Burger Kranzer 2009 Extreme High Efficiency PV Power Converters

Extreme High Efficiency PV-Power Converters

Bruno Burger, Dirk Kranzer FRAUNHOFER INSTITUTE FOR SOLAR ENERGY SYSTEMS ISE

Heidenhofstrasse 2 79110 Freiburg, Germany

Tel.: +49 / (0) – 761.4588.5237Fax: +49 / (0) – 761.4588.9237

E-Mail: [email protected]: http://www.ise.fraunhofer.de

Keywords «Photovoltaic», «Converter Circuit», «Efficiency», «IGBT», «Silicon Carbide».

AbstractPhotovoltaic (PV) inverters convert the DC current of solar generators into AC current and feed it into the grid. There are three basic inverter topologies: inverters with low frequency (50/60 Hz) transformer (LF), inverters with high frequency transformer (HF) and transformer-less inverters (TL). The European market is dominated by transformer-less types (80%), in Japan approx. 50% of the inverters are transformer-less and in USA up to now mostly transformer based inverters are used due to national standards. The efficiency of the transformer-less inverters is up to 98% in series products and in research 99% efficiency has been reached. Such extreme high efficiencies can be achieved with three level or multilevel inverter topologies and new power semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN).

IntroductionPV electricity is up to now expensive and therefore the feed-in tariffs are kept high to make PV installations attractive. In countries with low irradiation like Germany, the tariff is up to 0.45 € per kWh and in countries with high irradiation like Spain or California, the tariff is in the range of 0.20 € to 0.30 €.

Table I: Expected PV generation costs for roof-top systems at different locations

Sunshine hours 2006 2010 2020 2030

Berlin 900 0.45 € 0.35 € 0.20 € 0.13 €

Paris 1000 0.40 € 0.31 € 0.18 € 0.12 €

Washington 1200 0.34 € 0.26 € 0.15 € 0.10 €

Hong Kong 1300 0.31 € 0.24 € 0.14 € 0.09 €

Sydney/Buenos Aires/Bombay/Madrid 1400 0.29 € 0.22 € 0.13 € 0.08 €

Bangkok 1600 0.25 € 0.20 € 0.11 € 0.07 €

Los Angeles/Dubai 1800 0.22 € 0.17 € 0.10 € 0.07 €

Table I shows the expected PV generation costs for roof-top systems at different locations [1]. At sunny locations with high electricity costs like Los Angeles, grid parity is already reached.

0

0.1

0.2

0.3

0.4

0.5

0.6

2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020

Year

€ / k

Wh

Households (1200 kWh/a)

Households (3500 kWh/a)

Industry

PV rooftop < 30 kW

PV rooftop 30 kW to 100 kW

PV rooftop 100 kW to 1 MW

PV rooftop > 1 MW

PV freestanding

Fig. 1: Electricity costs and feed-in tariff in Germany.

In Germany, grid parity will be reached between 2011 and 2016, depending on different electricity tariffs (households or industry) and different feed-in tariffs (rooftop or freestanding) [2].

High efficiency single phase inverters Today most solar modules are manufactured according safety class II, so that there is no need for an additional galvanic isolation with a transformer in the inverter. A transformer causes approx. 2% additional losses and also component costs. So inverters without transformer are the better solution. They achieve higher efficiencies, are cheaper and have less weight than inverters with transformer. Additional safety mechanisms like isolation resistance tests and residual current measurement (RCD) make the transformerless inverters even safer than the inverters with transformer. In all basic circuits that are presented, ideal switches are printed. In practice, IGBTs with external anti-parallel diodes, MOSFETs or JFETs with external anti-parallel diodes can be used. Fig. 2 shows the ideal switch and the practical realizations with the same polarity.

= or or

Fig. 2: Ideal switch and real switches like IGBT, MOSFET and JFET.

In this paper only commercially used circuits are shown. Today, more than 40 circuits are used in series products [3]. In research, much more circuits have been developed.

S1 S3

S4S2

C1

L1

L2

Fig. 3: Single phase inverter in H4 topology.

Fig. 3 shows a single phase inverter in H4 topology. In principle, there are three switching methods for the bridge: bipolar switching, unipolar switching and single phase chopping. Additionally there is the

possibility to use only one inductor or two split inductors. But out of these six possibilities, only one, the bipolar switching with two inductors can be used. The five other possibilities cause rectangular voltages from the solar generator to earth and since the parasitic capacity of the solar generator to earth is quite high, high capacitive leakage currents would occur. With bipolar switching, inverter efficiencies between 96% and 97% can be reached for inverters up to 10 kW.

