Roadmaps explored: From wafers to electric drives, how automotive power electronics are a hotbed for innovation
The automotive sector will stimulate growth in the power electronics market and inspire further innovation.
For electrified vehicles power electronics play a crucial role in controlling the voltage levels, ensuring power is supplied to the electric motor and enabling plug-in vehicles to charge from the electricity grid. As the number of electrified vehicles on the road increases, the value of the power electronics inside our vehicles is expected to rise. As Figure 1 demonstrates, the value of power semiconductors in a vehicle is anticipated to grow 15-fold if you compare a conventional vehicle to a plug-in hybrid.
Yet despite the promising increase in value, the number of electrified vehicles on the roads still remains relatively low with automotive power electronics currently comprising a fraction of the wider power electronics market. Based on data from Oak Ridge National Laboratory in the US, (H)EV inverters were a fraction of the total market representing just $2.5 billion out of a total $46 billion in 2014. Moreover as you venture further down the automotive power electronics supply chain, it’s largely dominated by non-automotive companies who have innovated products for adjacent sectors. In fact many of the historic innovations in power electronics have emerged from sectors such as aerospace, renewable energy, consumer electronics or rail – current automotive technology has largely borrowed approaches from these sectors.
Nevertheless with the advent of electric vehicles the tables are beginning to turn. The challenging requirements of the automotive sector is creating a demand for higher temperature materials, faster switching frequencies, improved reliability and more power dense solutions. More importantly these solutions need to be scaled up to meet the demands of the automotive sector which creates a host of new technical challenges. In light of these pressing needs, this blog aims to capture the key themes of the Power Electronics Roadmap and highlights the innovation areas that will be most relevant for the automotive industry going forward.
Roadmap process is more granular than the 2013 roadmap
The updated Power Electronics Roadmap, released at LCV2017, builds upon the foundations of the 2013 Automotive Council roadmap and explores the specific technology challenges for automotive power electronics. The previous roadmap looked at motors and power electronics in tandem which provided clear insights on how to integrate motors and power electronics and their shared challenges. However a drawback identified early on in the update process was that the previous roadmap focussed on inverters – the system which supply power to the motor. However the updated Power Electronics Roadmap decided to broaden its remit and focus on other systems such as DC-DC converters (which manages power for lower voltage applications such as lighting) and on-board chargers (which enables the vehicle to be charged from the grid). Attention is also given to specific areas of power electronics development such as new semiconductor materials, packaging concepts and passive components to try and capture the breadth of innovation in the sector.
However what gives the roadmap a clear focus is its long term vision for power electronics development through a set of ambitious medium and long term technical targets. The table in Figure 2 detail the ambitious targets set for inverters, DC-DC converters and combined DC-DC and on-board chargers.
These technical targets are intended to stimulate research with respect to cost ($/kWh), power density (kW/l and kW/kg) and efficiency (%) and have been set for 2025 and 2035. Early on in the update process the community the Advanced Propulsion Centre convened recognised that power electronics solutions can be developed for two separate purposes. The first is a cost effective solution that will mainly be geared towards high volume applications such as passenger vehicles. The second purpose is high performance applications such as sports cars and heavy duty vehicles which may need to operate at much higher temperatures and voltages or need exceptional levels of reliability. So whilst commercialising an inverter that is $3/kW and 60kW/l by 2035 would be highly desirable, the technology choices needed to attain that power density would likely prohibit that cost point. This illustrates that trade-offs exist when designing power electronics. Introducing new semiconductor materials may result in smaller devices and could simplify cooling systems, but the cost of the material exceeds the incumbent silicon technology thereby increasing costs elsewhere. The themes of trade-offs and system level thinking underpin the new Power Electronics Roadmap and the article will attempt to highlight interactions between certain disciplines within power electronics.
