Background art

Generally, a known wireless inductive charging system, also known as inductive power transfer IPT system, comprises a DC power source such as a PV panel or a rectified DC input from AC mains. Further, a known charging system comprises a DC/AC inverter being fed by the DC power source and generating an AC charging signal for feeding a primary coil transmitting a wireless charging signal to a secondary coil of a device to be charged. This establishes a wireless power transfer from a charging unit to the device to be charged such as an electric car or a home appliance.

Typically, the DC/AC inverter, e.g. implemented as a pulse width modulated (PWM) inverter generates square wave voltage signals causing multiple harmonics in the electromagnetic spectrum. Radiation energy in the higher order harmonics induce or radiate high EMI emissions which is undesired and/or incompatible with health and safety standards.

US patent document US2014/0232197 describes systems, methods and apparatus for connecting and operating an AC source to a load, in the area of wireless power transfer. The topology allows for multiple sources to be operatively connected to a single conductive structure configured to generate a field, maintaining overall system power while lowering the power output of each source.

US patent document US2014/0361628 describes systems, methods and apparatus for connecting and operating an AC source to a load, in the area of wireless power transfer. The topology allows for a single source to energize one or more conductive structures configured to generate a field, improving power transfer to a power receiver.

US patent document US201 1/0080056 describes a method and apparatus for contactless power transfer relating to an impedance transformation network.

US patent document US2014/01521 15 describes systems, methods, and apparatus provided for tuning in a wireless power transfer circuit. The apparatus includes a field effect transistor configured to electrically engage a tuning element to an AC power path.

Summary of the invention

It is an object of the present invention to provide a wireless inductive charging system with reduced EMI emissions. Thereto, according to the invention, the system further comprises a resonance band pass filter eliminating harmonics from the AC charging signal generated by the DC/AC inverter before feeding the primary coil. According to the present invention, a wireless inductive charging system as defined above is provided, comprising a DC power source, a DC/AC inverter being fed by the DC power source and generating an AC charging signal, and a primary coil being fed by the AC charging signal and transmitting a wireless charging signal to a secondary coil of a device to be charged. The system further comprises a space harmonic elimination stage configured to eliminate harmonics from the AC charging signal generated by the DC/AC inverter before feeding the primary coil, wherein the DC/AC inverter and the space harmonic elimination stage are located within an EM shielded structure.

By controlled application of a resonance band pass filter in combination with an EM shielded structure higher order harmonics from the AC charging signal can be eliminated thereby also removing such higher order harmonics in the transmitted wireless charging signal. Then, EMI emissions are reduced considerably, especially if the DC/AC inverter and the resonance band pass filter are located within an EM shielded structure, such as a faraday cage. As an example, the second and higher order harmonics, or the third and higher order harmonics can be eliminated or suppressed. Preferably, only the fundamental component is allowed to pass. In principle, EMI emissions can totally be reduced.

The invention also relates to a method for wireless inductive charging.

Further embodiments are described by the dependent claims. Short description of drawings

By way of example only, embodiments of the present invention will now be described with reference to the accompanying figures in which,

Fig. 1 shows a circuitry of a wireless inductive charging system according to an embodiment of the present invention provided with a filtering transformer topology;

Fig. 2A shows a portion of the circuitry of Fig. 1 including an implementation of the filtering transformer topology;

Fig. 2B shows a circuitry of an embodiment of the present invention including an implementation of the filtering transformer topology with EM shielded structure;

Fig. 2C shows an alternative circuitry implementation of the embodiment shown in

Fig. 2B;

Fig. 3 shows a first diagram with voltage and current signals;

Fig. 4 shows a filtering transformerless topology;

Fig. 5 shows a diagram illustrating efficiency and power factor;

Fig. 6 shows a second diagram with voltage and current signals, and

Fig. 7 shows a flow chart of an embodiment of a method according to the invention. It is noted that the figures merely show exemplary embodiments according to the invention. In the figures, the same reference numbers refer to equal or corresponding parts. Description of embodiments

According to the present invention embodiments a wireless inductive charging system is provided, comprising a DC power source, a DC/AC inverter being fed by the DC power source and generating an AC charging signal, and a primary coil being fed by the AC charging signal and transmitting a wireless charging signal to a secondary coil of a device to be charged.

Figure 1 shows an example of a circuitry of a wireless inductive charging system 1 according to the invention provided with a filtering transformer topology. The system 1 comprises a DC power source 10, e.g. a 750 V PV panel, and a DC/AC inverter 12 being fed by the DC power source 10 and generating an AC charging signal.