Since three level switching methods like unipolar switching and single phase chopping can not be used for PV inverters, new circuits have been developed, that offer three level switching and avoid rectangular voltage from the solar generator to earth. Circuits with an additional freewheeling path with zero voltage have been developed to reduce switching and inductor losses. The first circuit of this category is the HERIC® topology [4]. It uses two additional switches for the freewheeling path, one for positive inductor current with D1 and S5 and one negative inductor current with D2 and S6 (Fig.4). During freewheeling, the four switches S1 to S4 are switched off and the solar generator is floating. This inverter needs an input voltage that is always higher than the peak of the grid voltage, e.g. 360 V for a 230 V grid.

S1 S3 S5

S4S2

C1

L1

L2S6D1

D2

Fig. 4: HERIC® inverter.

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0 10 20 30 40 50 60 70 80 90 100

Output power P/PN / %

Eff

icie

ncy

/ % HERIC, 1200V IGBT

Bipolar, 1200V IGBT

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Output power P/PN / %

Eff

icie

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/ % HERIC, 600V IGBT

Bipolar, 600V IGBT

Fig. 5: Efficiency of the HERIC® inverter with 1200 V IGBTs (left) and 600 V IGBTs (right) compared to conventional bipolar switching.

Fig. 5 shows a comparison of the efficiency curves of bipolar switching and HERIC switching for 600 V IGBTs and 1200 V IGBTs. At 10% load, the efficiency gain with the additional freewheeling path is 4% and at full load 1% for 1200 V IGBTs. With 600 V IGBTs, 98% efficiency have been reached, but the gain is smaller than for 1200 V devices. Since the HERIC topology was very successful at the market, other topologies with similar behavior have been developed. One possibility is to shift the additional two switches S5 and S6 of the HERIC topology from the right side of the H4 bridge to the left side [5]. For positive bridge voltage, S1, S4, S5 and S6 have to be switched on. Negative voltage is switched with S2, S3, S5 and S6. For zero voltage, S1 and S3 or S2 and S4 can be switched. Additionally the solar generator has to be disconnected to avoid voltage steps in the voltage from solar generator to earth. So when switching zero, S5 and S6 have to be switched off. This topology can be called H6 topology, since it uses 6 switches.

S1 S3

S4S2

C1

L1

L2

S5

S6

Fig. 6: H6 inverter.

The H5 topology [6] uses the switches S1 and S3 for switching zero voltage, while the switches S5, S2 and S4 are switched off to disconnect the solar generator (Fig. 7). By using the switches S2 und S4 for the disconnection of the negative pole of the solar generator, switch S6 can be saved.

S1 S3

S4S2

C1

L1

L2

S5

Fig. 7: H5 inverter.

The last three circuits are nearly similar in performance and achieve up to 98% efficiency in series products with IGBTs as switches.

S2

S3

S4

S1

S0

C1

C2

L1

1x - 3x 1x

C0

L0 D0

D1

D2

Fig. 8: Single phase inverter with three level neutral point clamped leg (3L-NPC) and boost converter (common negative).

The inverter with a three level leg (Fig. 8) [7] offers a freewheeling path with zero voltage and has therefore high efficiencyFehler! Verweisquelle konnte nicht gefunden werden.. For the transistors S1 to S4, 600 V devices can be used, that switch faster than 1200 V devices and have also lower forward voltage. But there are disadvantages concerning the voltage and size of the DC link capacitors. Since the DC link voltage has to be greater 700 V, a boost converter in the input is necessary. This lowers the efficiency by 1% to 2%, depending on the voltage difference between input voltage and DC link voltage.

S3

S1

S2

S0

C1

C2

L1

S4

L0 D0

C0

Fig. 9: Single phase inverter with three level leg (3L-SC) and boost converter (common positive).

Fig. 9 shows a single phase inverter, that uses another type of a three level leg [7], [8]. For the switches S1 and S2, 1200 V devices have to be used. For S3 and S4, 600 V devices are sufficient. The booster has a common positive line. This is advantageous for thin film solar modules.

The circuits of Fig. 8 and Fig. 9 have a connection of the center point of the capacitors to neutral. So the DC input voltage of the solar generator has a constant voltage to earth. These topologies are called „quiet DC rail”.

