Rohm and Fraunhofer demo SiC MOSFETs in a UPS
Rohm has collaborated with Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg to build a 10kW three phase UPS inverter using Rohm SiC MOSFETand Gate driver devices. The aim is demonstrate the performance of SiC devices at the application level.
SiC components offer great potential for reducing power converter system costs and decreasing the size of the heat-sink and inductive components by increasing efficiency and operating frequency.
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.
Nevertheless the hardware designer is confronted with some challenges when designing such systems, says Rohm. 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.
A UPS was chosen as the application as it is a growing market and also future UPS market will demand size and weight reduction. The results can also be easily adapted to drive inverters.
For this project the input voltage range is 700 V to 1000 V. The AC output grid voltage is 400V with a RMS current of ~15Arms. To have a good comparison 10kW 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 1200V is usually needed. A 3-level MNPC (Mixed Neutral Point Clamped) topology was chosen. Other names for this topology are Neutral Point Piloted (NPP) and T-type topology.
For a 400V grid, the blocking requirements of the four transistors lead to the applications of two 650V devices (S2206, 120mΩ, second generation SiC MOSFET and S6206, 12A, second generation SBD) and two 1200V devices (S2301, 80mΩ, second generation SiC MOSFET and S6301, 5A SBD). Simulations have shown that despite the high switching frequency of 100 kHz, low losses can be achieved in the semiconductor. The simulation results for one phase and for three phases are shown above.
The switching frequency of 100kHz leads to small passive filter components at the output and small input capacitors which - apart from smaller size and weight - also mean 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 optimise efficiency and thermal heat management.
To evaluate Rohm's new third generation SiC Trench MOSFETs additional modules have been ordered, including the third generation S4007 MOSFET (650 V, 80 mΩ). The new Trench devices feature a double Trench Gate structure which reduces the specific RDS, ON per area by 50 percent compared to the second generation.
At the moment the system is being built up and tested. During ISICPEAW taking place on May 26th - 28th 2015 in Stockholm the final results and real measurement data will be presented.
