in wound coils.

Litz Wire with Strain Relief

Very small litz wires or litz wires with a high demand for tensile strength or flex life performance can be reinforced with a high tensile mono or multifilament. For best performance those filaments are placed in the center of the litz wire. In some cases, it is sufficient to simply integrate the strain relief filament as a bunched non-conductive element within the litz wire.

Served Litz Wire

Smartbond Litz Wires

Self supporting coils can be produced with automated winding processes using litz wires provided with a 'Smartbond' adhesive. Very thin coils can be produced with Smartbond's unique self adhesive construction providing additional space for designers or to help achieve miniaturization objectives.

The use of litz wire for different applications is a very complex process, as it has to match best for very different technical problems. In this chapter, some basic considerations to help to design a litz wire are given.

The nominal outer diameter depends upon the method of twisting (directly, freely bunched or concentrically bunched), number of bunching steps, pitch direction, length of lay, and selected nominal diameter of single wires. The OD is also influenced by process specific factors.
Due to the litz wire’s natural flexibility, bending radius, and winding tension dependent dimensional stability, the nominal outer diameter is approximated by an average value in combination with a defined measuring method.
The nominal outer diameter of a bunched litz wire can be calculated guided by DIN EN 60317-11 with the following formula:

with packing factor k_PF, see table below

number of single wires

and nominal value for the outer diameter of the single wire

and increase in diameter by optional outer coating

with the square of calculated outer diameter of litz wire.

Calculation of High Frequency Litz Wire Losses

High frequency losses depend upon the cumulative influences of different loss mechanisms, as well as the expected working conditions of an individual application. Therefore, a simple differentiated formula-like calculation is not possible without a deeper understanding and additional tools.

Right-Hand Rule

A current I flowing through a straight conductor creates a magnetic field B, whose field lines are placed concentrically around the conductor. If a straight conductor is gripped with the right hand and the thumb points in direction of the flowing current I, then the fingers point in the direction of the circular magnet field B. The item B is also called magnetic flux density, which is proportional to the magnetic field intensity H and the material dependent magnetic permeability µ:

with magnetic field constant, permeability of free space

and relative permeability, ratio of the permeability of a specific medium to the permeability of free space

Please see the scheme “Right-hand-rule” below for illustration.

Reactance / Resistance RatioX/R

R_AC/R_DC

The complex resistance of a coil is defined as the impedance Z=R+jX which is a vector and consist of the resistance R representing the real component and the reactance X representing the imaginary component.

The current flows more and more along the outer surface of the conductor with increasing frequency. The measured alternating current resistance X, also referred to as R_AC rises compared with the direct current resistance R, sometimes referred to as R_DC. With increasing resistance values the ohmic losses through alternating current resistance rise and can even exceed the losses through direct current resistance at high frequencies.

The X⁄R ratio, sometimes referred to as R_AC⁄R_DC  ratio describes the alternating current resistance normalized to the direct current resistance (X⁄R ≥1) and is an indicator for the high frequency performance of a litz wire. The X⁄R ratio can be measured or calculated with sufficient accuracy in most cases for a given litz wire construction and is desired to be typically between 1-12 for the respective range of frequency. Along with the correct choice of the single wire dimension, the design of the litz wire construction plays an equally important role.

The graph

shows the calculated frequency dependent R_AC/R_DC trend of five different litz wire constructions with the same copper cross sectional area. It shows the alternating current resistance and the alternating current losses increase with the frequency and the thickness of single wire diameter. At a target frequency of 1 MHz the construction with 50 µm single wires has the best results. In this case, the related R_AC/R_DC ratio of 1.29 is still significantly higher from the optimum value of 1.0. In this case for example, a first step of improvement could be the selection of a smaller single wire diameter and/or optimization of the bunching construction.

Coil Quality Factor

The quality factor Q measures the freedom of losses of a swinging electrical or mechanical system.

As an example, a higher Q factor indicates a lower rate of energy loss relative to the stored energy of the resonator, the oscillations die out more slowly. A pendulum suspended from a high quality bearing, oscillating in air, has a high Q value. While a pendulum immersed in oil has a low Q value.

In an electrical oscillating circuit consisting of an air coil with inductance L, capacity C and ohmic resistance R, the Q factor measures the relationship between the total energy of an oscillation and its loss of energy per oscillation. An important characteristic of a high quality system is the use of a coil with high Q factor.

