A new compact power modules range for efficient solar inverters

Transcription

1 A new compact power modules range for efficient solar inverters Serge Bontemps, Pierre-Laurent Doumergue Microsemi PPG power module Products, Chemin de Magret, F Merignac Abstract The decrease of fossil fuel resources, the global climate and the environmental problems on the planet requiring a significant reduction of carbon dioxide emissions leads us to use the energy in a more efficient and intelligent manner. One way is to exploit a renewable energy and an infinite resource like the sun. Solar cells are used to capture the energy from the sun and generate a direct current that is converted into an alternating current compatible with the grid. The inverter performing the DC to AC conversion must exhibit the highest efficiency to not waste the energy provided by the source. A unique solution of power modules for unipolar switching DC/AC inverters has been developed to reduce the power loss. 1. Introduction In most cases the DC-AC inverter topology is based upon a full bridge topology fed directly by the solar cells as show in Fig.1. LOW VCESAT IGBT FULL BRIDGE LOW FORWARD VOLTAGE DIODE FAST RECOVERY OR SiC DIODE FAST IGBT LOAD Fig.1 Solar Inverter with full bridge configuration. However when the solar cell voltage is low or may vary significantly, a step up converter is inserted after the solar panels to power the full bridge circuit with a constant DC voltage as shown on Fig.2 STEP UP CONVERTER FULL BRIDGE LOW VCESAT IGBT LOW FORWARD VOLTAGE DIODE FAST RECOVERY OR SiC DIODE FAST IGBT Fig.2 Solar Inverter with boost chopper and full bridge circuit LOAD It is also possible simplifying the full bridge circuit into a phase leg combined with a capacitor divider. But the phase leg has to carry twice the current compared to a full bridge configuration for the same output power. The power modules proposed have a very symmetrical design such that this is very easy to parallel the two legs of the bridge to form a phase leg of twice the current capability. In this paper only the full bridge configuration will be considered. The size, performance, reliability and cost of the system are very important but the efficiency of the inverter is a key parameter. Meeting the highest possible efficiency of the solar inverter is not only fundamental to not waste a precious energy but it is also very important for reducing the cost of producing electricity. To meet this goal the full bridge section requires adopting unipolar switching DC to AC inverter topology. The bottom switches are operating at high frequency to minimize output filter size while top switches operate only at line frequency. By this way the inverter takes advantage of the high frequency operation with only two switches of the bridge exhibiting switching losses while the other two have only to dissipate conduction losses. These power devices are integrated into state of the art, low profile compact packages to offer high density and reliable inverter solutions up to 10 kw output power. 2. Power module range for full bridge solution. Two power module ranges are offered, 600V and 1200V, to address the two main AC grid voltages and meet a wide range of solar cells voltages. To

2 offer the most compact solution at a competitive price for a frequency operation in the range of 15 khz to 50 khz an IGBT technology is preferred. The bottom switches operating at high frequency are built with fast NPT IGBT devices. The top switches working at line frequency feature the lowest saturation voltage, offered by Trench and Field stop IGBTs. It would be possible implementing the fast switches in the top position of the bridge and the low conduction devices in the bottom but usually the opposite is achieved to facilitate the driving of the fast devices and avoid a floating position when placed in the bottom of the bridge. The diodes in these new power modules are matched to the power transistors for improved inverter efficiency as well. High speed, soft recovery Microsemi DQ series diodes are implemented in parallel with the top IGBTs to provide low recovery losses in combination with the bottom fast IGBTs. Low forward voltage diodes protect the bottom IGBTs during output zero crossings. These last diodes are much less stressed than the other ones that are submitted to high frequency reverse recovery which allows sizing them with a lower current rating which is appreciable for size and cost reduction. Product ranging from 30A to 100A for 600V and 15A to 50A for 1200V are proposed in the space saving compact SP1 and SP3 packages as listed in table V products are also available with Coolmos TM devices to operate at even higher switching frequencies with minimum switching and conduction losses. When the solar cell voltage can vary significantly it makes great sense implementing a step up converter feeding the full bridge section with a regulated DC voltage (400V to 800V). In order to offer a maximum flexibility in terms of solutions for the complete inverter a set of two modules is preferred, one for the boost switch, the other one for the full bridge. Since a full inverter using a boost stage and a full bridge circuit may be made of two individual packages it is very important that each package is first optimized in terms of size and performance but it must be also compatible in terms of mounting with the other one. To achieve this, up to 3KVA the association of a SOT227 package for the boost switch and a SP1 package for the full bridge will be preferred as shown on fig.3. The SOT 227 has a footprint of 25.4mm x 38.1mm while the SP1 package has a footprint of 40.8mm x 51.6mm. Both packages offer a 12mm height which allows them to be mounted side by side on the same heat sink and wired together with the same Printed Circuit Board. The SOT227 provides screw terminals while the SP1 is connected through solder pins. Fig.3a BOOST stage in SOT227 package (P<3KW). Technology Voltage IC package Part number (V) TC=80 C Trench/ Coolmos SP1 APTCV30H60T1G Trench/ NPT SP1 APTGV30H60T1G Trench/ NPT SP3 APTGV50H60T3G Trench/ Coolmos SP3 APTCV50H60T3G Trench/ NPT SP3 APTGV75H60T3G Trench/ NPT SP3 APTGV100H60T3G Trench/ NPT SP3 APTGV15H120T3G Trench/ NPT SP3 APTGV25H120T3G Trench/ NPT SP3 APTGV50H120T3G Table 1: List of full bridge modules in SP1 and SP3 packages. 3. Power module range for complete Boost and Full Bridge inverter configuration Fig.3b Full bridge in SP1 package (P>3KW). For output power higher than 3KVA the association of a SP1 package for the boost switch and a SP3 package for the full bridge is the best solution as shown on fig Individual modules for Boost and full bridge inverter

