Optimising Performance & Cost in Hall Effect Oriented DC Current Sensing Implementations

Author : Bruno Boury, Melexis

09 November 2023

Electric vehicles (EVs) are considered one of the key components in meeting ongoing sustainability objectives, as well as limiting the growing effects of climate change. Moving away from internal combustion engine (ICE) vehicles is a priority for the entire automotive industry. This shift presents new challenges, covering everything from the supporting charging infrastructure to vehicle performance and production expense.

One technology that is fundamental to many of these issues is the demand for high-performance current sensors. These devices must be reliable, accurate and cost-effective. 

Changes are underway in EVs’ high-voltage systems - with their powertrains going from the current 400V voltage level to 800V in next generation models. Ensuring EV batteries are functioning safely and in an optimal manner is absolutely paramount. For electric motors and actuators, as well as charging infrastructure, heaters and battery management system (BMS) deployments, current sensors play a vital role. To achieve fully efficient operation, all the constituent elements must be running within normal conditions and accurate feedback on current flow has to be obtained.

BMS within EVs
To understand the importance of DC current sensors within EV battery systems, one must appreciate the primary functions of the BMS. In simple terms, the BMS is involved in interpreting parameters that ensure the battery is operating within safe limits, as well as carefully balancing individual cells during charging/discharging cycles. Within EVs, the BMS is used to determine factors such as the state of charge (SoC), state of health (SoH) and state of function (SoF) - which are crucial for maintaining safety, estimating battery charge times and assessing the range that an EV can cover before needing to be recharged. 

Current sensors also ensure the EV battery remains within its optimal operating conditions. A typical EV powerpack has an ideal working temperature range between 15oC and 35oC. The US Office of Energy Efficiency & Renewable Energy found that in freezing temperatures, an EV’s range can drop by as much as 39%. Remaining within the recommended temperature range achieves optimal efficiency and reliability, while excessive deviations can lead to substantial drops in performance, as well as degradation of the cells. Most EVs utilise a combination of PTC-heaters, e-compressors and fans to manage thermal conditions. Current sensors provide the necessary feedback for battery control systems to precisely regulate thermal exchange systems. They also offer diagnostics to identify unusual operating conditions (such as excessive current draw). 

Typically, automotive manufacturers have used generic current sensing solutions, many of which are less than optimal for vehicle deployment - whether this is due to accuracy, packaging or cost. For this reason, EV manufacturers are now exploring new current sensing solutions.

The difficulties with EV DC current sensor design
DC current sensors are commonplace across a wide range of modern electronics, but the optimisation of EVs requires innovative solutions. Accuracy, packaging and cost are top of mind at EV manufacturers. There are additional thermal and electromagnetic interference (EMI) considerations too. In conjunction with these considerations, a well-designed current sensor needs to integrate with existing communication protocols, while operating efficiently with minimal power draw. It must also be operationally reliable, keep the processing load on vehicle control units down and being aligned with functional safety requirements (as outlined in the ISO 26262 standard).

Figure 1: The effects of internal climate controls and battery heaters on vehicle range
Figure 1: The effects of internal climate controls and battery heaters on vehicle range

In terms of existing current sensing solutions for BMS applications, OEMs can typically select fluxgate, shunt or Hall Effect based sensors. These solutions exhibit respective strengths and shortcomings. The issue is that, due to the number of aspects that have an impact on a sensor’s deployment (thermal changes, presence of EMI, physical constraints, weight limitations, cost concerns and communication requirements), finding a sensor which excels in all categories is difficult. While fluxgate technology offers high accuracy and the reliability needed for battery applications, at present they are just too expensive for widespread use. Shunt resistors offer a fast responding, cost-effective solution. However, their accuracy can be effected by system thermo-electric imbalances at higher currents. In many cases, getting Hall Effect sensors to attain the necessary accuracy for BMS high-voltage applications is challenging. 

Automotive OEMs are reassessing their battery support systems and looking for new DC current sensing solutions. But creating something that is capable of meeting the demands set by EV BMS implementations, while simultaneously remaining cost effective, is a sizable engineering task. It not only requires an accurate and robust device. It also needs to take into account system integration within the vehicle. So how do semiconductor suppliers bring together the strengths of present technologies, but also mitigate their inherent weaknesses?

The cost dimension
Within the average EV, the battery currently accounts for 72.5% of the development cost of the overall powertrain, with the electric drive modules and inverter covering a further 4.7% and 9.8% respectively. All 3 of these systems are reliant on numerous DC current sensors. For OEMs and tier 1s looking to reduce production costs, targeting these current sensors is therefore a sensible path for improvement.

Developing a new breed of Hall Effect sensors
The understanding of Melexis’ engineers in relation to the demands just outlined is what led the company to develop the MLX91230 for DC current sensing. This smart IVT Hall Effect sensor represents a radical new approach, where digital technology is embraced. Central to this sensor IC is the embedded microcontroller unit (MCU), which provides a full digital structure and advanced signal processing. By utilising it, automatic gain control is enabled, so that higher a dynamic range can be covered. 

Through the MCU (with its integrated flash memory), Melexis can provide support for bespoke software deployment. In addition, it allows OEMs to compensate for system imperfections, such as ferromagnetic saturation onset, non-linearities and hysteresis compensation (magnetic offset). 

The IVT design enables the measurement of 3 physical quantities - current, voltage and temperature. It also includes diagnostics, such as an on-chip over-current detection (OCD) which allows direct input to drive the Pyro-Fuse. Both LIN and UART communication are supported for simple integration into BMS installations.

Figure 2: Distribution of vehicle production costs gasoline/EV 2017/EV 2025
Figure 2: Distribution of vehicle production costs gasoline/EV 2017/EV 2025

As touched upon throughout this article, accurate current sensing is vital in modern EVs - as so many electronic systems are directly linked to a vehicle’s performance and safety. In response to this, the MLX91230 delivers 0.5% accuracy for thermal drift, or 1% all-in (lifetime drift and linearity errors). One area where this device can immediately add value is the battery’s SoC calculation. Here the increased accuracy allows greater utilisation of the installed battery capacity, increasing the range that the vehicle can travel.

Applications within EVs and beyond
With new solutions like the one outlined here proving more economically viable, as well as allowing for an increase in performance (through better battery optimisation, closer thermal regulation and reduction in wiring complexity), they are certain to appeal to EV manufacturers. Replacing a large number of existing sensors can have a measurable effect on battery optimisation and production costs. 

It is worth mentioning that the demand for cost-effective and high-accuracy DC current sensing is not just limited to EV models. With regard to alternative transportation, like e-Bikes, advanced current sensors will help increase performance. Whether this is through the optimisation of charging and discharging operations, increasing battery utilisation or reducing system complexity/costs, innovative DC current sensing solutions can be invaluable.

Conclusion 
In terms of the automotive industry's ongoing search for greater efficiency, higher accuracy measurement data will help to improve systems controlled through current regulation, such as motors. Better thermal management of batteries will also be pivotal, while smart MCUs and auxiliary measurement channels can help to lower wire harness complexity (with weight and component count reductions being derived). For automotive OEMs, evaluating the latest DC current sensor options against existing devices shows there is a real opportunity for measurable performance gains to be realised.


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