The basic loss factor of the coil is its resistance R_L. The resistance R_L increases with growing frequency, influenced by the frequency dependent skin and proximity effect.

The general relation of the quality can be described as

with different influencing factors which interfere with each other and lead to a frequency dependent trend of coil factor Q, as follows

Coil frequency f [Hz]

The Q factor increases with growing frequency and decreases again at a certain point due to disproportional rising high frequency losses and positive influence by litz wire construction features such as number of single wires, nominal diameter, and length of lay.

Coil inductance L [H]

The coil Q factor increases with growing inductance L (i.e. with increased number of windings N), the negative influence of the resulting increased coil resistance loss R compensates for this effect only at higher frequencies. The self-capacitance of the coil increases with the number of windings.

Coil Resistance R [Ω] dependent of frequency f

The coil resistance ohmic losses are influenced by the total conductor cross section A_CCS. The reduction of R leads initially to an increased Q factor, but at higher frequencies comes a stronger decrease in the Q factor due to increasing high frequency losses.

Positive Q factor influence by the litz wire construction may be possible (number of single wires, nominal diameter, length of lay, etc).

The graph

for different planar coils shows the influence of the litz wire construction and coil construction on the trend of coil Q factor by means of three measured planar coils with 12 windings and different Smartbond litz wire constructions.

By reducing the length of lay to 10 mm, indicated as a red line in graph, the coil Q factor can be increased over the complete frequency range in comparison to the blue solid line with length of lay of 26 mm. If the increase of coil Q factor is only necessary for a selective range of frequency like in this example until 150 kHz, it can be sufficient for longer lay length to increase the coil inductance L by choosing a higher number of windings, which are in this example at a range from 12 to 17. Here the Q factor increases for the indicated range of frequency, but drops faster for higher frequencies, compare blue dotted line with red solid line.

External Proximity Effect

The effect of current displacement can also be caused by the influence of external alternating magnetic fields of neighbouring conductors or other electrical components, please see the illustrating scheme “External Proximity effect”.

In contrast to eddy currents, which are induced by the skin effect, eddy currents induced by external proximity effect are not rotationally symmetric to the centre of the second conductor. The reason is the alternating magnetic field of the first conductor, which is created by an externally applied electric current through the first conductor.

Thus the induced eddy currents have nearly the same direction at any place on the affected conductor. The eddy currents cause ohmic losses, which lead to an apparent increase of the ohmic resistance as described in the previous section of skin effect. The necessary energy for moving these eddy currents is delivered by the causing magnetic field of the external current. Due to this general interference between eddy currents and its causing magnetic field, additional high frequency losses can also occur in any other neighbouring conductive material.

Internal Proximity Effect

The alternating magnetic fields of the single wires (strands) of a litz wire also create losses in neighbouring strands by eddy currents. Since these fields are created inside the litz wire by the strands itself, this effect is called internal proximity effect but formally seen as belonging to the skin effect, see scheme of current displacement below.

As a consequence, the electrical losses of a litz wire through the internal proximity effect increase with rising frequencies and can, in certain cases, even exceed the losses of a solid conductor with the same DC resistance.

The figure “Internal Proximity effect” shows the non-homogeneous distribution of current between neighbouring single wires (current density increasing from blue to red color).

This effect demonstrates that there is an optimal range of frequency for litz wires, in which the losses are lower than for a solid conductor. Beyond this range the use of multiple single wires such as a litz wire can have negative effects.

Both the skin effect and proximity effect are the most important aspects for considering high frequency losses in electrical conductors where the combined influence of inner proximity effect and outer proximity effect is dominating. For a specified working frequency in most cases only a litz wire construction can help to reduce these losses. In this case the construction parameters such as number of single wires, single wire diameter, number of bunching steps, length of lay (pitch) and lay direction have to be specified for each application. At the same time care has to be taken that each single wire occupies each place of the litz wire cross section consistently within a defined length so each wire maintains the same length and resistance. In combination with enamelled single wires litz wires are called high frequency (HF) litz wires in this context.