Silicon based semiconductor devices are fast approaching their performance limit so wide band gap materials are being explored in order to meet automotive demands
Semiconductor devices have driven the rapid growth of the telecoms, energy and consumer electronics industry over the past 60 years. But what are semiconductors? Semiconductor materials are those which have much lower resistance to the flow of electrical current in one direction compared to the other. Therefore the electrical conductivity of a semiconductor is between that of a conductor like copper and that of an insulator like rubber – hence the name semiconductor. Examples of semiconductor materials include germanium and gallium arsenic but the most widely used material for power electronics is silicon. However to turn a semiconductor material into a usable device such as a diode or transistor requires hundreds of manufacturing steps. Put simply, a thin slice of semiconductor material (called a wafer which is typically between 100-300mm wide) acts as a substrates on which hundreds (and in some cases thousands) of integrated circuits are layered on top of it. Contained within these integrated circuits are a number of individual transistors which are then sliced up and packaged to be assembled into discrete devices or as integrated circuits containing many transistors.
Since the 1960’s semiconductors have largely obeyed the principle of halving in size every two years. This is commonly referred to as Moore’s Law named after Gordon Moore’s observation of the trend whilst working at electronics giant IBM. Since Moore’s prediction half a century ago, silicon based semiconductor devices have provided the power electronics industry with a reliable and cost effective semiconductor material that achieves good levels of efficiency. However despites it’s reliability, silicon is fast approaching its theoretical performance limit with regards to size, temperature and switching frequency capability.
In light of the limitations of silicon, the updated Power Electronics Roadmap recognises the importance of new materials – called wide band gap materials – such as silicon carbide (SiC) and gallium nitride (GaN) for future automotive applications. Figure 3 highlights the significant performance benefits that can be attained with both SiC and GaN compared to silicon.
The spider graph demonstrates wide band gap materials enable higher operating temperatures, greater switching frequencies, higher endurance to electromagnetic radiation and a higher operational voltage for a given design. Moreover given the distinct benefits of both materials, respondents at the public workshop were keen to highlight that SiC and GaN can offer the most benefits in certain uses cases. For example SiC was deemed more suitable for higher voltage applications (>600V) and high power outputs whereas GaN was typically deemed more appropriate at lower voltages and where ultra-high frequencies are required. The different characteristics of the two materials therefore informs the initial applications. For example traction inverters ideally need to accept high current densities and be ultra-compact to fit alongside the motor. Therefore initial applications for SiC were predicted to be in traction inverters as SiC is better suited to high power and high temperature applications. Formula E’s early adoption of SiC based IGBTs for inverters illustrates the performance benefits of using SiC. ROHM Semiconductors, who supply the VENTURI Formula E team, claim that their new SiC based inverter for the Formula E’s fourth season is 43% smaller and 6kg lighter than the inverter for season two.
GaN on the other hand was identified as being slightly behind SiC on the automotive power electronics development curve but still important for the automotive sector. The roadmap identifies GaN as more being suitable for lower power applications that require higher switching frequencies (above 100 kHz which is the limit of silicon). Therefore initial applications of GaN semiconductors is likely to be in on-board chargers and DC-DC converters where the power requirements are lower and the higher switching frequency are desirable to increase efficiency. However this is a general principle and the choice whether to implement SiC or GaN can be more nuanced than this simple delineation. For example, new rapid charger converters closely resemble inverter circuit topologies so SiC could be suitable for the new range of high power chargers. Moreover the decision of using GaN or SiC isn’t an either/or decision. There is now an emerging trend in other sectors of using GaN-on-SiC transistors which leverages the advantages of both materials to create devices that can address extremely demanding applications like advanced radar systems.
Wide band gap materials do present considerable challenges
Despite the promising performance gains that can be achieved with wide band gap materials this doesn’t come without a cost. Alastair McGibbon, Power Electronics Business Development Manager at the Compound Semiconductor Catapult, reaffirms the importance of wide band gap materials for automotive applications but also illustrates the challenges:
Wide band gap materials such as silicon carbide and gallium nitride offer a number of benefits over the incumbent silicon technology and are central to reaching the automotive sector power density targets set for 2035. With effective system integration, wide bandgap materials will enable smaller, faster and lighter solutions. However, there are still challenges in commercialising these materials for automotive applications including achieving significant cost reductions and ensuring there is a consistent standard of reliability for wide band gap materials.
