"A 10kW 3 level UPS Inverter utilizing a full SiC module solution to achieve high efficiency and reduce size and weight"

Author: Christopher Rocneanu, BDM Power, ROHM Semiconductor 

Author: Andreas Hensel, Cornelius Armbruster, Fraunhofer ISE

The following article intends to describe the application of full SiC module in a 10 kW UPS inverter. 5-10kW is a widely used power rating for UPS and is also common for power modules. A module has been used to optimize the inverter system with respect to stray inductance and in order to use the full potential of the SiC devices. Target switching frequency is 100 kHz. For this project ROHM collaborated with Fraunhofer ISE in Freiburg, Germany to achieve a very high efficiency with best performance to volume, power density and weight ratio at lowest total system cost. Vincotech, a supplier of module-based solutions for power electronics has provided a Full SiC, MNPC topology module utilizing ROHM’s SiC MOSFETs and SBD.

Introduction:

To demonstrate the performance of the SiC devices on application level  ROHM has collaborated with Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg for their expertise in the power electronics world offering their experience on application level.

The department of power electronics at Fraunhofer ISE is an internationally leading R&D partner for many industries with activities ranging from a few watts up to megawatt level. The main development fields are photovoltaic grid connected inverters, storage systems, e-mobility and electricity grids. Approaches, technologies and solutions originally developed for photovoltaic applications are adopted and applied to other industries such as railway systems, automotive, etc. Since the very beginning Fraunhofer ISE has promoted SiC technology and demonstrated the advantages these devices offer for power electronics on system level by building up compact inverters with very high efficiency.

ROHM Semiconductor is a leading company in Power Analog IC, Low power MCU, Discretes and Power devices. To strive for excellence and high quality products ROHM has vertically integrated its supply chain for SiC. SiC MOSFETs, SBD, Modules and Gate Driver portfolio have been introduced in Bodo’s Power magazine April 2015 in more detail. Diodes are available with a breakdown voltage of 650 V, 1200 V and 1700 V and a current rating of 5 A-100 A in standard packages and bare die. MOSFETs are available with 45 mOhm, 80 mOhm, 160 mOhm, 280 mOhm and 450 mOhm in bare die and TO-247. For the new third generation MOSFET 1200 V, 40 mOhm has been released for PCIM and 1200 V, 30 mOhm and 22 mOhm will be following. At 650 V 30 mOhm, 22 mOhm and 17 mOhm will be available. To get more detailed information about ROHMs power products portfolio one can use (www.rohm.com), below link or QR code:

http://www.bodospower.com/restricted/downloads/bp_2015_04.pdf, (p. 32-37)

Technology comparison Si vs. SiC

Due to its high electric breakdown field, its high thermal conductivity and its wide band gap, SiC is nowadays one of the most promising semiconductor materials. New and better processing technologies which have been developed during the last ten years make it possible to produce different bipolar and unipolar power devices based on SiC. ¹ The physical material properties of this material (see Fig. 1) lead to very good static and dynamic performance compared to Si devices. Additionally the wide band gap combined with higher thermal conductivity allows the devices to operate at very high temperatures if the package allows it.

SiC components offer a great potential for reducing system costs of power electronic converters and decreasing the size of the heat-sink and the inductive components by increasing the efficiency and operating frequency. The higher efficiency results on the one hand in less energy drawn from the battery and on the other hand in reducing the size of the heat-sink. Furthermore, because of smaller switching energies of the SiC transistors, the switching frequency can be raised by three to nine times compared to conventional inverters, which reduces the size of the passive components by almost the same factor. ²

Nevertheless the hardware designer is confronted with some challenges when designing such systems. The high di/dt and du/dt ratio during switching requires careful design of all switching loops and nodes. Every additional parasitic inductance in the design leads to voltage/current spikes that can, in a worst case scenario, lead to EMC problems. Small and carefully designed switching paths are a premise for a good SiC layout and are much more required for SiC- than for Si-systems.

As the high switching speed does also have an impact on the gate drive circuits the applied driver should provide a high immunity against common mode noise.