Selection of Litz Wire Parameters

Litz wire design - following aspects are covered in this chapter:

I      Litz wire features: Main influences of constructional parameters

II     Selection of single wire diameter

III    Selection of bunching construction

IV   Example: Litz wire for HF-layer winding

V    Comparison: Preselection acc. Charles R. Sullivan

I Litz wire features: Main influence of construction parameters

The performance of a litz wire is determined by its electrical, mechanical, thermal, and chemical features. While the thermal and chemical demands are fulfilled by selection of suitable insulation materials, i.e. enamel, the electrical and mechanical features depend primarily on the chosen parameters of the bunching construction.

The following table

gives an overview of the mutual influence of the construction parameters on the most important electrical and mechanical features of a litz wire.

The table

shows an overview of litz wire criteria reduced to typical high frequency coil relevance.

Often there can be conflicting demands in each application that have to be thought fully worked out between Elektrisola and the customer. Elektrisola's expertise in litz wire design and litz wire construction coupled with the customers performance expectations for their application results in a final product exhibiting the best aspects of performance, processing ability, and cost effectiveness.

II Selection of single-wire nominal diameter

The correct choice of the single wire nominal diameter is one of the most important aspects in designing a litz wire since this directly affects the performance of the litz wire

At the same time, it also influences all mechanical features.

The relationship of the single wire diameter to the dominate operating frequency and the expected skin depth of the device is shown in table

In general, because of the skin effect, the higher the operating frequency, the smaller the nominal single wire diameter has to be. To consider interaction between several bundle diameters OD_Bundle with skin depths δ in a simplified way, the maximum single wire diameter should be smaller or equal of nearly a third of δ:

Example:  

f  = 200 kHz
δ ≈ 0,172 mm
Ø_SW  ≈ 0,063 mm

Regarding the influence to the mechanical litz wire performance for equivalent total copper cross sections you can assume the following: 

The smaller the nominal single wire diameter the

• more flexible and softer the litz wire
• smaller the minimal bending radius is
• better the flexlife performance
• larger the total outer diameter of the litz wire
• smaller the litz wire filling factor
• higher the single wire costs

III Selection of bunching construction

When the number of single wires is determined for the application, the specific bunching construction can be chosen. Finer litz wire with a smaller number of single wires (typical < 60) are bunched in one step, thicker and more complex litz wires are bunched in multiple steps.

The bunching construction is specified by definition of length of lay (pitch), bunching direction (S or Z), and the number of bundles and bunching steps. Proper selection of bunching parameters is required to en-sure optimum electrical, mechanical, and processing related

Number of bundles and bunching steps

Parameters like total copper cross section, electrical resistance, or current density define the required number of single wires, which can be divided in several bundles and bunching steps. In consideration of these factors, bundles of the first bunching step can be designed for optimal high frequency performance. In consideration of these factors, the number of single wires in one bundle is typically less than sixty.

There are 4 basic bunching constructions typically used in the final bunching step: The 3, 4, 5 and concentric 7 bundle con-struction.

3, 4 and 5 bundle constructions

These bunching constructions show a good bundling performance with statistically homogeneous distribution of the single wires across the litz wire cross section. These constructions are preferred for optimal high frequency performance. The 5 bundle construction is preferred due to its round profile, since roundness increases with higher number of bundles.

Concentric 7 bundle construction

These concentric constructions, also called “1+6 bundling”, show both high flexibility and good dimensional stability and roundness. One bundle always runs centrally, so this construction is less suitable for applications with demand for optimal high frequency performance due to the difference in total resistance between bundles. In order to compensate for differences in bundle lengths for the final bunching step, the pitch direction of the central bundle is opposite to the direction of the concentric outer bundles. Therefore, the pitch direction of the central bundle always represents the pitch direction of the final bunching step.

The previous basic constructions can be combined with each other independently from the number of bunching steps and complexity of electrical and mechanical demands. Special constructions are possible.

Selection of length of lay and pitch direction:

The length of lay determines the mechanical compactness and the high frequency performance of a bundle. A measure for the tightness of a bunching step is the so called bunching factor. It proportions the length of lay to the outer diameter of the bundle and is typically in the range of 15 mm -20 mm:

Bunching factor

The bunching factor f_B can be calculated as follows

Depending upon the pitch direction, the bunching factor for pre-bunching steps is often chosen higher in many cases.