The system 1 includes a resonant inductive power transfer system 18 that can be implemented as a serial-serial SS (as shown in the exemplary embodiment of Fig. 1 ), serial- parallel SP, parallel-serial PS, parallel-parallel PP or LCL circuitry.

In the shown embodiment, the DC/AC inverter 12 is implemented as a DC/AC phase shift inverter (e.g. a PWM inverter) having a multiple number of boost Mosfets 12a-d that are arranged in parallel with reversely oriented diodes 12e-h and connected in a full bridge structure for generating a high frequency AC charging signal. In principle, the frequency of the AC charging signal can be any arbitrary value. As an example, the AC charging signal frequency can be in a range from circa 70 kHz or lower to circa 100 kHz or higher. In practice, the AC charging signal frequency can be circa 85 kHz so that the signal can be touted for wireless electric vehicle charging. The Mosfets 12 a-d may include SiC Mosfets or other suitable Mosfet switches. The diodes 12e-h can e.g. be implemented as SiC Schottky diodes or other suitable diodes. The inverter can also be operated as a single Active Bridge for voltage cancellation.

For the purpose of providing DC power to the DC/AC inverter 12, the system 1 is further provided with a transient absorber circuitry 14, in the shown embodiment including a St.St. Mosfet 14a and a transient absorber resistor 14b arranged in parallel. However, also other transient absorber circuitry could be applicable. Further, the system 1 includes a DC/DC Boost converter 16 having a Boost inductor 16a, Boost Mosfet 16b, diode 16c and Boost Capacitor 16d. The transient absorber circuitry 14 and the DC/DC Boost converter 16 are located between the DC power source 10 and the DC/AC inverter 12. In principle, the system 1 can be provided without transient absorber circuitry 14 and/or without DC/DC Boost converter 16.

Further, the system 1 includes a resonant inductive power transfer system 18 with a primary coil L _P that is fed by the AC charging signal. During operation of the system 1 , the primary coil L _P that transmits a wireless charging signal to a secondary coil L _s of a device 20 to be charged. In one embodiment, both the primary coil L _P and the secondary coil L _s are arranged in series with a primary and secondary capacitor C _P , C _s , forming a resonant SS transmitting and receiving circuitry, respectively. However, also other transmitting circuitries and/or or receiving circuitries can be applied.

In Fig. 1 an equivalent resonant transformer SS topology is shown, also known as air core resonant transformer SS topology, is shown including a mutual inductance coil L _m , the primary coil L _P , (shown as series circuit of L _P -L _m and L _m ). the secondary coil L _s (shown as series circuit of L _s -L _m and L _m ), the primary capacitor C _P , and the secondary capacitor Cs. Here, a so- called environment series resonance is formed by L _P -L _m and C _P in series, while a so-called pickup series resonance is formed by L _s -L _m and Cs in series. The mutual inductance coil L _m represents an air core mutual inductance.

In the circuitry shown in Fig. 1 , the device to be charged 20 is provided with an

AC/DC full bridge rectifier 22 including four diodes 22a-d connected in a bridge structure, and a capacitor 22e arranged in parallel. Further, the device to be charged 20 is electrically represented by a load resistor 24.

The system 1 also includes a resonance band pass filter 19 or space harmonic elimination stage 19, also referred to as filtering transformer topology, which is arranged for eliminating harmonics from the AC charging signal generated by the DC/AC inverter 12 before feeding the primary coil L _P . In the shown embodiment, the filtering transformer topology 19 is located between the DC/AC inverter 12 and the primary coil L _P .

In the embodiment shown in Fig. 1 , the filtering transformer topology 19 is also arranged for stepping down the voltage of the AC charging signal so that a relatively high voltage and, therefore, a relatively low current can be applied in the DC/AC inverter 12. However, in principle, the filtering transformer topology 19 can be implemented without a stepping down functionality. The resonance band pass filter 19 may be arranged for eliminating second and higher order harmonics from the AC charging signal, thereby reducing EMI radiation emission. However, also other harmonics could be eliminated, e.g. third and higher order harmonics, or fourth and higher order harmonics.

