Q&A: LEM’s bi-directional automotive RCM type B sensor

I recently spoke with Clément Amilien, Head of Global Product Management Automotive at LEM, to find out more.

Can you explain the significance of transformer-less on-board chargers (OBCs) in the electric vehicle industry, and how they differ from traditional designs?

Transformer-less OBC is a hardware architecture where the galvanic isolated transformer between grid connection and the DC battery is removed. This architecture saves on paper weight space and cost and offers a higher efficiency versus traditional transformer-based architecture. But it is coming with some technical challenges such as managing isolation barrier safety and leakage current flowing in the car from the DC battery to the grid. This is where LEM RCM is helping to monitor them.

What are the key challenges system engineers face when designing bi-directional OBCs, and how does LEM’s CDT sensor address these issues?

With bi-directional OBCs, the OBC can behave as an energy source, similarly to a solar inverter transforming DC solar energy in AC energy for the grid. Similarly to a solar application, it needs to have a residual current measurement that complies with international standards. As an energy source it becomes responsible for none, injecting DC leakage into any AC load on V2L, V2H, or B2G outlets and protects from any leakage in the load to avoid electrical hazards.

Challenge #1: dealing with AC and smooth DC

CDT is a RCM type Bmeaning that, compared to standard switch board protections that are only valid for AC signal, the CDT sensor protects against all AC and DC leakage, whatever type.

Only a type B RCM can measure and detect AC and smooth DC. The new CDT sensor has been designed to meet this demand by combining best-in-class automotive grade with a level of accuracy of ± 0.5mA @ 5mA, thanks to LEM’s multi-patented fluxgate technology.

Challenge #2: compliance with ISO26262 and new ISO5474

With the rise of bi-directional OBCs also comes challenges on the standard compliance side. To ensure end-user safety, the system design has to be compliant to ISO26262 and ISO5474

LEM’s new sensor takes away the worries of designing-in solutions within constrained automotive environments while also speeding up design architecture by being ISO26262 Automotive Safety Integrity Level B (ASIL B) ready.

More importantly, the new RCM type B sensor enables designers to react quickly to the new ISO5474 Part 2 standard for AC power transfer.

Challenge #3: adapt the same design to different regional regulations

Other features specific to LEM’s latest sensor include features like advanced diagnostic functions and the ability to work equally effectively with single-phase and 3-phase AC. CDT also offers advanced functions on the secured Serial Peripheral Interface (SPI) bus including monitoring of T°C, leakage value, and supply monitoring (a secured bus ensures encrypted and authenticated data transmission between devices), and dynamic fault selection.

This last feature allows system engineers to set up different tripping levels depending on the local norms, application or regional standards, without having to change anything in the system and its design.

Could you elaborate on the role of bi-directional OBCs in enabling V2X applications, such as V2L, V2G, and V2V?

V2X, or Vehicle to Everything, allows the energy stored in EV batteries to be used not just for powering the vehicle, but also for feeding other vehicles, homes, appliances, and even the grid. This paradigm shift has significant implications for the design and operation of EV charging infrastructure. Bi-directional OBC systems are essential for enabling V2X capabilities.

Their role is to allow current to flow into the EV vehicle battery pack as am AC to DC converter and out the battery pack, similarly as an inverter with DC to AC converter, into an external device.

They also interface with the end user loading or unloading the battery of the EV. These systems have to protect EV end-users from electrical shock and require advanced safety solutions.

The CDT sensor is described as being "easy to design-in." What specific features or considerations contribute to this ease of integration for automotive engineers?

• Built in SPI bus with all diagnostics embedded in, with app note and algorithm ready to be set with specific standards
• Dynamic Fault level selection (allowing to select standard to apply on sensor)
• End to End safety monitoring communication protocol
• DC Fault, AC Fault value
• Leakage value (CH1 and CH2 in SF version)
• Fault diagnostic (T°C, Supply, Overload)
• Sleep mode (for low power mode)
• Soft upgrade (bootloader)

All these features give easier implementation, faster development, and higher value from the sensor than classic industrial devices.

Could you share any insights into how LEM plans to further develop or expand its product line to support the evolving needs of the EV market?

LEM is supporting the development of the EV market with three main product ranges where we show intense R&D effort.

• A product range for motor control solutions which is evolving rapidly
• Battery management with CAB, the new SMU, and some newcomer to be announced very soon in 2025
• On board charger where we have a lineup of integrated current sensors that are available and some exciting new launches coming in 2025
• On Residual Current sensors, our strategy is to extend our lineup to all use cases from our customers

In all these areas, we are concentrating our efforts on smaller, smarter, cheaper, and co-developing with our customers to find the best solutions for their systems.