For the selection of length of lay and pitch direction the following basic statements can be assumed:

The smaller the length of lay,

• the more compact, stiffer, and more dimensionally stable the construction
• the larger the outer diameter of the bundle
• if optimal high frequency performance is demanded, an optimal combination of aligned pitch directions for all bunching steps must be chosen
• counter rotating pitch directions of multiple bunching steps are to be preferred for complex litz wire constructions, where high mechanical flexibility is demanded
• for wound coils the length of lay should be in the range of the smallest winding diameter

The table

shows an overview of different designs of a litz wire 270 x 0.071 mm and its features.

IV Example: Litz wire for high frequency layer winding

In many cases high frequency coils are layer wound with a small number of windings. Usually those litz wires are served with silk or nylon, since exact winding in layers is only possible with litz wires which keep their round shape on the coil bobbin also with applied winding tension. In some cases also unserved litz wires

can be used.  Here special attention is required to select solid and dimensionally stable constructions. Nevertheless, since small elliptical deformation is not avoidable, this has to be compensated by reducing the total outer diameter appropriately. For this reason, with a given maximum outer diameter in this case a served litz wire can show a higher copper cross section than a non-served construction.

Example

An example shows the simplified preselection of a litz wire construction for a layer winding with 30 windings and an operating frequency of 200 kHz. Assumed is a winding window with an effective usable size of width by height: 25.8 mm x 8.0 mm.

Layer construction

Depending on winding technology, layer windings can be constructed in layers with equal or alternating number of windings. For preselection it is possible to roughly calculate with the same number of single wires per layer. This results in 3 layers with 10 windings each for the winding window, and a calculated max. outer diameter for the processed litz wire of d_LW=25.8 mm ⁄ 10 = 2.58 mm.

Single wire diameter

The higher the applied operating frequency the smaller the single wires will become. At the same time the costs for the single wire will increase with smaller nominal diameter d_SW, as well as for the bunching process with increasing complexity of the bunching construction. Regarding the interaction between the thickness of the sub-bundles and the frequency dependent skin depth δ the ratio d_SW ≤ δ/3 can be taken approximately as indicator for the choice of the nominal single wire diameter. In practice it represents a working compromise between frequency performance and costs. Depending on application and technical demands also variations are allowable and common.
In this case a nominal diameter d_SW = 0.063 mm is sufficient for a first approach (see example above, section II).

Bunching construction

The total outer diameter of a litz wire depends on the dimensional stability of the individual wires bunched during the winding process. To take this into consideration as empirical value the calculated maximum outer diameter of the processed litz wire d_LW=2.58 mm, see above, should be reduced by 10% to d_SW=2.32 mm for the serving material and by 15% to 20% to d_SW=2.19 mm for the non-served construction.

The non-served litz wire should be bunched compactly, for example that means a small lengths of lay and same pitch direction per bunching step. Constructions with 4 bundles or 5 bundles are preferred.

The table

shows a comparison between suitable served and non-served litz wire constructions for operation frequencies of 50 kHz,125 kHz and 200 kHz and a winding window of width x height = 25.8 mm x 8.0 mm.

In this case, for the desired layer winding

• Copper filling factor of

is a little bit smaller compared with the basic litz wire. The number of single wires and thus the total copper cross section of the unserved litz wire still increases.

• Copper filling factor of the winding window lies typically in the range of 25% to 30%. It is higher for the served litz wire compared with the unserved basic litz wire due to its higher total copper cross section.
• A construction with 5 bundles enables a symmetrical litz wire structure with sub bundles of significantly less than 60 single wires.

If layer winding is not necessary and a randomly wound coil can be used, it is possible to produce a very flexible and soft litz wire. In this case the coil windings cling to each other, intermediate spaces are filled optimally and thus the copper filling factor of the winding window can be increased once again. Alternatively the usage of

is possible as well. It is also important to ensure that the copper cross section of the design allows for the required current capacity of the application to be achieved.

V Comparison: Preselection acc. to Charles R. Sullivan

Another method of simplified preselection of litz wires for RF coils is proposed by Charles R. Sullivan from the Thayer School of Engineering in Dartmouth, USA in his study

The parameters used are the skin depth, operating frequency, number of windings of the winding window, the width of the winding window, and from this calculated constant k is required. This method then proposes a number of suitable litz wire constructions consisting of nominal single wire diameter, a maximum number of single wires for the first bunching step, and the number of bundles for any further bunching step.