High Efficiency three phase inverters Three phase inverters have several advantages on single phase inverters. Due to the three phase feed-in, the power flow is constant and there is no 100/120 Hz voltage ripple at the DC link capacitor like it is the case for single phase inverters. So the size of the DC link capacitor is quite small and film capacitors can be used to get longer lifetime than with electrolyte capacitors. Fig. 10 shows a three phase inverter in B6 topology. The input stage consists of one to three boosters for the connection of the different PV strings. The neutral wire is connected to the center point of the DC link capacitors to get a „quiet DC rail”. This inverter achieves efficiencies in the range of 95% to 97%, depending on the level of the DC input voltage.

S1 S3

S4S2

S5

S6

S0

1x - 3x 1x

C2

C3

L1

L3

C1

L0 D0

Fig. 10: Three phase inverter with B6 bridge and boost converter.

Especially the switching losses for commutating of the diodes can be saved by the circuit of Fig. 11[9]. For each phase, two legs in parallel are used and they are coupled with two small inductors (L1 to L6). The main inductors L7 to L9 have higher inductivity. This circuit is especially designed for higher power levels in the range of several hundred kilowatts. The maximum efficiency is above 98%.

S1 S3 S5 S7

S6 S8S2 S4

S11S9

S12S10

C1

C2

L7

L9

L1

L6

Fig. 11: Three phase inverter with soft switching.

The same efficiency can be reached with three level topologies. The circuit of Fig. 12 consists of a boost converter and a three level neutral point clamped inverter with diode clamping. The Neutral wire is not connected to the center point of the split DC link capacitor. So only approx. 600 V DC link voltage is needed instead of approx. 700 V, if it is connected. The disadvantage of the circuits of Fig.11 and Fig. 12 is, that there is a 150/180 Hz ripple in the DC solar generator voltage to earth. This may cause leakage currents, depending o the size and technology of the solar generator and also depending on humidity and rain on the PV modules.

S2 S6 S10

S3 S7 S11

S4 S8 S12

S1 S5 S9

S0

C1

C2

L1

L3

2x 1x

C0

L0 D0

D1 D3 D5

D2 D4 D6

Fig. 12: Three phase, three level neutral point clamped (3L-NPC) inverter with boost converter (common negative).

The inverter circuit of Fig. 13 has a connected Neutral wire. Therefore a higher DC link voltage of approx. 700 V is needed. The advantage is the constant DC voltage to earth, so called „quiet DC rail”.

S2 S6 S10

S3 S7 S11

S4 S8 S12

S1 S5 S9

S0

C1

C2

L1

L3

2x - 3x 1x

C0

L0 D0

D1 D3 D5

D2 D4 D6

Fig. 13: Three phase, three level neutral point clamped (3L-NPC) inverter with boost converter (common negative) and connected Neutral wire.

S2 S6 S10

S3 S7 S11

S4 S8 S12

S1 S5 S9

S0

C1

C2

L1

L3

2x - 3x 1x

C0

L0 D0

D1 D3 D5

D2 D4 D6

Fig. 14: Three phase, three level neutral point clamped (3L-NPC) inverter with boost converter (common positive).

The inverter of Fig. 14 is almost similar to Fig. 13, but the booster is in the negative line. This common positive topology is better suitable for thin film amorphous Silicon (a-Si) PV modules.

The inverter of Fig. 15 uses two symmetrical boosters with common connection to the center of the split capacitor. So 600 V transistors instead of 1200 V transistors can be used in the boosters.

S2 S6 S10

S3 S7 S11

S4 S8 S12

S1 S5 S9

S02

S01C1C01

C2C02

L1

L3

1x - 2x 1x

L01

L02

D01

D02

D1 D3 D5

D2 D4 D6

Fig. 15: Three phase, three level neutral point clamped (3L-NPC) inverter with symmetrical boost converters.

Fig. 16 uses a different version of the three level neutral point clamped inverter, that is also first described in [7]. It is often called three level stacked cells (3L-SC) topology. The transistors S1 to S6 need 1200 V blocking capability, while the transistors S7 to S12 only need 600 V. For positive freewheeling current, the transistors S8, S10 and S12 are switched permanently on and the inverse diodes of S7, S9 and S11 conduct the current when the switches S1, S3 and S5 are switched off. When these switches are switched on, the diodes block the current. The center point of the split DC link capacitor is connected to Neutral, so that the DC voltages to earth are “quiet rail”.