As alluded to above, the two biggest challenges the roadmap identifies for the adoption of wide band gap materials are the higher upfront costs and understanding the reliability issues surrounding wide band gap materials. The higher raw material costs of wide band gap materials (especially SiC) coupled with relatively low manufacturing capacity contributes significantly to the increased cost of these devices. For example substrate material accounts account for approximately half the cost of SiC based devices whereas in traditional silicon based power devices, substrates account for only 5–7% of the cost. Nevertheless whilst SiC substrates and epitaxy layers are currently more expensive, they can still compete with silicon devices on a cost/area basis due to lower specific on-resistance of the SiC material. Furthermore Figure 4 predicts dramatic cost reductions for various SiC devices illustrating that they will become increasingly viable in the early to mid-2020s.
The situation is slightly more nuanced for GaN as the raw material is relatively cheap and the devices can be made smaller using similar processes to silicon. GaN HEMT’s for RF applications for example are already cost comparable to their silicon counterparts. However GaN requires improvements in manufacturing efficiency and speed to achieve the economies of scale in order to be cheaper than silicon in automotive power electronics.
Secondly there is considerable uncertainty on how and why wide band gap materials fail. This necessitates the creation of new reliability standards as the qualification tests in place for silicon are not entirely suitable for wide band gap devices. This is a paramount concern for the automotive sector as if a component fails it can compromise performance but more importantly safety.
Nevertheless whilst the roadmap strongly acknowledges the role SiC and GaN will play, it doesn’t neglect the potential role for silicon in the future. Silicon has amassed an enormous manufacturing capability which means it’s a very cost effective option. In the public workshops, it was felt silicon could still remain a popular choice as the use of wide band gap materials may not be cost effective for some applications. For example in lower voltage applications such as 48V mild hybrids, keeping the costs down is vital and the vehicle level efficiency improvements by using wide band gap materials may not outweigh the up-front costs.
Current semiconductor packaging technology will continue to be improved but new wide band gap materials will require radical packaging concepts as well as development of other components
The roadmap identifies that current approaches to packaging semiconductors and converters will undergo incremental innovations to improve the mechanical, thermal and electrical performance of automotive power electronics. The requirement for higher performance semiconductors will drive higher temperature capable thermal interface materials (e.g. greases, phase change materials and thermal tapes); innovations in power semiconductor substrates (e.g. better ceramic materials, bonding techniques, implementation of new substrate concepts); improved encapsulation and insulation materials (e.g. parylene) as well as higher temperature polymers and dielectrics. The roadmap also acknowledges there will be steps to integrate component functions in circuits by integrating sensors, gate drives and filters into semiconductor packages.
As highlighted earlier, the introduction of wide band gap materials presents numerous opportunities but from an application perspective requires a significant amount of system engineering. Mark Johnson, Professor of Power Electronics at the University of Nottingham and the APC’s Spoke Lead for Power Electronics, believes the introduction of wide band gap materials has significant opportunities in changing the way we design power electronics:
When introducing wide band gap materials it’s not as simple as replacing silicon with silicon carbide or gallium nitride. To realise the full potential of these materials in a system, you need to optimise the components that sit around the new devices. For example, the number and size of passive components can be reduced when using wide band gap materials, but the passives you do use need to be chosen to meet the requirements for higher frequencies, more compact layout and possibly higher temperatures. Moreover existing packaging materials and concepts that have proved effective when using silicon based devices can limit the power densities and efficiency. Therefore improved packaging designs that offer higher levels of integration with better thermal and EMI management will be needed to unlock the potential of wide band gap materials.
One approach being actively investigated is using converter-in-package concepts alongside wide band gap materials. This approach aims to transition away from single and multi-chip modules housing just semiconductor components to fully integrated power modules. Traditional converter architectures design approaches have tended to result in electrical, mechanical and thermal aspects being treated in isolation by separate teams. As a result each component in a converter is designed separately, cooled individually and has its own operational requirements which adds complexity.
