April/May 2021

26 AUTOMOTIVE POWER www.gansystems.com Issue 2 2021 Power Electronics Europe www.power-mag.com topologies. The power rating ranges from 3.3 kW to 22 kW. An on-board DC/DC Converter converts the high voltage input from the battery to low output voltage (14V or 24V) for auxiliary systems. The power rating typically ranges from 1KW to 3KW. The Traction Inverter drives the motor through the high voltage DC battery pack. A 3-phase, 6-switch motor drive topology is present with a front-end boost converter as an option. The power rating normally ranges from 80 kW to 300 kW. Normally the OBC, DC/DC converter, and traction inverter in an EV share one liquid cooling system where the maximum coolant temperature is 70°C with ambient air temperature ranging from -40°C to 80°C. Technology innovation is happening quickly and new requirements of future powertrain systems need to consider: Reliability. This is a key factor affecting the adoption of new technologies in EVs. For automotive power semiconductors, the Automotive Electronics Council (AEC) establishes baseline reliability testing requirements (AEC-Q101) however, further assessment is required. Semiconductor vendors must implement testing procedures that extend stress testing beyond standard qualification conditions to predict Time-To-Failure (TTF). Automotive customers target a 15+ years lifetime in their specific mission profiles for products with far lower Failure in Time (FIT<<1) rate. Stress testing involves switching accelerated lifetime testing with hard switching to provide a direct demonstration that lifetime requirements are achieved. Recharge mileage. Longer range and fewer charging cycles are now must haves in EVs. To extend the mileage range per one charging cycle, higher efficiency and lower weight is required in powertrain systems especially for 100 kW and above traction inverters. The target peak efficiency for current traction inverters is above 99.5% with power density beyond 15 kW/l. Smart charging. A unique aspect of an EV battery powered system is that it requires multi-directional power flow. With OBCs, the battery loading should be able to interconnect between power systems from power grids, solar panel systems, and other standalone loads such as home appliances. The energy is distributed based on the end user’s requirements. EV traction inverters also require bidirectional power transfer unlike traditional industrial motor drives. During regenerative braking, switches are controlled to allow the same inverter to act as a rectifier, while the motor acts as a generator, thereby allowing power to flow back to the battery pack for fuel economy. System cost reduction. About 70-80% of overall electrification costs in EVs are due to the battery and power electronics. Reducing costs in this area can be done by increasing the efficiency of the traction inverter. Therefore, the capacity of the battery can be drastically reduced while maintaining the same driving mileage. Moreover, higher efficiency means the ability to downsize the cooling system, again lowering overall system cost with less heatsinks. Besides bringing the component costs down, the integration of an all-in-one OBC, DC/DC converter, and traction inverter into one cooling case is also a cost effective and reliable approach. Power transistors for EVs So far, EV powertrain systems have used Silicon solutions. For OBCs and DC/DC converters where the normal operating frequency is about 100 kHz, Si MOSFETs were used and Si IGBTs were the main switching device traction inverters. But that is changing. Wideband Gap (WBG) devices such as GaN and SiC are the most Table 2: Examples of OBC, DC/DC converter, and traction inverter products featuring GaN power transistors