This is accomplished in the following steps: 

1. Determination of the skin depth δ calculated from the specific conductor resistance ρ, operating frequency f and the permeability µ₀.

2. Definition of available width b_W of the winding window and the requested number of windings N_W of a given coil construction. As an option, construction with an air gap can be considered.

3. Calculation of approximate values for the recommended total number of single wires n_SW depending on several nominal single wire diameters d_SW. The effectively applied number of single wires for a specific nominal diameter can deviate from the calculated value up to ± 25 %.

4. Selection of single wire nominal diameter and number of single wires is made. Following this selection, a determination is made as to which of the tabular single wire diameters (and number combinations) according to a given number of windings fits into the winding window. A winding window copper filling factor range of 25% to 30% is assumed. Demands regarding litz wire resistance and current capacity have to be determined. Alternative constructions with bigger single wires are also possible.

5. The interaction between skin depth and bundle diameter is taken into account: Calculation of the maximum number of single wires n_SW1max of the first bunching step is dependent upon the frequency influenced skin depth δ and the chosen nominal single wire diameter d_SW.

6. A part of the calculated total number of single wires, see (3), on several bunching step combinations of 3, 4, and 5 bundle constructions.

A recommendation of certain bunching lengths or bunching directions of the constructions is not given in this context. It is left to the litz wire manufacturers.

The linked table

compares the previously given practice-related selection of Elektrisola`s typical constructions with those according to the method of Ch. R. Sullivan. It is related to a layer wound coil and a winding window of 25.8 mm x 8 mm and operating frequencies of 50 kHz,125 kHz and 200 kHz.

The table shows that the litz wires, selected with the practice-related approach, correspond closely with those constructions selected with the Sullivan-method. They cover implicitly the recommended basic features:

• the total number of single wires of the practice-related samples lies within the range suggested by the Sullivan method.
• the combined application of 3, 4 or 5 bundle constructions are an integrated part of Elektrisola typical litz wire designs (see table 5).
• the single wires of the basic bundles in the first bunching step are independent from the respective construction and Elektrisola-typical free selectable within a number of 60 single wires (see table 5).
• cost reductions are possible with thicker single wires (d_SW≤ δ/3) designed constructions (see table 5), which show that the Sullivan recommended ideal basic bundling of ≤ 64 to 36 single wires.
• apart from cost reduction these constructions can additionally increase the filling factor of the litz wire and of the winding window (see table 5).
• Through careful selection of bunching length and direction, the product can be optimally specified for each unique application

Therefore, Elektrisola's applied design concepts for high frequency litz wires typically include both practical and theoretical requirements.

Elektrisola supplies high frequency litz wire for the following applications including certifications and requirements in accordance with the standards of each sector.

Content

1. Automotive
   1. EV components containing litz wire
      1. On-Board Charger
      2. DC/DC converter
      3. Wireless Charger
      4. Charging Station
      5. Electric Motor
      6. Basic Relations between Litz Wire and Power Electronic
   2. Interior Heating
   3. Interior Electronics

2. Industry
   1. Switched Mode Power Supply
   2. Sensors
3. Medical
   1. Hearing Aids
   2. Other Applications
4. Appliance
   1. Inductive Cooking
5. Renewable Energy
   1. Solar Inverter
6. Consumer Electronic
   1. Switched Mode Power Supply
   2. Smart Textiles
   3. Wireless Charging
7. Special Cables

• On-board charger (OBC)
• DC/DC converter
• Wireless charger (WC)
• Charging station
• Electric traction motor

Litz wire is necessary for the listed components to reduce copper losses at higher frequencies in order to increase efficiency.

Higher power densities are possible due to the efficiency improvements from the use of high frequency litz wire. Lighter and cheaper constructions are able to be used which lead to increased battery range for electric vehicles. In addition, cheaper production through material savings is also possible.

Elektrisola offers a wide range of litz wires, which are core materials and determining factors of several electric vehicle components, described in the following chapters.

General

Electric vehicles have at least two different voltage networks, one with a low voltage battery for all automotive peripherals and another with a high voltage battery for drive train components. The DC/DC converter ensures the bidirectional energy transfer between both networks by voltage conversion with galvanic separation.

Technical Details of DC/DC Converter

The low voltage EV network is typically based on lead acid batteries with voltage levels between 12 V and 48 V.