Figure 1: Physical material properties of Si and SIC suitable for power electronic semiconductors. ³

In the last article (see above link) a short description about various UPS Systems has been made. For this project the input voltage range is 700 V – 1000 V. The AC output grid voltage is 400 V with a RMS current of ~15 Arms. To have a good comparison 10 kW is a suitable output power since the system will be three phase, easy to handle and is well suited to the power rating of available SiC devices. Also a breakdown voltage of 1200 V is usually needed. There are various options to choose from different topologies and levels e.g. Half-Bridge, Neutral Point Clamped (NPC) and Mixed Neutral Point Clamped (MNPC) (see Fig. 2). Table 15 shows a comparison overview of half-bridge, NPC and MNPC and why a 3-level MNPC (Mixed Neutral Point Clamped) topology has been chosen. Other names for this topology that could be found in literature are “Neutral Point Piloted” (NPP) and “T-type” topology. This topology offers the possibility to achieve the highest efficiency compared to other three-level topologies. ⁴

Table 1: Topology comparison between half-bridge, NPC and MNPC ⁵

Figure 2: NPC and MNPC topology ⁷

Choice of devices: For a 400-V-grid the blocking requirements of the four transistors lead to the applications of two 650 V devices (S2206, 120 mOhm, 2. Generation MOSFET and S6206, 12 A, 2. Generation SBD) and two 1200 V devices (S2301, 80 mOhm, 2. Generation MOSFET and S6301, 5 A, 2. Generation SBD). Simulations have shown that despite the high switching frequency f _s of 100 kHz low losses can be achieved in the semiconductor. The simulations results for one phase and for three phases are shown in Fig. 3.

Figure 3: Simulation results of the loss simulation of the semiconductors at nominal load (10kVA, cos ( φ) =1, VIN =700VDC, 380VAC, fs=100 kHz)

The switching frequency is factor three times higher than in commercial systems and allows the application of small passive components for the output filter. The whole 10 kW system with all required periphery, heat sink and safety features measures only 23 cm x 21 cm x 10 cm which corresponds to less than 5 l of volume. The weight is only 4.5 kg.

To evaluate the new third generation SiC MOSFET additional modules have been ordered, including the S4007* MOSFET (650 V, 80 mOhm, 3. Generation MOSFET). The new Trench device features a double Trench Gate structure which reduces the specific RDS, ON per area by 50% compared to the second generation. For the third generation the VF of the body diode could be decreased and the maximum VGS has been increased from -6 V to -10 V while the threshold shift has been kept stable. S4007 has a total gate charge of only 50 nC and an input capacitance of 577 pF. This will lead to a higher efficiency at a smaller chip size.

As mentioned above the high switching speeds require high common mode noise immunity to avoid failure signals in the gate driver circuit. ROHM’s simple and complex gate driver portfolio features high output current (3 A _rms, 5 A_peak), small delay time (75 ns) and due to the coreless transformer design there is no distortion since the turn on and off delay time are independent of temperature compared to photo coupler. All gate drivers are UL1577 recognized and featuring a common mode noise immunity of 100 kV/us. To optimize space BM60015FV was chosen which has the package size of 3,5 mm x 10 mm x 1,8 mm. Additionally BM60015FV has an isolation voltage of 3,75 kV (VDE 0884-10 reinforced) and samples as well as an evaluation board are available upon request.

Packaging:

In addition to the topology, designers always have the choice of which package to choose.

The IGBT has been developed roughly 35 years ago and has been improved by each generation since. But there has been not a lot of innovation at discrete packages as the IGBT is limited by switching frequency and thus there has been no push from the market for further package development. One can choose TO-247, TO-220 or Surface Mount Devices like TO-252, TO-263 or TO-268. Those packages are widely used since they are inexpensive but they have a large stray inductance (~10nH) in case of the TO-247 and TO-220. Therefore they are not perfectly suitable for SiC devices since the stray inductance will cause high dv/dt and thus voltage overshoots or extra losses during switching event. Compared to a Si IGBT the SiC MOSFET chip area is smaller by a factor of up to ~10 times (dependent on which chip you choose). This increases the energy density and thus the need to cool the device and dissipate the heat. High cost is involved with developing new packages and furthermore there is a danger that the power electronic market will not adapt to this package due to a missing second sourcing.

