How to meet the design requirements of on-board chargers with dedicated microcontrollers

Addressing "range anxiety" is critical for engineers focusing on electric vehicles. Consumer expectations based on the range and fuel experience of internal combustion engine (ICE) vehicles are difficult to change.

Battery capacity is one consideration. It is increasing in size and voltage as designers strive to optimize product range by expanding energy storage capacity and incrementally improving efficiency. The size and weight of the vehicle electronics, especially the wiring harness, is also a target for optimization. These factors have a significant impact on vehicle range per charge; however, they are a double-edged sword. Larger batteries take longer to charge; a four-hour stop at a charging station is not an option on a cross-country trip.

Higher DC link voltages require the use of different energy conversion technologies; while vehicle modules must demonstrate cutting-edge performance for safety and reliability, such as ISO 26262. In addition, the targets for certain key performance indicators, such as increased energy density (kW/L) and specific power (kW/kg), make the design of systems like OBCs more challenging.

OBC Architecture

The on-board charger (OBC) is a key part of the energy "value chain". The size of the battery determines the output power level of the OBC; its main role is to convert energy from the grid into a DC current that the battery management system uses to charge the battery pack. The OBC must do this while complying with strict emission requirements and meeting its main production targets.

Designers use different architectures to achieve their goals, choosing between various approaches based on several objectives, including the nature of the incoming power source (number of stages), cost/efficiency metrics, and whether the design needs to support vehicle-to-grid (V2G) energy transfer, which requires a bidirectional structure. On the other hand, the size and weight of the module are mainly determined by discrete components such as capacitors, inductors, and transformers. These components limit energy density performance.

The emergence of higher voltages in electric vehicles of 800V or higher has driven the use of wide bandgap semiconductor technologies in energy conversion applications, especially those connected to the DC-LINK bus, including OBC, BM, and inverter. For OBC, silicon carbide (IC) or barium nitride (GAN) is becoming the preferred technology to support higher voltage and power levels.

The SIC is ideal because it supports efficient operation at very high voltages and temperatures. It also reduces cost and size because it requires a smaller and less expensive cooling device. When combined with a faster control loop, the WBT device can significantly reduce the space requirements of the discrete components shown in Figure 3. Second, advanced microcontroller architectures with enhanced digital control capabilities can support faster switching and control loops, providing a level of integration that helps achieve design goals such as energy density and cost.

Disadvantages of traditional microcontrollers

Of course, electric vehicle systems present unique challenges that must be addressed with targeted solutions. This is evident in the choice of microcontroller. Traditional automotive MCUs, such as those designed for ICE vehicle powertrains, are not designed to support the basic digital, analog, and system-level capabilities required to support electrified design requirements. For example, most traditional automotive MCUs cannot support the high switching frequencies required to gain the benefits of WBG technology.

Many traditional automotive MCUs support PFC switching frequencies of less than 150 kHz and lack the PFC resolution to utilize WBC technology that is critical in the power factor correction (PFC) and DC-DC converter stages. For example, some 200 MHz MCUs offer an input clock as low as 80 MHz. In this case, if the desired PFC frequency is 150 kHz, the MCU will only support 9-bit PFC resolution.

For OBC, this capability is not suitable for silicon hybrid based implementations, let alone WBT devices. Figure 4 highlights the importance of switching frequency, while the pressure wave M resolution is also an important aspect because it mainly determines the time when the switch starts/stops operation based on the input parameters measured by the analog-to-digital converter (ADCS).

To fully realize the potential of SIC/GAN devices, the design must optimize the control loop. This requires higher resolution, precise dead-time control, faster ADCs, and faster calculations to reduce the control loop time. In addition, the ADC samples should be synchronized with the corrugator output control. Therefore, the functionality of the MCU has a great impact on the weight, footprint, and cost of the OBC.