The EV high voltage network often includes a lithium based battery and the voltage level varies between 200 V and 1000 V.

Typical power of DC/DC converters is between 1.5 kW and 5 kW.

The power electronics typically use soft switching topologies with a resonant transformer stage with galvanic separation. Bidirectional energy flow is realised by buck/boost topologies between the two stages.

Inductive High Frequency Components

The efficiency of the DC/DC converter is the key to ensure a minimum construction volume and low weight. High switching frequencies help to reduce inductor and transformer sizes. Modern semiconductor technologies make higher frequencies possible, which enables even smaller inductors and transformers.

The switching frequencies range from 100 kHz to 550 kHz. The High Frequency litz wire products are optimised for the application to guarantee high filling factor for high efficiency while also maintaining high voltage durability.

Widely used diameter of single wire strands is 0.05 - 0.1 mm.

Typically values of temperature class varies between B(130°C) and F(155°C).

Litz wire products with taping, extrusion and serving are common.

Which one to choose depends on the individual application parameters e.g., available space and breakdown voltage requirements.

See examples of HF transformers in the following pictures:

General

Charging stations supply electric energy for the recharging process of the traction battery of plugin (H)EVs. Therefore, two different kinds of charging stations are available to convert AC from the power grid into DC for the EV battery:

AC charging station, which acts as energy source for the OBC.

DC charging station, which acts as direct energy source for the EV battery. The charger hereby is part of the charging station.

Technical Details of Charging Stations

AC charging station

For the AC charging process, the chargers are installed on board the electric vehicle (OBC), and details are described in chapter 1.1.1. A special version of AC charging stations are wireless chargers, which are described in chapter 1.1.3.

The on-board charger can be plugged in for electric power connection at either public charging stations or residential charging stations.

Public Charging

Public charging stations are often combined with public parking lots. They are owned by commercial or private ventures, sometimes in partnership with a parking lot owner. The following table gives an overview of the different charging stations.

Residential Charging

Residential or private charging stations, often referred to as “wall box” chargers are home charging stations, which can be installed at private facilities by an EV owner to charge the vehicle at home, but they are often limited by output power as either higher voltage sockets are not available or the current is limited.

A comparison of different charging options can be seen in the following table:

DC Charging Station

DC charging stations can achieve power higher than 43 kW because there is no limitation on space and weight inside the charger, as it is the case with the OBC installed in the vehicle.

The charger is integrated into the DC charging station, while the power electronic technology in external chargers is the same as in OBC.

Often modular constructions are used in DC chargers. Thereby the power can be easily increased by adding modules in a parallel circuit to the charging station.

A comparison of different charging options can be seen in the following table:

Inductive High Frequency Components for Charging Stations

For future types of EVs, the industry is planning for charging power up to 450 kW with voltages at 800 V. State of the art technology in power electronics development make voltage levels up to 1000 V possible. These high voltage levels allow for faster and more efficient charging.

These trends lead to high requirements with regard to the insulation of high frequency litz wire in the inductors and transformers of the chargers.

External DC chargers, in comparison to on-board chargers, can supply higher charging power, as the available space is not limited. On-board charging power is limited by weight and space of the components.

Typical litz wire constructions of high frequency transformers in DC chargers use single wires of 0.07 - 0.1 mm diameter, optimised for switching frequencies between 50 and 100 kHz.

Relation of Switching Frequencies and Inductor Size

The coil size in transformers, inductors, and chokes can be significantly reduced by higher switching frequencies.

Switching frequencies are limited by thermal losses, which occur during the switching process. This is why switching speed had to be shortened to minimise switching losses in power electronics and to reduce coil sizes for higher power densities and to lower volume.

Developments in power transistor technologies have reached significant alteration in switching speed and voltage range due to new semiconductor materials.

Shorter switching times mean that the voltage rising flanks are getting higher, according to the following table:

Relation of High Voltage Trend and Litz Wire Insulation Materials

The trend of increasingly higher voltages set demanding requirements for the insulation materials of litz wire. In addition, the high voltage potential between the single turns of the transformer needs consistently high insulation from nearby turns.

This is caused by the high frequent switched voltages, especially in converter topologies with resonant circuits.