S7 S9 S11

S1 S3

S4S2

S5

S6

S0

C2

C3

L1

L3

S8 S10 S12

L0 D0

C1

Fig. 16: Three phase, three level (3L-SC) inverter with boost converter (common positive).

The only five level topology, which is currently on the market, is shown in Fig. 17 [10]. It uses two B6 bridges, one directly connected to the solar generator voltage, the other one connected to a higher bipolar DC link voltage, which is built with two symmetrical boosters. The voltage waveforms of this inverter are shown in Fig. 18. For small instantaneous values of the grid voltages, the inner B6 bridge with the transistors S11 to S16 is pulsed. Zero voltage is switched with the transistors S31 to S36 and their inverse diodes. For higher instantaneous values, the outer B6 bridge with the transistors S21 to S26 is pulsed. Zero voltage is again switched by S31 to S36 and their inverse diodes. The diodes D11 to D16 are required to provide a short circuit between the higher DC voltage and the lower DC voltage through the transistors S21 to S26 and the inverse diodes of the transistors S11 to S16.

D01L01

D02L02

S31 S33 S35

S11

D11

S21

S22

S23

S24

S25

S26

S01

S02

S13

D13

S14

D14

S12

D12

S15

D15

S16

D16

C1

C3

C4

C2

L1

L3

S32 S34 S36

Fig. 17: Three phase, five level inverter with two bipolar DC voltages.

Fig. 18: Voltage waveforms of the inverter of Fig. 17.

Efficiency ratings with conventional IGBTs

The inverter efficiencies are strongly related to the type of the power electronics circuit used for the inverter. Inverters with bipolar switching like the H4 and the B6 bridge achieve maximum efficiencies in the range of 96% to 97%. Highest maximum efficiencies up to 98% are achieved with tree and five level topologies like HERIC®, H5, H6 and NPC topologies. Additional boosters cause additional losses in the range of 1% to 2%, depending on the level of the input voltage. Inverters with line frequency transformer achieve 94% to 96% maximum efficiency, depending on their rated output power. Inverters with high frequency transformer get 93% to 96% maximum efficiency, depending on their output power and the used power electronics circuit. Fig. 19 shows typical efficiency curves for all different inverter types.

0 0.2 0.4 0.6 0.8 190

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Relative AC Power

Effi

cie

ncy

/ %

Fig. 19: Typical inverter efficiency curves for transformer-less three level inverters (red), transformer-less two level inverters (light blue), inverters with 50/60 Hz transformer (dark blue) and inverters with HF transformers (green).

The inverter efficiency also depends on the level of the DC input voltage. Fig. 20 shows the efficiency dependence of a transformerless HERIC inverter from the AC power and the DC voltage. The highest efficiency is reached at 350 V DC voltage and part load of 1 kW to 2 kW. At the maximum input voltage of 700 V, the efficiency is approx. 1% lower.

98%

97.5%

97% 96.5%

0 1000 2000 3000 4000 5000

400

AC Power / W

DC

Vo

lta

ge

/ V

500

600

700

Fig. 20: DC voltage dependence of inverter efficiency of a transformerless inverter.

Efficiency ratings with SiC transistors

The first SiC diodes were introduced in PV inverters in research projects in 2001 [11] and in series products in 2005. The first application of SiC-MOSFETs in PV-inverter was reported in 2007 [12]. Now MOSFETS, normally-on JFETs, normally-off JFET sans Bipolar Junction Transistors (BJTs) are available as engineering samples. Especially normally-off JFETs are very promising. Their ratings are 1200 V / 15 A / 125 m at 25°C. The normally-off SiC-JFET is a current driven device, which differs in the gate drive circuit compared to a MOSFET or IGBT. The low gate threshold voltage of 2.5 V of

the normally-off JFETs can be easily affected by EMI and therefore a negative bias is mandatory to increase the signal-to-noise ratio and to prevent uncontrolled switching. Its application is shown in single and three phase inverters.

Single Phase Inverters with normally-off SiC-JFETsThe measurements were taken in a single phase H4 inverter (Fig. 3) with bipolar switching and in a HERIC® PV-inverter (Fig. 4) at a DC-Link voltage of 350 V and 230 V / 50 Hz output voltage. The switching frequency of the inverter is 16 kHz. The losses caused by the on board power supply, relays, control circuit and DSP were included in the efficiency measurement.