Figure 2A shows a portion of the circuitry of Fig. 1 including an implementation of the filtering transformer topology 19, depicted at the left-hand side in Fig. 2A. In the shown circuitry portion 2, the air core resonant transformer SS topology 18 is depicted at the right-hand side connected in parallel to a load equivalent 24' representing the load resistivity 24 and the AC/DC full bridge rectifier 22 of the device to be charged 20 shown in Fig. 1.

In the shown embodiment, the filtering topology 19 includes a serial resonance filter 19a eliminating harmonics from the AC charging current signal as well as a parallel resonance filter 19b eliminating harmonics from the AC charging voltage signal. The serial resonance filter 19a has an inverter series resonance inductor L and an inverter series resonance capacitor C, in series, providing an eliminating functionality for the AC charging current signal. Further, the parallel resonance filter 19b includes an inverter parallel resonance capacitor C arranged in parallel with the primary coil L _P , providing an eliminating functionality for the AC charging voltage signal. Thus, in the shown embodiment, the resonance band pass filter 19 includes a combined series and parallel resonance filter eliminating current and voltage harmonics from the AC charging voltage signal. Further, a step op or step down transformer function is implemented in the filtering topology 19 (depicted by the transformer equivalent circuitry of inductances LM, Lp-M and Ls-M).

Generally, the filtering topology 19 may include both the serial resonance filter 19a and the parallel resonance filter 19b. Alternatively, the filtering topology may merely include either the serial resonance filter 19a or a parallel resonance filter 19b, for eliminating harmonics in the AC charging current or voltage signal, respectively.

According to the present invention embodiments a wireless inductive charging system as defined above is provided, comprising a DC power source, a DC/AC inverter being fed by the DC power source and generating an AC charging signal, and a primary coil being fed by the AC charging signal and transmitting a wireless charging signal to a secondary coil of a device to be charged, wherein the system further comprises a space harmonic elimination stage 19 configured to eliminate harmonics from the AC charging signal generated by the DC/AC inverter before feeding the primary coil, wherein the DC/AC inverter and the space harmonic elimination stage are located within an EM shielded structure.

Figure 2B shows a circuitry including an implementation of the filtering transformer topology with EM shielded structure. As shown in the embodiment of figure 2B, the space harmonic elimination stage 19 includes a coil Li and capacitor Ci arranged in series as the serial resonance filter 19a eliminating harmonics from the AC charging current signal, as well as a parallel resonance filter 19b provided with a second coil l_2 and a second capacitor C2 arranged in parallel eliminating harmonics from the AC charging voltage signal. An equivalent resonant transformer topology 18, also known as air core resonant transformer topology, is shown including the primary coil L _P , the secondary coil L _s , the primary capacitor C _P , and the secondary capacitor Cs. Thus, in the shown embodiment, the resonance band pass filter 19 includes a combined series and parallel resonance filter eliminating current and voltage harmonics from the AC charging voltage signal

The large reluctance associated with the air-gap in loosely coupled coils such as the shown primary coil L _P and secondary coil L _s arrangement, results in poor efficiency and large leakage reactance. Reactive power compensation is implemented by adding the primary and secondary capacitors C _P , Cs to the primary and secondary coils L _P , L _s as shown. This system of double-resonating coils in effect acts as a band-pass filter effectively allowing only power at resonant frequency to be transferred to the load (device to be charged 20) at high efficiency. Depending on the connection of the capacitors C _P , Cs to the coils L _P , L _s there are four basic compensation strategies as already discussed above: Series - Series (SS), Series - Parallel (SP), Parallel - Series (PS) and Parallel - Parallel (PP).

More generic, in exemplary embodiments of the present invention the system includes a resonant inductive power transfer system 18 comprising the primary coil L _P and the secondary coil L _s , the resonant inductive power transfer system 18 being implemented as a series-series (SS), a series-parallel (SP), a parallel-series (PS), a parallel-parallel (PP) circuitry or as an LCL-circuitry, etc.. All components on a primary side of the resonant inductive power transfer system 18 with the exception of the primary coil L _P are located within the EM shielded structure.

The DC/AC inverter 12, the space harmonic elimination stage 19, and the components of the air core resonant transformer 18 upstream of the primary coil L _P (i.e. capacitor Cp) are located inside an EM shielded structure 1 1 , e.g. implemented as a Faraday cage. By providing a band type filtering topology in combination with the EM shielded structure 1 1 , only base harmonic signals in a pre-defined spectrum band may pass thereby removing signals having a spectrum outside said pre-defined spectrum band.