Elektrisola designs the litz wire individually for each power transformer for optimum efficiency, high voltage protection, thermal conductivity, and filling factor.

Relation of Switching Methods, Breakdown Voltage and Partial Discharge

Voltage is controlled by semiconductor switches to impress current into inductors in all power electronics. These consist primarily of litz wire in high frequency applications. The switching methods differ depending on the application between hard switching and soft switching.

Hard switching means that the switching operation is done at the time when the current and the voltage are not equal to zero. Thereby switching losses are high and durability is low. Partial discharge risk is high because of the significant voltage increase per time on inductors.

Soft switching means that the switching operation is done at the time when the current and the voltage are zero. Switching losses are low inside the transistors and improve efficiency and durability of the power electronics and inductive components. The risk for partial discharge is low.

General

Wireless charging of cell phones is an increasingly common addition to the passenger cabin as it removes the need to handle a charging cord for the customer.

In this market, two technologies for wireless charging systems are widely used:

Inductive Wireless Charging System

The wireless transmission of power is performed by mutual induction of magnetic fields between a transmitter and receiver coil. In the transmitter coil an alternating current creates a time varying magnetic field, which is radiated in all directions. The highest power transfer efficiency in this system is achieved when a receiver coil with identical dimensions to the transmitter coil is placed precisely on top and a few millimetres away from the transmitter coil.

The main advantage of inductive wireless charging systems is a relatively high efficiency, when transmitter and receiver are tightly coupled.

This is preferred when a high efficiency from the charging system is needed.

Resonant Wireless Charging System

This system is still “inductive”, in the sense that a magnetic field generated by the transmitter coil induces a current in the receiver coil. However, the principle of transmitting energy is done at higher frequencies than inductive systems and relies on both transmitter and receiver coils operating at the same resonant frequency. In a resonant wireless charging system, a type of energy tunnel is created between the coils, which allows an energy transfer at larger distances, between multiple coils, and in multiple directions.

There is some loss of efficiency in the system due to flux leakage even at closely positioned coils.

Therefore, this system is preferred when convenience of easy positioning is required from the system.

Technical Standards

There are 2 main standards for wireless charging for consumer electronics:

• Qi

(Developed by the Wireless Power Consortium “WPC”)

Technology: Inductive and Resonant

Operating Frequency: 85 kHz - 205 kHz

Transmitter Power Levels: 5 W -15 W

• Rezence

(Developed by the Alliance for Wireless Power “A4WP”)

Technology: Resonant

Operating frequency: 6.78 MHz

PTU (Power Transmitter Unit) Power levels: 2 W - 70 W

PRU (Power Receiver Unit) Power levels:

3.8 W - 50 W

Wireless Charger Components in the Qi Standard

Power Transmitters in the Qi standard are specified from design types from A1 to A34 (having one or more Primary Coils) and design types from B1 to B7 (having an array of Primary Coils). The different design types differ from one another among other things in coils´ size and shapes (round, oval, and square), all having litz wires with differ-ent numbers (from 24 to 180) of single wires of 0.08 mm.

Even though Power Receivers are not defined by the Qi standard, it sets some examples using similar litz wire configu-rations as the Power Transmitters.

Typical Litz Wire Products

For typical litz wire products for wireless charging designs see the following links:

• “Self Bonding Wire”
• “Basic”
• “Served”
• “Smartbond”

General

The optimal High Frequency (HF) performance of enamelled litz wire and the wide range of possibilities for unique designs make litz wire an optimal material for sensors and antennas.

Examples are

• Inductive proximity sensors
• Metal detection sensors
• Coils for RFID-Systems
• Coils for Near Field Communication

Technical details

Radio Frequency Identification (RFID)

The reading device creates a high frequency field such as a gate that acts on the antenna of the transponder. Voltage is induced in the antenna; the chip is activated and creates an individual response by field attenuation. The response is received by the reader.

Near Field Communication (NFC)

NFC defines different communication protocols for a communication between two electronic devices over a distance of 4cm.

The low speed connection can be used for the following exemplary applications:

• Contactless payment systems
• Information exchange
• Identity and access tokens
• Smartphone automation
• NFC tags

Inductive Proximity Switches

The function of the sensor is based on an oscillating circuit consisting of a coil and capacity as characterized by the Q-factor. The coil itself creates an electromagnetic field that covers the area of detection. In the presence of a conductive element in the area, the Q-factor is changed due to eddy current losses.