3 IGBT boost stage in SP1 package meets this target. A summary of Boost stages is given in table 2. Fig.4a BOOST stage in SP1 package (P>3KW). Fig.4b Full bridge in SP3 package (P>3KW). For the highest output power levels it may be required using SP3 packages for both the input boost stage and the output full bridge module. In combination with the 40.8mm x 51.6mm SP1 footprint the SP3 occupies an area of 40.8mm x 73.4mm. Both SP1 and SP3 modules offer 12mm height and can be connected to the same PCB using wave soldering assembly technique. The boost converter must operate at high frequency, ideally 100 khz, to minimize the size of the magnetic components particularly the boost inductor. For 600V application, Coolmos TM devices offer the best performance at high frequency. Products are available with 45 mohms in SOT227 package and 24 mohms in the SP1 module. In all cases the matching diode is built with the latest DQ soft and fast recovery diode. For high voltage application MOSFETs are the best candidate should the boost stage work at high frequency. As the Rdson dramatically increases as the MOSFET voltage increases, a large area of MOSFET device is required as output power increases. Boost chopper products are available in the SP1 package with 180 mohms and 300 mohms for respectively breakdown voltages of 1000V and 1200V. To minimize the cost of the boost function, a fast IGBT can replace the MOSFET device in case the switching frequency can be reduced to 25 khz. 50A to 100A, 1200V fast Technology Voltage Rdson package Part number (V) (mohm) Coolmos SOT227 APT40N60JCU2 Coolmos SOT227 APT60N60JCU2 Coolmos SP1 APTC60DAM35T1G Coolmos SP1 APTC60DAM24T1G MOSFET SP1 APTM100DA33T1G MOSFET SP1 APTM100DA18T1G MOSFET SP1 APTM120DA56T1G MOSFET SP1 APTM100DA30T1G Table 2a. MOSFET and Coolmos TM modules Technology Voltage IC package (V) TC=80 C Part number Boost Fast IGBT SP1 APTGF50DA120T1G Fast IGBT SP1 APTGF75DA120T1G Fast IGBT SP1 APTGF100DA120T1G Table 2b. IGBT Boost modules These Boost power modules can be associated with the SP1 and SP3 full bridge modules described in the previous full bridge section. The voltage, technology and current rating of the devices will be chosen according to the output power and the switching frequency chosen for the inverter. 3.2 Integrated module for Boost and full bridge The combination of both the boost stage and the full bridge in the same package leads to further reduce the size of the inverter. Two products are offered in 600V and 1200V. For each voltage rating the two lowest power devices are offered in the 40.4mm x 93mm footprint SP4 module (See Fig.5). The two higher power ones are integrated in the low profile SP6-P housing (62mm x 108mm footprint see Fig.6). For 600V products the boost stage is made of Coolmos TM transistors while for 1200V products fast IGBTs have been adopted for space and cost savings reasons. Table 3 summarizes the available modules with integrated Boost and Full bridge configuration. Module Boost technology Full Bridge technology Voltage Current Package APTGV50H60BG Coolmos TM Trench/NPT 600V 50A SP4 APTGV100H60BTPG Coolmos TM Trench/NPT 600V 100A SP6-P APTGV25H120BG IGBT Trench/NPT 1200V 25A SP4 APTGV50H120BTPG IGBT Trench/NPT 1200V 50A SP6-P Table 3: Solar modules with integrated Boost and Full Bridge.