It is noted that the air core resonant transformer 18 may be implemented to provide a step up or step down conversion. Figure 2C shows an alternative embodiment of the circuit implementation shown in Figure 2B, wherein the step-up or step down conversion is implemented upstream from the primary coil L _P . In this exemplary embodiment, the parallel resonance filter 19b is implemented using a transformer T2 in place of the second coil l_2 of the embodiment shown in Figure 2B. In more generic terms, embodiments may be provided wherein the space harmonic elimination stage is further arranged for stepping down or stepping up the voltage of the AC charging signal.

Figure 3 shows a first diagram with voltage and current signals related to the circuit diagram shown in Figure 2A (and equivalent locations in the circuit diagrams of Fig. 2B and 2C). Specifically, on the left-hand side, the upper sub-diagram shows a block type charging voltage signal 31 that is input to the filter topology 19, the second sub-diagram shows the corresponding charging current signal 32. Further, the third sub-diagram shows a voltage signal 33 across the inverter series resonance capacitor C, while the fourth sub-diagram shows a voltage signal 34 across the inverter series resonance inductor L. On the right-hand side, the upper sub-diagram shows a voltage signal 35 across the primary inductor L _P , while the second sub-diagram shows a current signal 36 across the inverter parallel resonance capacitor C. Further, the third sub- diagram shows a filtered charging voltage signal 37 at the output of the filter topology 19, while the fourth sub-diagram shows a corresponding filtered charging current signal 38. Clearly, the output signals, voltage and current, have a pure sinusoidal behaviour. The first harmonic of the block type charging signal has passed through the filter 19 while second and higher harmonics have been eliminated.

Figure 4 shows a filtering transformerless topology 19' of the space harmonic elimination stage 19 of the wireless inductive charging system 1 according to a further embodiment. Here, no stepping down transformer functionality has been included. The filtering topology 19' includes a serial resonance filter 19'a provided with a coil Li and capacitor Ci arranged in series eliminating harmonics from the AC charging current signal as well as a parallel resonance filter 19'b provided with a second coil l_2 and a second capacitor C2 arranged in parallel eliminating harmonics from the AC charging voltage signal. According to the present invention, embodiments may be provided wherein the space harmonic elimination stage 19 comprises a series resonance filter 19a; 19'a to eliminate current harmonics, or a parallel resonance filter 19b; 19'b to eliminate voltage harmonics. Furthermore, the space harmonic elimination stage 19 may comprises a combined series and parallel resonance filter eliminating current and voltage harmonics, respectively, from the AC charging voltage signal. Alternatively, or additionally, the space harmonic elimination stage 19 may be implemented as a cascade circuitry of a plurality of filter stages (such as series resonance filter 19a and parallel resonance filter 19b).

Figure 5 shows a diagram 40 illustrating an efficiency 41 and power factor 42 of the filtering topology 19' shown in Fig. 4.

Figure 6 shows a second diagram with voltage and current signals. Here, on the left-hand side, the upper sub-diagram shows a block type charging voltage signal 51 that is input to the filter topology 19', while the lower sub-diagram shows the corresponding charging current signal 52. Further, on the right-hand side, the upper sub-diagram shows a filtered charging voltage signal 53 at the output of the filter topology 19', while the lower sub-diagram shows a corresponding filtered charging current signal 54.

In principle, the wireless inductive charging system 1 can be used for charging any device to be charged, e.g. a dynamic device such as an electric vehicle. The wireless inductive charging system 1 can also be used with an electric appliance for home, industry, office or a communication application, such as a mobile phone. In the latter case, the wireless inductive charging system 1 can be included in a kitchen, home or office furniture.

Generally, a wireless inductive charging system is provided with a filter topology to eliminate harmonics presented by the PWM inverter. To achieve this, various components may be needed. The circuitry shown in Fig. 1 indicates charging/transferring power wirelessly from a PV panel, it could also be a rectified dc input from ac mains. The input dc voltage line is fed into the boost converter, the inverter is located such that it receives the boosted DC output of the converter. The inverter is a full bridge using SiC Mosfets and Reversed SiC Schottky diodes in order to invert the DC input to high frequency, typically circa 85kHz AC. The two legs of the full bridge inverter are driven by the Texas Instrument C2000 microcontroller, creating 4 PWM signals to the gates of the Mosfets with adjustable phase shift by a control loop according to the voltages on specific pin of the microcontroller.

Then, the output of the inverter, is connected to the input of a filter, and then to a resonant SS system with the a primary coil and a capacitor, in series.

