As EV designs continue to evolve, however, the SiC devices that are specified for them will likewise need to be enhanced.
What’s happening in the power management space amid the never-ending drive to lower power consumption in more and more complex technologies and applications? What about in applications dealing with higher and higher voltages? This month’s In Focus highlights the various design developments and manufacturing strategies happening in the power management segment.
Over the course of the last decade, we’ve seen wide bandgap semiconductor technologies emerge and start to finally take hold in certain application areas where their performance advantages proved to be of most value. Among these have been back-up power systems and renewable energy generation sites. In the following article we will give details about how silicon carbide (SiC), which has already shown itself to be pivotal in some aspects of electric vehicle (EV) design, is now ready to take things to the next level.
The need to combat air pollution and reduce our dependence on rapidly depleting fossil fuels has brought great impetus to the progression of the EV sector, with governments around the world announcing initiatives and legislative measures to assist with this. A recent report by IHS Markit shows that EV sales are forecast to rise sharply in the coming year. It predicts that there will be an increase in sales of close to 70% during 2021, which translates into a total of approximately 4.25 million units. Greatest demand is expected to come from China, followed by Europe, North America then Japan/South Korea. If the industry analyst’s longer-term projections are accurate, then by 2025 annual sales will have surpassed 12 million. Looking ahead still further, consultancy firm Wood Mackenzie estimates that there will be at least 320 million EVs on our roads by 2040.
Though all this sounds very promising, encouraging people to migrate from the combustion engine cars that they have driven throughout their adult lives and enter into the EV realm has several major challenges. Firstly, there is the convenience of charging. Large investments are already being made into the mass rollout of charging infrastructure, which will mean the EV users have greater assurance that they will be able to recharge their vehicles wherever they are travelling to. The speed with which charging can be completed is another key concern, as the time it takes for the EV battery to be replenished cannot lead to user frustration. In response to this, most new charging points being deployed are capable of supporting rapid charging functions. Vehicle affordability still remains an obstacle though and moving forwards EV manufacturers need to find an effective way of addressing this. It will predominantly be done by curbing the expense associated with the component parts and better system design with advanced technologies
The SiC Factor
SiC is seeing increasing uptake by EV manufacturers. It has the properties necessary to help make charging cycles quicker to complete, and to also undertake power conversion activities in a much more efficient manner. As EV designs continue to evolve, however, the SiC devices that are specified for them will likewise need to be enhanced.
By making EVs’ power conversion operations more efficient, the range that these vehicles can travel before they need to be recharge may be substantially increased. In relation to on-board chargers (OBCs), support for higher voltages is going to be necessitated. 650V-rated FETs will need to be replaced with devices with elevated voltage parameters, while not having to accommodate the additional costs that devices with ratings above 900V will call for. Finally, by making improvements to the drive train system, the traction inverters employed will have less financial impact. All three of these points will be of interest from a consumer perspective – and by implementing them into their vehicles, manufacturers will be able to gain a competitive edge over their rivals.
A new breed of SiC devices
Through architectural improvements to SiC devices, it will be possible to raise power density levels and mitigate some of the power losses that are experienced in current EV systems. At the same time, there should also be potential to bring the system bill of materials costs down, so that EVs can be positioned at more attractive price points. Semiconductor vendors need to implement these improvements within as short a timeframe as possible.
Appreciating the urgency with which viable solutions to the problems previously outlined need to be found, UnitedSiC has introduced the company’s fourth generation (Gen 4) SiC technology. By leveraging unmatched expertise in wide bandgap semiconductors, UnitedSiC engineers were able to develop the UJ4C075018K4S. This AEC-Q101 compliant 750V-rated SiC FET device incorporates a high-density trench SiC JFET structure. The JFET is co-packaged with a low voltage MOSFET that is Silicon (Si) based, to form a cascode. The JFET element is very compact, and this enables extremely low on-resistance characteristics to be witnessed relative to the area occupied. Alternatively, via UnitedSiC Gen 4 SiC technology, a FET with smaller dimensions could be utilized while keeping the on-resistance at an acceptable figure, thus allowing downsizing to be achieved.