4 100 Fig.5 3D view of the SP4 package Trench 50Hz, NPT20kHz All Trench 20kHz All NPT 20kHz Fig.7 Efficiency for Trench and Field stop, NPT and a mix Trench/NPT at 20 khz. Fig.6 3D view of the SP6-P package 3. Performance comparison Various combinations of technologies are compared in terms of efficiency as a function of output power. The study is also performed with different operating frequencies to better measure the impact of the switching losses of the different fast switches. In order to have a fair comparison between solutions the efficiency is given for the normalized output power. In order to avoid any audible noise and minimize the size of the magnetic components an operating frequency of 20 khz for the fast switches is generally adopted. Fig.7 gives the efficiency as a function of for: - A configuration using only Trench and Field stop IGBTs for all 4 switches of the bridge, - A configuration using only fast NPT IGBTs - The optimized configuration with fast bottom NPT IGBTs and low conduction Trench and Field stop top IGBTs. 600V Trench and Field stop IGBTs are designed to operate up to 20 khz. Low VCEsat voltage combined with reasonable switching times allow reaching efficiency between % and 97%. Despite higher conduction losses, fast NPT IGBT devices permit to improve the efficiency thanks to improved switching losses. The combination of Fast IGBTs for their low switching losses and Trench and Field stop IGBTs for low conduction outperform the previous versions by almost 1% with an overall efficiency above % over a wider range of input power. In order to further improve the efficiency it is still possible decreasing the operating frequency down to 16 khz, limit of audible noise without impacting the size of magnetic components (see Fig 8) Trench 50Hz, NPT16kHz All Trench 16kHz All NPT 16kHz Fig.8 Efficiency for Trench and Field stop, NPT and a mix Trench/NPT at 16 khz.

5 The decrease from 20 khz to 16 khz allows getting efficiency above 97% for Trench and Field stop IGBTs, very close to % for fast NPT IGBTs and well above % for the mix of IGBT technologies. In some cases it makes great interest increasing the operating frequency in the range of 50 khz to further decrease the magnetic components, particularly on the output filter. 600 V fast NPT IGBTs with low turn off energy are fully capable of operating up to 100 khz and therefore can achieve acceptable efficiency in the range of 50 khz. MOSFET devices with even faster switching times can offer lower switching losses than the fastest NPT devices. As long as the MOSFET devices can also offer low conduction losses they can easily exhibit lower overall losses. For 600V, Coolmos TM transistors offer very low RDson for minimum conduction losses with fast switching times Trench 50Hz, CoolMOS 50kHz Trench 50Hz, NPT50kHz Fig.9 Efficiency for fast NPT/Trench IGBTs and Coolmos TM / Trench switches combination at 50 khz. The combination of fast NPT and Trench IGBTs allows efficiency still higher than 97% at 50 khz. The combination of Coolmos TM transistors with Trench IGBTs provides an even higher efficiency than the previous combination. If the operation at high frequency is not a must to reduce the size of the inverter and a 16 khz frequency is acceptable then the association of Coolmos TM devices and Trench IGBTs will lead to the best efficiency. Despite the low 50Hz line frequency operation of the Trench and field stop IGBTs it is recommended using FREDFET devices or Coolmos TM transistors with faster intrinsic diode to minimize the EMI disturbance of the system. Another important characteristic for a solar inverter is the life time and the reliability. Also critical is the EMI/RFI generated by the inverter. SiC diodes feature essentially zero forward and reverse recovery losses that provide a significant advantage in terms of switching noise generation and performance improvement compared to standard fast silicon diodes. The diode recovery current affects significantly the switching turn on energy within the power switch when hard switched. Such behaviour will generate a significant amount of turn on losses both in the power switch and the diode, with increasing switching frequency. It has to be noted that at the end of the recovery phase some oscillations can appear, leading to a significant amount of noise in the system that may be difficult to cancel by expensive and bulky input filters. Faster recovery results in much lower switching loss both in the switch and the diode. The small peak current observed while a SiC diode turns off is due to the capacitive junction of the Schottky barrier device rather than to reverse recovery characteristics. As opposed to the configuration using conventional FRED diodes, no ringing or oscillations can be measured. Such quiet switching is of prime interest to reduce the size and complexity of input filters and a great help to meet severe EMI/RFI requirements. The recovery behaviour of SiC devices is not only excellent at room temperature but also constant over a wide temperature range. Fig.10 shows the reverse recovery behaviour of a 10A/600V Cree SiC diode versus a silicon diode of same current and voltage rating for a range of junction temperatures. CSD10060 TJ = 25, 50, 100, 150 C 600V, 10A Si FRED TJ = 25 C TJ = 50 C TJ = 100 C TJ = 150 C E E E E E E E-07 Fig.10 Reverse recovery behaviour of SiC and Si diodes for various junction temperatures.