96

96,5

97

97,5

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99,5

100

0 500 1000 1500 2000 2500 3000 3500AC-Power / W

Eff

icie

ncy / %

350V HERIC 16kHz

350V H4 bipolar 16kHz

Fig. 21: Efficiency versus AC-power of a single phase HERIC PV-inverter and a H4 inverter with bipolar switching.

The achieved efficiencies shown in Fig. 23 are remarkable and a maximum efficiency of 99% at 350 V was achieved with the HERIC topology compared to 98.6 % with the H4 topology and bipolar switching. Commercial inverters with Si-IGBTs achieve 98% efficiency. So in this case the losses have been halved from 2% to 1% by the use of the SiC JFETs.

Three Phase Inverter with normally-off JFETs In three phase inverters, the efficiency and system improvement related to the chip area of the SiC transistors are much higher compared to those of a single phase inverters. Since the DC-link voltages are higher, the performance of the SiC transistors fit the requirements of this applications better. A PV-inverter in hard switched three-phase B6 topology with bipolar switching was developed at Fraunhofer ISE. It was designed for the application of normally-off SiC-JFETs with a low inductive PCB layout and high switching frequencies up to 144 kHz. Its nominal power is 5 kW at 400 VRMS.The split DC-link capacitor with connection to the neutral conductor suppresses common mode voltages, but leads to at least 650 V DC-link voltage. The measurements were taken at a DC-link voltage of 750 V and 400 V AC voltage. In case of the IGBT the switching frequency was 16 kHz. With the normally-off SiC-JFETs a switching frequency of 48 kHz was used. This frequency was chosen because the standards for EMC start at 150 kHz. Therefore the still dominant third harmonic of the switching frequency is below this limit. The increase of switching frequency leads to a reduction of volume and mass of the choke. The weight of a common three-phase choke for an inverter of 5 kW is about 8 kg. Although the switching frequency

was tripled, the performance with normally-off JFET is better than with the 4th generation IGBT over the whole power range. The European Efficiency with IGBTs was 95.4 %, whereas with the normally-

ff JFET an efficiency of 96.6 % has been reached. o

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500AC Power / W

Eff

icie

ncy

/ %

normally off SiC-JFETs (48 kHz)

Si-IGBT Generation 4 (16 kHz)

Fig. 22: Efficiency of the three-phase full bridge inverter (B6) with normally-off SiC-JFETs switched t 48 kHz and 4th generation IGBTs switched at 16 kHz.

phase PV-Inverter with an efficiency improvement of 1.2 % for several locations and feed-in tariffs.

ds. So a balance of production costs, inverter lifetime and

energy harvesting has to be kept in mind.

Trends in PV inverters

or

Maximum feed-in tariff / kWh Financial Gain / 10 years

a

Table II: Financial gain over 10 years for the 5 kW three

Freiburg, Germany 0.45 EUR 270 EUR

Marseille, France 0.44 EUR 520 EUR

Almeria, Spain 0.55 EUR 590 EUR

The increase in efficiency by 1.2 % means a reduction of losses by 20 %! Depending on the land sitean additional benefit between 270 EUR and 590 EUR can be gained over an inverter lifetime of ten years (see table 1). A further increase of the switching frequency is not profitable for PV-inverters, as the main benefit is given by the efficiency improvement and the feed-in or net-metering tariff [2] annot by the reduction of the production cost

The PV inverters started with low power levels of approx. 1 - 2 kW. New inverter series have up t11 kW power in single phase topologies and up to 30 kW in three phase topologies. These powe

levels will rise in the future. There is also a trend to higher efficiency ratings. Therefore specialinverter circuits like HERIC® or H5 for single phase inverters and three level or Neutral Point Clamped (NPC) topologies for three phase inverters will win market shares. The maximum efficiency

ing ave to change their regulations and standards, so that transformer-less

here from the adapted UPS inverters towards special PV inverters with higher

efficiency ratings.

of the inverters will rise from 98% in 2007 to 98.5% in 2010 and up to 99% in 2015 (Fig. 23).The trend to transformerless topologies will continue since these inverters offer higher efficiencies at lower costs. Countries with national standards, which dictate the use of transformers or the groundof the solar generator will hinverters can be installed.The market for central inverters is today dominated by UPS inverters, which are adapted for PV. These inverters have approx. 2% lower efficiency than inverters especially developed for PV. Twill be a trend away

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1990 1995 2000 2005 2010

year

max

. eff

icie

ncy

/ %

Fig. 23: Maximum efficiency of PV inverters since 1990 and trend curve.

for

dred kHz, so that the today used big and expensive inductors an be replaced by much smaller ones.

rsy

r to improve the quality of grid electricity by controlling reactive power or tering harmonics.

