How to Rapidly and Efficiently Implement Flexible EV Charging Systems

The trend toward e-mobility relies on expected availability of an electric vehicle (EV) charging infrastructure of public service stations, augmented by suitable charging systems at users' homes and workplaces. Although their core design requirements remain largely consistent, each type of system brings specialized demands, a complication that is compounded by regional differences in factors ranging from communications platforms to compliance requirements.

The challenge for designers of charging infrastructure, therefore, is to meet the core requirements with sufficient flexibility in their design to meet the widest possible range of end-use and regional requirements, while balancing cost and time to market.

This article describes the diverse nature of public charging station design requirements. It then introduces a flexible solutions platform from NXP Semiconductors that can be used to launch designs equipped to meet those requirements.

Meeting diverse design challenges

Efforts to accelerate the transition to EVs require ready availability of efficient electric vehicle sourcing equipment (EVSE), more commonly known as EV charging systems. Local driving needs can get by with the onboard AC-DC chargers built into vehicles for home or office charging, but these charging systems are unable to relieve the EV range anxiety that continues to limit EV adoption. Long-range e-mobility depends on the availability of public EV DC charging systems able to charge an EV much more quickly than the built-in AC-DC chargers. At the same time, these different EV charging systems need to conform to a number of standards and regulations for safety, security, and privacy.

For developers creating EV charging system solutions, the need to supply effective solutions for each specific use case presents both tremendous opportunities and significant technical challenges. Among the challenges, developers need to deliver a broad set of capabilities in a range of designs able to deliver required performance and efficiency while meeting the specific requirements for each application. Meeting this need requires adapting the fundamental architecture underlying all EV charging system designs.

Adapting the basic EV charging system architecture

Regardless of their specific target application, EV charging systems comprise two major subsystems—a power delivery front-end and a power management backend controller—separated by an isolation boundary (Figure 1).

Figure 1: The basic architecture for EV charging systems includes separate subsystems for the power socket interface and controller separated by an isolation boundary. (Image source: NXP Semiconductors)

On the front-end facing the vehicle and energy source, the power socket interface subsystem manages power delivery to the vehicle. On the other side of the isolation barrier, the controller subsystem handles safety, communications, and other high-level functions. Implementation of these subsystems typically depends on a few fundamental building blocks to meet specific requirements for metrology, control, functional safety, security, and communications associated with each specific application.

Each building block contributes critical functionality to the overall EV charging system design. The metrology unit needs to ensure safe energy transfer as well as accurate, tamper-resistant energy measurement for billing purposes. The control unit ensures reliable execution of the various protocols required for downstream energy transfer and upstream data transfer, building on capabilities for functional safety and security while supporting local and region-specific requirements for secure payment and communications protocols used to communicate with cloud-based resources.

In the past, developers needed to adapt the basic EV charging architecture design to their requirements by implementing each needed building block, typically using custom designs incorporating a wide range of general-purpose devices. NXP’s family of solutions for EV charging offers an effective alternative, enabling developers to combine off-the-shelf building blocks to quickly create EV charging system designs for a wide range of target applications.

Implementing the EV charging system front-end

NXP’s solutions for EV charging revolve around a number of processor families designed specifically to deliver the performance and functionality required in demanding applications like EV charging system designs. Among these processor families, members of NXP’s Kinetis KM3x series microcontrollers (MCUs) are designed specifically to provide a certifiable accurate power delivery measurement. Based on an Arm® 32-bit Cortex® M0+ core, Kinetis KM3x MCUs integrate an extensive set of functional blocks for measurement, security, communications, and system support, along with on-chip flash and static random access memory (SRAM) (Figure 2).

Figure 2: The Kinetis KM3x series integrates a complete set of functional blocks needed to implement a certifiable accurate power delivery measurement. (Image source: NXP Semiconductors)

To simplify metrology implementation, the KM35x MCU measurement front-end integrates a highly accurate sigma-delta analog-to-digital converter (ADC), multiple successive approximation register (SAR) ADCs, up to four programmable gain amplifiers (PGAs), a high-speed analog comparator (HSCMP), a phase compensation logic block, and a high-precision internal voltage reference (VREF) with low-temperature drift. To protect the integrity of the metrology unit, the on-chip security functionality supports both active and passive tamper detection with time stamping. Used in combination with external sensors, relays, and other peripherals, these on-chip blocks provide all the functionality needed to rapidly implement a sophisticated metrology subsystem for an EV charging system power socket front-end (Figure 3).

Figure 3: With a Kinetis KM MCU, developers need only a few additional external components to implement an EV power socket subsystem. (Image source: NXP Semiconductors)

Implementing the EV charging system controller

As noted above, an EV charging systems controller orchestrates the wide variety of functional capabilities needed in every system. The demands of this subsystem dictate the use of a processor able to deliver both the real-time performance needed to ensure precise control of the charging system, as well as the processing throughput needed to support diverse protocols while minimizing design footprint and cost.

Based on the Arm Cortex-M7 core, NXP’s i.MX RT series of crossover processors deliver the real-time capabilities of embedded microcontrollers with application processor-level performance. With an operating frequency of 600 megahertz (MHz) and a full complement of peripherals, i.MX RT processors such as the i.MX RT1064 are able to meet demands for low-latency real-time response. At the same time, features like a large on-chip memory, an external memory controller, a graphics subsystem, and multiple connectivity interfaces meet application demands (Figure 4).