6 The SiC devices exhibit temperature independent switching behaviour and offer very stable operation even up to elevated junction temperatures. Switching losses using SiC diodes will remain stable compared to silicon devices where the switching losses dramatically increase with temperature. By this way the use of SiC diodes can significantly reduce the overall losses of a solar inverter and contribute to reach record efficiencies. Lower losses mean also lower operating junction temperatures that obviously will enhance the life time of the inverter which is crucial in solar applications. Based on this approach the best efficiency performance is obtained with an optimized mix of power devices technology; Low conduction loss IGBTs operating at 50Hz, Fast switching devices operating at high frequency and SiC diodes combined with the fast transistors. Targeting a minimum 16 khz switching frequency will lead to the maximum possible efficiency as shown on Fig Trench 50Hz, NPT16kHz Trench 50Hz, NPT 16kHz, SiC diodes Trench 50Hz, CoolM OS 16kHz Trench 50Hz, CoolMOS 16kHz, SiC diodes Fig.13 Efficiency for fast NPT/Trench IGBTs and Coolmos TM / Trench switches combination at 16 khz with SiC diodes. The efficiency of the different configurations has been compared in this paper at 75 C heat sink temperature. But the inverter efficiency can easily be reduced by up to 1% when operated at maximum ambient temperatures. With temperature superior characteristics, SiC diodes will increase the gap of efficiency in these extreme conditions compared to conventional silicon devices. Thermal performance can be further enhanced by the use of Aluminium Nitride substrates having a much better thermal conductivity than the existing alumina substrates used for standard modules. Better junction to case thermal resistance of the power devices allows getting lower operating junction temperatures. For Silicon devices higher junction temperature means higher conduction and switching losses while for SiC devices it leads only to higher conduction losses. Therefore AlN substrate can contribute to further improve the efficiency and extend the lifetime of the solar inverter. COOLMOS comprise a new family of transistors developed by Infineon Technologies AG. COOLMOS is a trademark of Infineon Technologies AG. 5. Conclusion This paper has demonstrated that the combination of low conduction loss and fast power devices technology in an innovative full bridge configuration is key to meet high efficiency targets of modern solar inverters. Microsemi Power Product Group is offering a wide range of dedicated power modules integrating these topologies with all the various power devices technologies described in this paper. Any of the products listed can be offered with FRED diodes but also with SiC diodes for improved characteristics. These products feature a base plate for an excellent heat transfer to the heat sink that further contribute to enhance the performance, quality and reliability level of state of the art solar inverters. The availability in a very near future of SiC switching devices, MOSFETs or IGBTs will permit to get efficiencies better than 99% and reach the maximum that is technically feasible. 6. Bibliography 1. Khairy Fathy Interleaved ZCS boost DC-DC converters using quasi-resonant switch blocks for PV interface and its performance evaluations, PCIM Serge Bontemps, Alain Calmels "Low profile power module combine with state of the art MOSFET switches and SiC diodes allows high frequency and very compact threephase sinusoidal input rectifiers", PCIM Mike Meinhardt improvement of Photovoltaic inverter efficiency-targets, methods, limits, PCIM Mickael O Neill, SiC puts new spin on motor drives, Power Electronics Technology magazine, January 2005.