First, the output power signal is connected to a filter topology to filter the square wave voltage output of the inverter which is the reason for high EMI emissions.

In a first embodiment, a Step Down Filter Transformer is included in the filter, while, in a second embodiment, a Resonant Filter S and P are included in the filter.

Generally, Inductive Power Transfer IPT, also referred to as Air core resonant transforming, is the process of transferring power between circuits without wired interconnects by the process of electromagnetic induction in the near-field. Near-field inductively coupled systems include two coils separated from each other by an air-gap with the magnetic flux created by the exciting coil/primary inducing an emf onto the pickup/secondary. This establishes wireless/inductive power transfer between the coils separated in air. In the main air core transformer, the primary coil is transferring power to the secondary, which is also using an S topology of a coil and capacitor. The high frequency AC voltage and current that is created in the secondary part, is connected to a full bridge active or passive rectifier, in order to be transformed into DC and this is fed into the battery of a device to be charged, such as an electric car through the charger that is included in the battery system.

According to a first embodiment, a step down/up filter transformer is included in the filter topology. Specific laboratory implementation design conditions can be determined as follows. The input voltage of the system implemented is 750V, but this would lead to higher current values to achieve 1 1 kW power, that may destroy the Mosfets and Diodes. For that reason, the voltage was raised with the Boost converter up to 1.1 kV (actually 1 kV in the lab), so that the current value is reduced. So, after the inverter, a step down transformer should be introduced in order to lower the voltage around at around 350-400Vrms and transfer the power through to the air core transformer that has a turn ratio of 1 : 1. Again, this was chosen to happen inside the inverter and not outside (so not stepping down the voltage in the air core transformer that is located in the environment), so that the Faraday box isolation can be achieved and used. It must be kept in mind that the car may be charging at 400-420V DC, so the output of the filter should force this voltage on the charger of the car. The step down filter transformer that is suggested is presented in the schematic shown in Fig. 2.

In an alternative embodiment the primary coil L _P and the secondary coil L _s are arranged as a loosely coupled transformer. This can be implemented e.g. using an air gap transformer, or any transformer with a coupling factor having an absolute value \k\ in the range of 0.01-0.99, e.g. in the range of 0.01 to 0.5.

The filter topology is designed to enforce a sinusoidal voltage current source through the SS resonant topology creating a sinusoidal current, that is fed from a square wave voltage source inverter switches. Then a sinusoidal current source will induce a voltage on the secondary coil, that is resonating with a series capacitor that is forcing to "receive" a sinusoidal voltage scheme. By that way, there is a natural way to have both sinusoidal current and voltage, created inside the inverter box, that will feed the air core transformer that is located in the environment, eliminating almost all the radiated and conducted EMI. A question for the schematic is, why not using the Primary coil of the transformer as SS resonance and what kind of transformer this should be. The answer to the second question, comes very easily by the fact, that the voltage that is needed in the secondary, is now forcing a very high current. It is interesting to note that the filtering topology Is not achieved by using a tight transformer, the reason being saturation of the core under resonant voltage conditions. For that reason, an air core transformer may be used, in order to avoid naturally the saturation. The value of the coupling factor that will be used is k=0.6, which is quite easy to succeed in the lab, due to the very small distance between the coils and the value of the turns ratio n and mutual inductance can be calculated. To answer the first and the rest of the second question, a mathematical approach will be presented. To start with, the turns ratio n needed, will be calculated and the coil values will be also estimated in order to have sinusoidal waveforms. The output of the inverter, is a square wave voltage with 1 kV amplitude. Because the power is transferred with the 1 ^st harmonic of the output voltage, this means that the output of the inverter has:

4

- lkV

Vo rms = ^1L -=- = 900V

1 t 2

harm

From the rectifier part, the impedance that the secondary resonance system (in the environment) sees is equal to 12.09Ω for 1 1 kW power. So the rms voltage of the specific spot is:

Vss secondary = VllkW * 12.09Ω = 365V

and this voltage is reflected to the primary resonant system, due to 1 :1 ratio of the main air core transformer.

So the turn ratio of the filter, should decrease the voltage from 900V to 355V, leading to the ratio of the filter:

Vi 900

n ⁼ vT 355 = ² - ⁵

Since the ratio is defined and the coupling factor was considered to be k=0.6, now the coil values can be defined:

The value of the mutual inductance M is very crucial for the whole operation of the system and various factors should be considered first, before assigning a value that will define the final physical coils.