Figure 4: The i.MX RT1064 crossover processor combines peripherals and memory with an Arm Cortex-M7 processor subsystem designed to deliver both real-time execution and application processor-level performance. (Image source: NXP Semiconductors)

Besides meeting critical real-time and performance requirements, EV charging system designs need to ensure security on multiple fronts including tamper detection and authentication of power connections and payment methods. For data protection, secure boot, and secure debug, developers can take advantage of the i.MX RT processor's integrated security features including high assurance boot, hardware cryptography, bus encryption, secure non-volatile storage, and a secure Joint Test Action Group (JTAG) controller.

To further harden security in an EV charging system controller, a design would typically complement the i.MX RT processor's security capabilities by including an NXP EdgeLock SE050 secure element. Designed to provide end-to-end lifecycle security, the SE050 provides hardware-based security accelerators for a range of popular cryptography algorithms, trusted platform module (TPM) functionality, secure bus transactions, and secure storage. By using this device to provide a root of trust (RoT) for the execution environment, developers can secure critical operations including authentication, secure onboarding, integrity protection and attestation.

Using an i.MX RT processor and EdgeLock SE05x device, developers need only a few additional components to implement a controller subsystem designed to run a high-performance, real-time operating system (RTOS) (Figure 5).

Figure 5: With their integrated functionality and performance capabilities, i.MX RT MCUs simplify design of controller subsystems for EV charging systems. (Image source: NXP Semiconductors)

Flexible solutions for diverse EV charging system applications

By combining the power subsystem and controller subsystems mentioned above with optional blocks for payment and communications options, developers can quickly implement a single-phase EV charging system capable of delivering up to 7 kilowatts (kW) (Figure 6).

Figure 6: Used in combination, a KM3 MCU and i.MX RT crossover processor provides an efficient hardware foundation for EV charging systems. (Image source: NXP Semiconductors)

With relatively modest modifications to the analog front-end, this same design can be extended to deliver a three-phase EV charging system capable of delivering up to 22 kW (Figure 7).

Figure 7: Developers can quickly adapt a design based on a KM3 MCU and i.MX RT crossover processor to support a variety of applications. (Image source: NXP Semiconductors)

Although this combination of KM3x and i.MX RT devices will suit many use cases, other EV charging system applications may require developers to optimize other facets of their designs. For example, residential chargers intended to deliver faster charging times than are possible with on-board chargers will require solutions that optimize cost and footprint. For these applications, developers can implement a lower-cost, entry-level controller using a cost-effective MCU such as the NXP LPC55S69.

In contrast, commercial EVSE chargers intended for public service stations will bring more stringent requirements in terms of high-speed application processing and real-time performance. These are needed to safely control battery storage systems operating at levels ranging from 400 to 1000 volts and delivering charging levels at 350 kW or more. Here, the ability to execute both application-level software and real-time software is critical to performance and functionality. For these systems, use of a processor such as the NXP i.MX 8M processor allows developers to more easily implement charging solutions able to provide both the Linux-based application processing and the RTOS-enabled real-time performance needed in these complex designs (Figure 8).

Figure 8: For more complex applications such as ultra-fast EV charging, developers can extend the basic EV charging architecture using high-performance processors such as i.MX 8M processors to support more complex controller requirements. (Image source: NXP Semiconductors)

Rapid implementation of cloud-connected EV charging systems

NXP processors including the Kinetis KM3x, i.MX RT, LPC55S69, and i.MX 8M provide a flexible platform for meeting specific requirements of different EV charging system applications. For more complex applications, however, delays in deploying the hardware foundation can impose significant delays in the development of the end-to-end EV charging system application.

To avoid such delays, NXP offers a fast path to development using a set of boards and evaluation kits based on the devices previously discussed. For example, the NXP TWR-KM34Z75M module provides a complete metrology platform that combines a Kinetis MKM34Z256VLQ7 metrology MCU with a full complement of support components. Similarly, NXP's i.MX RT1064 evaluation kit combines an MIMXRT1064DVL6 processor with 256 megabits (Mbits) of SDRAM, 512 Mbits of flash, 64 Mbits of quad SPI (QSPI) flash, all on a four-layer board, complete with an extensive set of peripheral connectors, including an Arduino interface. In addition, NXP's OM-SE050ARD board provides ready access to the EdgeLock SE050, and NXP's PNEV5180BM evaluation board provides a drop-in NFC front-end development board.

By combining the NXP TWR-KM34Z75M board for metrology, the i.MX RT1064 for control functions, and the OM-SE050ARD and PNEV5180B boards, developers can quickly implement a full-function hardware platform for building EV charging system applications (Figure 9).

Figure 9: Developers can rapidly implement complete end-to-end EV charging solutions using NXP boards and evaluation kits with available cloud services like Microsoft Azure. (Image source: NXP Semiconductors)

Used in combination with Microsoft Azure cloud services, NXP's board-level solutions enable developers to rapidly prototype a complete end-to-end EV charging system solution and use the platform as the basis for designing more specialized applications.

Conclusion

The ready availability of EV charging systems is a key enabler for e-mobility, but cost-effectively implementing the different solutions needed in homes, offices, and public service stations remains a barrier. Using a platform of specialized devices and board solutions from NXP Semiconductors, developers can quickly implement designs with the performance needed to meet the full range of EV charging applications and the flexibility to adapt to emerging requirements.