The body diode of the UJ4C075018K4S has very impressive values in relation to both its forward drop voltage (VFSD) and its reverse recovery charge (QRR). The SiC die has been thinned to improve its electrical and thermal properties. The die is then attached to a copper (Cu) lead frame, with a silver (Ag) sintering material applied (which has thermal conductivity that is far superior to conventional solders). All this results in enhancements when it comes to the junction-case thermal resistance. Its minimal gate drive losses means that this FET can be switched at a much faster rate in soft-switching applications than competing devices, without any risk of overheating the existing gate drive ICs.
By supporting 750V operation, UJ4C075018K4S FETs can cope with larger battery voltages better than the standard 650V devices available from other vendors. This means that the better performance levels can be attained, without resorting to the need to specify more costly devices with elevated voltage ratings (e.g. 900V or 1200V). In Table 1, a comparison is given of the key performance parameters of power discretes from a variety of different vendors. These are a Si-based Superjunction FET, plus several 650V-rated SiC-based MOSFETs. Even though the UJ4C075018K4S has a far superior voltage rating (100V to 150V higher) than the other devices cited, its on-resistance/unit area is still much better – with the 650V SiC MOSFETs having values that are 2-3X that of the UJ4C075018K4S, and the Si-based FET being a whole order of magnitude worse.
Table 1: UnitedSiC 750V SiC FETs compared to current 650V SiC MOSFET and 600V Superjunction FET options
Applying G4 SiC Devices to EV Applications
Having already discussed the performance parameters that UnitedSiC’s G4 technology enables, let’s look at how it will be implemented into EV designs. Figure 1 describes a typical EV power system. This comprises an on-board charger which connects with the AC charging point, along with a DC-DC converter which takes the power from the 400V intermediate bus and delivers it to the numerous 12V sub-systems within the vehicle. When the EV is being charged, power flows from the AC charging point via a totem-pole type power factor correction (PFC) arrangement. A full-bridge CLLC resonant converter is used at the DC-DC conversion stage. The 750V UJ4C075018K4S FETs are highly suited to both the PFC unit and the primary side of the LLC/CLLC unit. For higher battery voltages, 1200V-rated UnitedSiC Gen 4 SiC FETs can be specified.
Figure 1: The on-board charging and DC-DC conversion circuitry found in a typical EV
Through use of the G4 SiC FETs with their low switching loss and industry-leading QRR, it is possible for the front-end PFC stage to run at significantly higher frequencies than increasingly outdated IGBT-based arrangements would be able to support—with 100kHz being reached. Likewise, much higher frequencies can be applied to the DC-DC stage. The efficiency improvements derived in this system have advantages from a thermal management standpoint. Less engineering effort and bill of materials budget needs to be devoted to this, as there is less heat to be dissipated.
Now, let’s look at the inverter element of the EV. This is what powers the drivetrain and represents a big proportion of an EV’s overall cost. Most EV inverter manufacturers are actively investigating SiC-based solutions.
By being able to use inverters that have smaller overall dimensions, major benefits can be derived. EVs will have appeal from a financial perspective, as car buyers won’t have to part with as much cash. The weight of the vehicle in which the inverter is situated will be reduced, and the lower losses mean that the range that it can cover before needing to be recharged will also be extended.
Though larger EVs for commercial usage will require SiC FET with higher voltage capabilities, there are many opportunities for 750V-rated FETs to be incorporated into the drivetrains of personal EVs. UnitedSiC G4 SiC FETs have been shown to outperform the best-in-class IGBT modules in this context, with far smaller power losses being witnessed and the ability to run at far higher frequencies. Other advantages include much lower rectifier losses during regenerative braking, as well as short circuit robustness which is closer in line with what would be expected from IGBTs. The capacity of SiC to withstand high temperatures will also mean that inverter hardware could be integrated directly into the motor housings—resulting in space and cost savings, as well as improved motor operation..
The performance benefits of devices using wide bandgap process technologies is already recognized, and their value in relation to expanding the prevalence of vehicle electrification is beyond question. Through further innovations at an architectural level, there is the clear prospect to make EV operations even more efficient and minimizes losses. The added convenience, alongside the cost reductions resulting from this are certain to lead to greater EV uptake in the years ahead.
About the Author
Anup Bhalla is the vice president of engineering at UnitedSiC.