Power Electronics Europe Issue 4 - November 2022

www.lem.com AUTOMOTIVE CURRENT SENSORS 23 www.power-mag.com Issue 4 2022 Power Electronics Europe Solving the Current Sensor Footprint Problem when Designing Compact EV Traction Inverters Electric vehicles (EVs) are said to be the future of transport as the trend for electric mobility moves forward. This article is focused on the challenges of current sensing in high-power integrated traction inverters and highlights the benefits of using compact magnetic core-based sensors. Sofiane Serbouh, Product Manager of Large Drives, LEM, Switzerland Globally, according to the International Energy Agency’s Global EV Outlook 2022 report, the number of electric cars on roads by the end of 2021 was around 16.5 million and EV sales around the world in just the first five months of 2022 are reported to have exceeded 3.2 million. Challenges of current sensing in high- power integrated traction inverters Of course, with this increased popularity of EVs comes greater demand on their reliability, most notably their ability to be driven longer distances between charges. To achieve this, components such as high- power integrated traction inverters – which are essential to the vehicle’s battery range and the whole driving experience – have needed to become as compact and efficient as possible. A traction inverter converts DC current from the EV’s battery into AC current which powers the vehicle’s propulsion system. Another of the inverter’s functions is to capture energy from regenerative braking and send it back to the battery. The solution to the requirement for high-power integrated traction inverters has been to develop reliable power module packages and take advantage of the ability to slash the footprint of such components as capacitors, inductors, transformers and filters using the benefit of Silicon Carbide MOSFETs to switch faster and increase battery voltage. Engineers involved in the design of EV traction inverters understand that a key component is the current sensor and for it to meet the requirements of the e-mobility market it must offer a combination of high accuracy, affordability, high integration and the ability to operate in a demanding and rugged environment. Integration of coreless current sensors in EV not yet mature Coreless current sensors represent a promising solution for the future, because they will enable smaller and lower cost components to be used, but there are still many challenges with this technology before it can be widely adopted by the market. For example, the strong variation of the magnetic field in space makes it necessary to place the coreless sensors on the busbar with high accuracy and with no option to move them following assembly and calibration. A tenth of a millimetre variation can quickly lead to a degree of error that is not acceptable in high-power traction inverters. Tolerances on assembly, mechanical handling, vibrations and thermal expansion are all potential causes of displacement. Together with the need to overcome skin effect to achieve high bandwidth, these factors mean that significant efforts in mechanical design need to be carried out in order to achieve reliable current measurement. Furthermore, with traction inverters getting more compact, conductors are getting closer with a complex magnetic field distribution. Coreless sensors with their differential measurement are immune to homogeneous external fields but not to field gradient, which can introduce an extra degree of error in the measurements. Overall, significant constraints on the mechanical design to achieve the desired accuracy, combined with time-consuming calibration steps at the inverter level, reduce the attractiveness of the coreless sensor solution - for the moment. Meeting the challenges with magnetic core current sensors Until these technical barriers are overcome, fully calibrated current sensors with a magnetic core will remain the preferred means of achieving highly accurate current measurements in EV traction inverters. Not only does this technology have many years ahead of it but there is still massive potential for development and innovation in this area. In operation, the magnetic core concentrates and amplifies the magnetic field to sense with a reduced output noise, while also shielding the measurement from external disturbing fields. As a result, there is a high signal-to-noise ratio (SNR) over a wide bandwidth. Also, reliable and stable measurement is made possible even under tough vibration scenarios due to the tightly controlled assembly and calibration of the magnetic core, the Hall- effect based ASIC and the busbar. The problem is that open loop core- based sensors tend to be bulky and present challenges in terms of integration at inverter level. That’s why LEM has focused on developing compact and affordable current sensors with magnetic cores, to offer reliable current To receive your own copy of Power Electronics Europe subscribe today at: www.power-mag.com