In a typical PFC or DC-DC control loop, the microcontroller measures voltage and current. Next, the microcontroller and DSP run an algorithm on these measurements and then control the duty cycle of the PWMS. The control loop timing depends on:

Voltage/current sampling rate

Calculate throughput

Response time

Controlling/monitoring voltage/current in an OBC requires high ADC sampling rates, coupled with good CPU throughput (DMIPS) and math accelerators. These determine the execution time of the algorithm. The number of channels and the associated resolution determine the speed and accuracy of output control, as well as the level of integration of converter stages possible in the device. For example, parallel output stages are used to increase output power; this configuration requires sampling of current and voltage at both stages simultaneously. This requires four ADC instances; therefore, not only the number of channels is important, but also the number of instances.

Silicon MOSFT requires longer dead times to reduce switching losses, while SiC/GAN allows for shorter dead times. Short dead times can increase the power from input to output in one cycle. Most conventional MCUs cannot support these small dead times.

The OCS must include protection against overcurrent, overvoltage, and overtemperature. Analog comparators are often used to detect these faults and control the output as quickly as possible to avoid damage. These comparators require very fast response times. Microcontrollers that are not dedicated for these applications may not have comparators, or their response time is too long, making them unsuitable for implementing protection in the OBC. Even if external comparators are used to implement the protection mechanisms, they require digital-to-analog converters (DACs) to generate the references, and most microcontrollers do not usually have any or enough external DACS. In addition, using external comparators increases the size and cost of the solution footprint.

Override control loop mechanism

In addition to control and protection mechanisms, other aspects should also be carefully examined.

Radio firmware update support

Functional safety (ISO26262)

Safety measures

Automotive design cycles are accelerating, and OEMs must continually deliver new features to keep up with the competition; therefore, vehicles are becoming "software defined." This enables companies to monetize the features they enable. These aspects need to support sales after firmware upgrades, so the MCU must support OTA updates.

Functional safety is also required in automotive designs. While the design requirements for each OBC may vary, in most cases the system must support AISR-B through AISL-D. Not all MCUs support single-level cores, while others prohibit the use of independent execution. The designer's ability to choose independent execution or independent execution cores provides greater flexibility to support various safety integrity levels. This allows the design to be optimized based on cost and scalability.

Moreover, the risk of cyberattacks is greater for connected cars. Therefore, OBC may need Evita or Evita's medium-sized security to deal with these threats. This security is especially important for vehicles connected to the power grid.

To facilitate electrification, some MCU suppliers offer devices that meet these new requirements. One example is the Stellar E1 (SR5E1) which integrates standard MCU and DSP functions into one device, providing a single-chip solution for OBC.

The Star E1 is an AEC-Q100 qualified MCU that includes 2x ARM Cortex M7 cores, so one core can be used for one PFC loop and one for the DC-DC stage in a bidirectional OBC implementation. To support fast control loops, the Star E1 contains a CORDIC math accelerator. The MCU contains 12 high-resolution timers with 104-ps resolution, supporting PWB switching frequencies greater than 1MHz with precise dead-time control. Combined with fast computational capabilities, the high-resolution timers replace external DSPs.

The devices also include fast comparators on chip for protection. In addition, they offer a 2.5MSP 12-bit synthetic aperture radar application package that provides up to 5MSP in dual mode for improved control loop performance. The two MCU cores in the device can operate independently (for a Space-B system) or in sync when greater safety is required.

The E1 microcontroller implements A/B exchange OTA firmware upgrades, facilitating field upgrades. In addition, a Hardware Security Module (HSM) subsystem provides the highest security to the Evita media management network.

Microcomputer with a purpose

Higher switching frequencies can improve power density in OBCs, reducing weight, space, and cost. MCUs customized for OBCs do not require external DSP/DSCs and include peripherals capable of high-speed switching and diagnostics. OBCs require fast control loops involving complex calculations and tightly coupled feedback through various sensors; therefore, math accelerators and fast ADCS are essential.

Other features that are often required include high-speed comparators and support for firmware upgrades, safety, and security. Here, microcontrollers designed specifically for e-mobility, such as the Star E1, can address key aspects of OBC system design.