For this project the choice was made to use modules giving the advantage of easy and quick mounting, low and constant stray inductivity and flexibility for customized tailored solutions e.g. free choice of pin position. For this project a MNPC topology in a flow 0 package from Vincotech has been chosen. One module contains one leg of MNPC topology. Press-fit Pins have been chosen in order to enable a solder free assembly, design flexibility and easy PCB repair and re-use. ⁸ This module features a stray inductance of 5 nH (DC+/- vs. Neutral), a Al₃O₂ baseplate and RTH of 0,7 K/W from chip to Heatsink. Optional a phase change material can be used which is optimizing the thermal interface (up to 20% RTH reduction) once it is heated up and is reducing the risk of DBC cracks.

Figure 4: 10kW UPS Inverter with size of 23 cm x 21 cm x 10 cm and a weight of 4,5 kg

Outlook: A 10kW three phase UPS inverter has been developed by Fraunhofer ISE with ROHM SiC and Gate driver devices and can be seen in figure 4. The high switching frequency of 100 kHz leads to small passive filter components at the output and small input capacitors which –apart from smaller size and weight – also leads to lower system costs. Low losses in the semiconductors enable the integration of one small active heatsink cooling all three phases.

An external SBD has been chosen to optimize efficiency and thermal heat management.

As an additional option one could also think about using the body diode of the MOSFET - which is fully qualified and rated at the same current like the MOSFET - as FWD. The high inverting efficiency of the system reduces waste of energy during charging of the system and can thus reduce costs over the whole lifetime of the system.

The small size and the low weight of the inverter show the benefits on system level that can be achieved by the application of SiC devices in the power range of 10kW in a MNPC UPS inverter.

UPS as application has been chosen because it is a growing market and because it is well suited to demonstrate the advantages of SiC devices. The future UPS market will also demand size and weight reduction. The results can be easily adapted to drive inverters. At the moment the system is being built up and tested. During ISICPEAW taking place on May 26 ^th - 28^th 2015 in Stockholm the final results and real measurement data will be presented.

*S4007 is under development. For further information contact your ROHM Sales representative or the author.

Sources:

1. HENSEL, Andreas ; WILHELM, Christian ; KRANZER, Dirk: Development of a boost converter for PV systems based on SiC BJTs. Power Electronics and Applications (EPE), 2011

2. WILHELM, Christian; KRANZER, Dirk; BURGER, Bruno: Development of a Highly Compact and Efficient Solar Inverter with Silicon Carbide Transistors. International Conference on Integrated Power Electronics Systems (CIPS), Nuremberg, Germany, 2010, pp 297-302

3. Grafik Fraunhofer ISE, Daten aus Dirk Schröder, “Leistungselektronische Bauelemente”, Springer 2006

4. SCHÖNER, Christian; DERIX, David; HENSEL, Andreas: Comparison and Evaluation of different three-level inverter topologies for PV systems. 16th European Conference on Power Electronics and Applications (EPE), 2014.

5. “Advantages of NPC Inverter Topologies with Power Modules”, Michael Frisch, Vincotech GmbH (Germany), Temesi Ernö Vincotech Kft. (Hungary)

6. UPS-Flyer_Vincotech.pdf, Download: 01.04.2015 http://www.vincotech.com/fileadmin/downloads/power/ApplicationFlyers/UPSflyer_Vincotech.pdf, Download: 01.04.2015, Marketing der Vincotech GmbH

7. CHOW, T.P., KHEMKA, V., FEDISON, J., RAMUNGUL, N., MATOCHA, K., TANG, Y., GUTMANN, R.J.: SiC and GaN bipolar power devices, Center for integrated Electronics and Electronics Manufacturing, Rensselaer Polytechnic Institute, Tray, NY 12180-3590, USA, 27.02.1999, revised 27.06.1999; accepted 02.08.1999

8. Application Note, AN_2010-10_001-V05_Press-fit-Application Note.pdf, Download: 01.04.2015,  http://www.vincotech.com/fileadmin/downloads/power/ApplicationNotes/AN_2010-10_001-v05_Press-fit-Application%20Note.pdf, Marketing der Vincotech GmbH