# Novel Thermal Management Strategy for Improved Inverter Reliability in Electric Vehicles
## 1 Introduction  
Transport is one of the main sources of greenhouse gas emissions of anthropogenic origin. For example, road transport accounts for 27% of total emissions in Europe, being the greatest contributor among all sectors [1]. There is almost a consensus on the idea that a shift to electro-mobility will be mandatory to meet the Paris Agreement goals, as is confirmed by the meta-analysis conducted in [2]. In this context, a comprehensive evaluation of CO\({}_{2}\) emissions generated by battery powered electric vehicles (EV) vs. internal combustion engine (gasoline and diesel) vehicles has been conducted in [3]. A life-cycle assessment (LCA) was carried out considering not only emissions during vehicle operation, but also the ones generated from fuel extraction, refining, power generation, raw material extraction, vehicle manufacturing, regional particularities (electrical mix, driving distances, etc. ), maintenance and end-of-life phase. The authors in [3] conclude that, with some particular exception, the longer the EV's driven distance, the more the CO\({}_{2}\) emissions would be reduced (note that battery manufacturing significantly penalizes EVs in terms of emissions), performing research focused on improving reliability and maximizing the life-cycle of EVs extremely important. In particular, EVs inEurope would outperform gasoline and diesel vehicles regarding emissions from a range of 76,545 km and 109,415 km, respectively. However, for the general adoption of battery powered EVs by end users, features need to be comparable to the ones of internal combustion engine based vehicles. Thus, ambitious technological targets have been set for both electric machines (power density of 5.7 kW/L, overall efficiency of 93% and cost of 4.7 $/kW) and power electronics (power density of 13.4 kW/L, overall efficiency of 97% and cost of 3.3 $/kW) [4]. With regard to high power density and efficiency requirements, surface mounted or interior permanent magnet synchronous machines (PMSM) are commonly adopted [5; 6; 7; 8]. Current trends on PMSM technology focus on multiphase configurations [9; 10], high speed electric machines (HSEM) [11; 12; 13], and on in-wheel direct drive solutions [14; 15]. In this context, wide band-gap semiconductor technologies are being investigated for their introduction in PMSM drive power inverters [16; 17] due to the following reasons:  
1. Both HSEM and in-wheel multi-pole direct drive solutions lead to high electrical stator frequencies. These trends exhibit regulation issues to be addressed [13; 18; 19], which can be overcome by introducing power electronics technologies that can operate at high switching frequencies with reasonable power losses [20]. 2. Cost reduction trends consider sharing the high temperature cooling loop of the electric machine (at 105 \({}^{\circ}\)C) with power electronics [21; 22], or also using air cooling technologies that adopt convenient power converter placement to take advantage of circulating air during EV motion [23]. Thus, the utilization of power semiconductor technologies that can withstand higher junction temperatures is required [20]. As a consequence of such stringent operation conditions, reliability of power converters can be jeopardized. According to [24], approximately 60% of power semiconductor breakages are a consequence of thermal issues. Moreover, an increment of 10 \({}^{\circ}\)C in the operation mean temperature of a power semiconductor can double its failure-ratio [25]; consequently, special care of inverter thermal management must be taken. Such failures are a consecuence of the mismatches between the coefficients of thermal expansion of the materials that constitute power modules, which suffer from significant thermo-mechanical fatigue during thermal or power cycling [26]. In such a context, operation point-dependant predefined current and/or switching frequency derating schemes are commonly used in the traction industry to avoid excessively high junction temperatures.