The loss of field energy is detected by electronic circuits of the proximity switch and the element is confirmed.

RFID devices and proximity switches are used in logistics, process automation, and railway technology.

Typical Litz Wire Products

Elektrisola offers the following design options:

• High frequency litz wires with selfbonding enamelled wires allow for the smallest bending radiuses with high copper filling factors.
• Litz wire variants with Smartbond overcoat ensure easy hot air bonding with the highest copper filling factor.
• Thermal bondable silk or nylon served litz wires from Elektrisola guarantee optimal form stability and windability for air coils.

For detailed information see:

General

The solar inverter is the heart of a solar power system. It converts DC current from the solar panels into the grid alter-nating current.

At the same time electronic control in the inverter monitors the whole solar power system and the grid.

A solar inverter has the following tasks:

1. Efficient energy conversion from lower voltage DC to higher voltage AC
2. Power optimisation
3. Monitoring of power and temperature data
4. Communication to smart energy systems
5. Temperature management to avoid overheating of solar panels

Technical Details of Solar Inverters

Solar inverters can be categorized, dependent on three properties:

1. Power

The power ranges from a few kW to MW range. Typical values for private households are 5 kW, for industrial 10-20 kW and 500 kW and up for solar power plants.

1. Module interconnection

on the DC side solar inverters can be connected to a string, multistring, or central topology, dependent on the power and efficiency requirements.

1. Circuit topology

The inverter can be designed for single phase AC grid or three phase AC grid, and can be either with or without galvanic separation.

Galvanic separation is realised with a transformer between the DC and AC side. By doing this the module stack can be coupled to the ground to prevent alternating voltage potentials, which is compulsory in some countries.

Inverters without galvanic separation have the DC and AC side electrically connected, which leads to the advantage of higher efficiencies, but the disadvantage of alternating voltage potentials against ground which limits the lifetime.

Litz Wire Requirements for Solar Inverters

• Temperature index TI = 155 °C
• High mechanical robustness
• Good flexibility
• High dielectric breakdown voltage

Litz wire is used for special applications in Smart Textiles. Generally, very thin litz wires are used, as the wire must blend into the fabric and not disrupt the base texture. At the same time, rough handling in daily use, as well as washing or cleaning, creates a very demanding environment.

General

• Sports and casual clothings
• Protective gear for security forces, RFID Identification
• Building security technology
• Festive clothes/uniforms
• Mechanical engineering/monitoring of multi material systems, medical engineering

Technical Details

• Small outer diameter of litz wire
• Textile characteristics, fine structures
• High tensile strength, splice, and flex life performance
• Chemical stability

• Color retention

For use in textile applications special enamelled wires are needed, see

Typical Characteristics of Smart Textile Wires

• Single wire diameter: 0.02 mm - 0.071 mm
• Number of single wires: 2 - 100
• Conductor material: copper and alloys with and without silverplating
• Insulation: Polyurethane based enamels, extrusion coatings with low melting points
• Optional: Bare single wires
• Optional: Strain reliefs
• Optional: Nylon or silk serving for mechanical support

General

Litz wires can be used in similar ways as a cable because the individual strands of enamelled wire have a very strong insulation. Extrusion of the litz wire can be used to further strengthen the mechanical and electrical properties, but also adds weight and volume. There are some applications that use a litz wire with enamelled wires directly as a cable, having advantages on diameter, weight and volume. Colored single wires help to differentiate the individual strands in the litz wire for the further processing.

Typical Litz Wire Components

Examples for special litz wire conductors and cables are

• High frequency litz wires with superfine and very complex bunching construction, for example 600 x 0.010 mm or 25.000 x 0.20 mm.
• Micro cables with the smallest possible outer diameters of mere millimetres, for example 7 x 0.010 mm.

• Diagnostic cables with thermally fixed selfbonding enamelled wires. For example twisted pair wires (e.g. 2 x 0.020 mm) or planar Multifilar Wires.
• Trailing cables with maximum flexibility and low friction constructions and enamel coatings.
• Vibration resistant signal line with conductors based on special alloys (e.g. 7 x 0.04 mm).
• Litz wire for headset cables which are extruded together to get a thin light weight cable.
• High frequency litz wire with special color coding.