Converter-in-package designs could accept much higher currents and temperatures, contain fewer interfaces and contain multifunctional sub-components and materials (e.g. multi-functional printed circuit boards or phase change materials for example). This can reduce bill of material costs, simplify cooling approaches and create smaller and lighter converters which increase power densities.
However the roadmap acknowledges these approaches are fairly early in the development stage and is still a long way from mass market adoption. A considerable challenge identified in commercialising converter-in-package is that existing manufacturing processes cannot create these concepts in high volumes. Therefore these more complex designs need to be designed with manufacturability in mind or will require the widespread adoption of new manufacturing processes such as additive layer manufacturing.
Successfully integrating power electronics into the wider vehicle could enable both cost effective and power dense solutions
Finally the roadmap identifies two broad strategies the automotive industry are pursuing regarding the integration of power electronics. The first strategy is the closer coupling of electric machines and power electronics. Ryan Maughan, Managing Director of AVID Technology, believes closer integration of electric motors and power electronics is the next practical step in removing redundancy and reducing costs:
There are significant opportunities in the deep integration of motors and power electronics for future electric vehicles. This allows interconnections to be deleted, a move from cables to busbars for motor phases and common cooling to be achieved improving power density, eliminating potential failure points and reducing costs. Delivery of a fully integrated and tested motor drive unit to the high volume production line also eliminates any production issues that might occur trying to sync motors and inverters during the vehicle build process.
However despite the aforementioned benefits of integrating motors and power electronic, challenges to this approach were raised during the workshop process. Ensuring integrated drives can be manufactured cost effectively at higher volumes was cited as a challenge as well managing temperature differences between the inverter and motor. In the longer term, the roadmap offers even more dramatic possibilities than close coupling and shared cooling systems to power electronics embedded onto the motor using additive layer manufacturing (i.e. electronics printed into the rotor).
The second trend in power electronics integration is combining converter functions to remove the need for multiple converters in a vehicle. In fact this is already emerging in mass market applications with the newer models of the Nissan LEAF combining the on-board charger and DC-DC converter function into a single unit. This enabled Nissan to reduce costs as well as offset some of battery weight increase as well as free up packaging space.
Another approach seen in the mass market has been introduced by Renault called the “Chameleon charger” which is used in the ZOE. In this design Renault opted to use the inverter and motor to charge the vehicle rather than build a separate on-board charger. This approach means that the ZOE could charge from a 3kW single phase charger right through to a 22kW three phase using the supplied type-two cable provided. In fact for higher-powered, higher-speed charging, the ZOE could charge at up to 43kW AC three-phase from a dedicated, tethered AC quick charging station. However this approach can put additional pressure on the electronics, with the converter needing more EMI and line filters to limit the ripple power. However a significant benefit of this approach can be seen in Continental’s AllCharge configuration where customers have 230V of AC power available for on board use if needed which can be used to power a range of portable electrical devices.
Power electronics are at the heart of mobility
The Power Electronics Roadmap aims to capture the breadth and depth of potential innovations in this fast moving sector and illustrates that there is enough work for the research and manufacturing community to tackle. A key theme running through the roadmap is that the imminent introduction of wide band gap materials has implications on organisation working on a device level right through to companies integrating power electronics into a vehicle platform. More importantly for the UK supply chain, the window of opportunity to act is narrowing. Strong competitors from Europe, Asia and the United States are amassing considerable capability in automotive power electronics and the UK must build upon its strengths and protect areas where considerable value can be achieved. With this in mind, we hope the updated roadmap will help facilitate discussions between interested companies and aid those organisations in adjacent sectors wishing to understand the automotive power electronics landscape.
The Advanced Propulsion Centre would like to thank all the participants who attended the Power Electronics workshop. Your input at the workshop was valuable in delivering the updated Power Electronics Roadmap. Special thanks also go to members of the steering committee who ensured the APC represented the challenges facing the community.