Outlook

heno the developing

countries. Off-grid systems and energy storage will then get more important.

New semiconductor materials like SiC or GaN will be introduced in PV inverters first, since the electricity costs of PV are quite high and higher efficiency will pay off earlier for PV inverters than UPS inverters or drives. Besides higher efficiency, the new materials offer the possibility of higher switching frequencies up to several hunc

The main objectives for all new types of inverters will be the extension of lifetime to at least 20 yeaand cost reduction. Research for the medium term focuses on the possibility of using functionalitbuilt-in to the invertefil

Since the resources of oil and gas are limited and the CO2 problems with global warming will get moreand more serious, there will be no alternative to renewable energy sources. Big growth rates will only be possible for wind energy and PV. The annual growth of the PV market will be in the range of 20%to 40% for the next 10 to 30 years. PV power will be able to compete with the conventional energy sources in the next 5 to 10 years, depending on the irradiation level of the site. Due to the learning curves of PV modules and inverters according Fig. 24 [13], the prices will drop in the future. Wthey are low enough, the market will swap over from the industrial countries t

Inve

rter

co

sts

in €

/W

cumulative power in MW

2010

Fig. 24: Learning curves for PV modules (top curve) and PV inverters (bottom curve).

References

[1] „Solar Generation IV – 2007”, Solar electricity for over one billion people and two million jobs by 2020, European Photovoltaic Industry Association EPIA & Greenpeace, 2007

[2] „Energiedaten“, Zahlen und Fakten, Bundesministerium für Wirtschaft und Technologie, Referat III C 3, 26.05.2009

[3] Burger, Bruno: „Power Electronics for Photovoltaics“, OTTI Seminar, Munich, Germany, May 25/26, 2009 [4] Schmidt, Heribert; Siedle, Christoph; Ketterer, Jürgen: „DC/AC converter to convert direct electric voltage

into alternating voltage or into alternating current“, Patent US 7 046 534 B2, filed February 9, 2004 [5] Gonzalez Senosiain, Roberto; Coloma Calahorra, Javier; Marroyo Palomo, Luis; Lopey Taberna, Jesus;

Sanchis Gurpide, Pablo: „Single-phase inverter circuit for conditioning and converting DC electrical energy into AC electrical energy“, Patent EP 2 053 730 A1, filed 31.07.2006

[6] Victor, Matthias; Greizer, Frank; Bremicker, Sven; Hübler, Uwe: „Method of converting a direct current voltage from a source of direct current voltage, more specially from a photovoltaic source of direct current voltage, into alternating current voltage“, Patent US 7 411 802 B2, filed June 16, 2005

[7] Nabae A., Takahashi I., Akagi H.: „A new neutral-point-clamped PWM inverter”, IEEE Trans. Industry Applications, Vol. IA-17, pp.518-523, Sept./Oct.1981. [8] Knaupp, Peter: „Inverter“, Patent EP 1 861 914 B1, filed October 10, 2005 [9] Hesterman, Bryce L.; Ilic, Milan; Malinin, Andrey B.; Siddabattula, Kalyan N.C.: „Soft switching

interleaved power converter“, Patent US 6 979 980 B1, filed August 24, 2004 [10] Hantschel, Jochen: „Wechselrichterschaltung für erweiterten Eingangsspannungsbereich“, Patent

Application DE 10 2006 010 694 A1, filed March 8, 2006 [11] Phlippen, Frank; Burger, Bruno: “A New High Voltage Schottky Diode Based on Silicon-carbide (SiC)’’,

9th European Conference on Power Electronics and Applications – EPE 2001, Graz, Austria, 27.-29.08.2001

[12] Stalter, Olivier; Burger, Bruno; Lehrmann, Sascha: „Silicon Carbide (SiC) D-MOS for Grid-Feeding Solar-Inverters”, 12th European Conference on Power Electronics and Applications - EPE 2007, Aalborg, Denmark, 2.-5.09.2007

[13] Meinhardt, Mike; Burger, Bruno; Engler, Alfred: „PV-Systemtechnik – Ein Motor der Kostenreduktion für die photovoltaische Stromerzeugung“, Jahrestagung des Forschungsverbunds Sonnenenergie, Hannover, 26. -27. September 2007