Here, the quality factor Q of the resonant topologies may be relevant. Quality factors are the loaded quality factors and not native quality factors. Hence values can be 3, 4, 5. In principle, every quality factor Q > 3 can produce a pure sinusoidal waveform, no matter if it is a series or parallel resonance. For the parallel and series resonance, that are located in the secondary and primary side respectively inside the inverter box, the quality factors are:

o _ H O — ^Ω ° ^ ^ΕΧ1;Γ3

Vparallel f and ^series n

where R are the impedances seen by each resonant topologies that can be considered almost the same (before and after the step down filter).

At the same time, the capacitors that will be used should be equal with:

1 1

Cs _econc j _ar y 2 3nd Cp _r j _mar y 2

ωο ^L S ^ω ο ^L extra

In case the series resonance was used with the Primary coil and not with an added coil, then this would be a SP resonant topology and the primary capacitor would have been equal with: ^Primary

M_ ²

ω ₀ ² (¾> - ^-)

Ls ^"

Having this in mind, it can be proven, that the quality factors needed cannot be succeeded at the same time in case the C _Primary was compensating the primary coil L _P . To prove that case, consider:

nkR ω ₀ Ι_, _Ρ ω ₀ ηΜ

^parallel and Qseries ^— so,

0) _n L ω ₀ Μ Rk

Qparallel Rk

= a -

Qseries ω ₀ Μ M ²

That proves that the quality factors cannot be at the same time higher than 3 because

i

both are depended on— , and an added degree of freedom is needed, which is also proven by simulations. So an extra coil (L _extra ) is used to create the SS resonance current source, independently.

Coming back to the initial question, the M should be estimated. To succeed that, two main sections should be discussed. To define it easily, the quality factor is a way to measure the selectivity of the filter and actually how much influence this filter "is not accepting" from the rest of the circuit. So someone could say that, a SS resonance with Q=3 will produce the same waveform as a filter with Q=15 for example. Then why not using the lower quality factor, so that smaller coils will be used?

This topology, although it is a filter, is still a transformer and has all the characteristics of a normal transformer. So the secondary keeps being reflected back to the primary and "carries" all the behaviors of the secondary, on the primary circuit. With that case, the voltage source that is created from the Parallel resonance is reflected back on the Primary system that is already acting as a current source. Also, for frequencies higher than the resonant frequency the Parallel resonant topology is acquiring a capacitive behavior and the Series Resonance is acquiring an Inductive behavior. So the reflected behavior back to the primary is purely capacitive and it is compensating part of the inductive behavior of the Series resonance by the turns ratio times the quality factor of the Parallel. So for the kind of load that the inverter "sees", the quality factors may be qualified as shown in Table 1 below.

It can be proven, that the Capacitive load cannot be achieved, because then, the quality factor of the parallel resonance (voltage source) that will be reflected with -n ratio- to the primary side, will be higher than the quality factor of the series resonance (current source) and that will distort the voltage input of the series resonance, which means that it will distort the square wave voltage output of the inverter. So the switches of the inverter will not give a DC pulse of 1 kV, but will acquire an 1 kV DC offset that will fluctuate over that value, due to the reflected sinusoidal voltage from the secondary. To make this clearer several simulation result will be presented, keep in mind that n=2.46. Also, for the current topology the mutual inductance M and the extra coil L of series resonance, will be chosen as a factor of Lm which is the Mutual inductance of the air core transformer that is located outside. The importance of the resonance in the equivalent current sources that are located in the main aircore transformer is so high that the values of the quality factors should obey to a rule that will be defined.

According to a second embodiment, a filtering topology including a resonant filter S and P is provided without step down function as shown in Fig. 4 is provided. Since the resonant Series topology is forcing a sinusoidal current and a Parallel topology is forcing a sinusoidal voltage, that is of course depended on the on the loaded quality factor of the system (Q>2.5 is needed), then a combination of these two topologies will act as an LC filter with resonant characteristics, that will filter the square wave voltage into the Faraday box and have an almost sinusoidal voltage and current output as a result.

In view of the above description, a further embodiment of the present invention relates to a wireless inductive charging system the loaded quality factor Q of each filter stage of the space harmonic elimination stage 19 is equal to or higher than 2.5. In further advantageous embodiments, the loaded quality factor Q of subsequent filter stages of the space harmonic elimination stage 19 diminishes from the DC/AC inverter 12 towards the primary coil L _P .

Because the mutual inductance of the air core transform in an embodiment is 22.5μΡ, this mean that the quality factor of the filter is lower than 2.5, which is needed to have a pure sine. Although, the quality factor is turning to be a crucial factor on the IPT technology, because it from the one hand it forming a very good sine waveform, on the other hand, the better this sine is then the bigger the drop of voltage on the resonant capacitor and coil. For that reason, an inductance of the 3 times the mutual will be used so that there is a quality factor, slightly higher than 2.5, around Q=2.57. Also, the parallel resonant topology is causing a power factor close to 1 , when the driving circuit is producing a switching frequency equal with the resonant frequency. So, the idea why filter works, is because the S resonant topology is producing a sinusoidal current, as a current source, that is seen by the P and is order to create power factor of 1 , the voltage should always be in phase with the current, forcing the voltage to become sine also.

Figure 7 shows a flow chart of an embodiment of the method according to the invention. The method 100 is used for wireless inductive charging. The method 100 comprises the steps of providing 1 10 a DC power source, generating 120 an AC charging signal from DC power fed by the DC power source, and transmitting 140 a wireless charging signal to a device to be charged, using a primary coil being fed by the AC charging signal, further comprising the step of eliminating 130 harmonics from the AC charging signal before feeding the primary coil by a space harmonic elimination stage, wherein the DC/AC inverter and the space harmonic elimination stage are located within an EM shielded structure.

The invention is not restricted to the embodiments described above. It will be understood that many variants are possible.

As an example, the DC power source may be implemented as a 750 V PV panel or as another PV panel. Further, the DC power source may be implemented by providing a rectified DC input from AC mains such as an AC grid with so-called power factor corrector PFC topology or by another DC power source.

The present invention has been described above with reference to a number of exemplary embodiments, and can also be described by the following interrelated embodiments: Embodiment 1. A wireless inductive charging system, comprising:

- a DC power source,

- a DC/AC inverter being fed by the DC power source and generating an AC charging signal, and

- a primary coil being fed by the AC charging signal and transmitting a wireless charging signal to a secondary coil of a device to be charged,

the system further comprising a resonance band pass filter eliminating harmonics from the AC charging signal generated by the DC/AC inverter before feeding the primary coil.

Embodiment 2. A system according to embodiment 1 , wherein the resonance band pass filter is also arranged for stepping down or stepping up the voltage of the AC charging signal.

Embodiment 3. A system according to embodiment 1 or 2, wherein the resonance band pass filter is arranged for eliminating second and higher order harmonics from the AC charging signal.

Embodiment 4. A system according to any of the preceding embodiments, wherein the resonance band pass filter includes a serial resonance filter eliminating harmonics from the AC charging current signal.

Embodiment 5. A system according to any of the preceding embodiments, wherein the resonance band pass filter includes a parallel resonance filter eliminating harmonics from the AC charging voltage signal.

Embodiment 6. A system according to any of the preceding embodiments, further comprising a capacitor arranged in series with the primary coil forming a resonant SS transmitting circuitry. Embodiment 7. A system according to any of the preceding embodiments, wherein the resonance band pass filter includes a combined series and parallel resonance filter eliminating current and voltage harmonics from the AC charging voltage signal.

Embodiment 8. A system according to any of the preceding embodiments, wherein the system includes a resonant inductive power transfer system implemented as a SS, SP, PS, PP or LCL circuitry. Embodiment 9. A system according to any of the preceding embodiments, wherein the DC/AC inverter is implemented as a PWM inverter.

Embodiment 10. A kitchen, home or office furniture, comprising a wireless inductive charging system according to any of the preceding embodiments.

Embodiment 1 1. A method for wireless inductive charging, comprising the steps of:

- providing a DC power source,

- generating an AC charging signal from DC power provided by the DC power source, and

- transmitting a wireless charging signal to a device to be charged, using a primary coil being fed by the AC charging signal,

further comprising the step of eliminating harmonics from the AC charging signal before feeding the primary coil.

Embodiment 12. A method according to embodiment 1 1 , wherein the device to be charged as an electrically driven vehicle.

Embodiment 13. A method according to embodiment 1 1 , wherein the device to be charged is a kitchen, home, office or communication appliance.

These and other embodiments will be apparent for the person skilled in the art and are considered to fall within the scope of the invention as defined in the following claims. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments. However, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.