On-board charger design’s call for purpose-built MCUs

Article By : John Johnson and Sachin Gupta, STMicroelectronics

An MCU tailored for OBCs eliminates the need for external DSP/DSCs and includes peripherals capable of high-speed switching and diagnostics.

Addressing “range anxiety” is vital for engineers focused on electric vehicles (EVs). Set by the range and fueling experience of internal combustion engine (ICE) vehicles, consumers’ expectations are hard to change.

Battery capacity is one consideration. It is increasing in both size and voltage as designers strive to optimize range through expanded energy storage capacity and incremental improvements in efficiency. The size and weight of vehicle electronics, particularly wiring harnesses, are also a target for optimization. These factors have a significant impact on vehicle range per charge; however, they are a double-edged sword. A bigger battery takes longer to charge; and parking at a charging station for 4 hours on a cross-country trip is a non-starter.

Higher DC-link voltage requires employing different technologies for energy conversion; and vehicle modules must exhibit cutting-edge performance that is safe as per ISO 26262 and reliable. Additionally, certain key performance indicator (KPI) objectives like improving energy density (kW/l) and specific power (kW/kg) make the design of systems like the OBC more challenging.

Figure 1 The OBC is a key part of an EV’s energy value chain. Source: STMicroelectronics

The OBC architecture

The OBC is a key part of the energy ‘value chain’ (Figure 1). The size of the battery drives the OBC’s output power rating; and its primary role is to convert energy from the power grid into the direct current the battery management system (BMS) uses to charge the battery pack. The OBC must do so while complying with stringent emissions requirements and meeting its KPIs.

Figure 2 Power designers employ different OBC architectures in EVs. Source: STMicroelectronics

Designers employ different architectures to achieve their goals (Figure 2). They choose among the approaches based on several objectives, including the nature of the incoming power (number of phases), cost/efficiency targets, and whether the design needs to support vehicle to grid (V2G) energy transfers, requiring a bi-directional architecture. On the other hand, module volume and weight are largely dictated by discrete components such as capacitors, inductors, and transformers (Figure 3). These components limit energy-density performance.

Figure 3 The above example shows a SiC-based OBC reference design. Source: STMicroelectronics

The emergence of higher voltages in 800-V or higher EVs drives the use of wide bandgap (WBG) semiconductor technologies in energy conversion applications; especially those connected to the DC-link bus, including OBC, BMS and traction inverter. For the OBC, silicon carbide (SiC) or gallium nitride (GaN) are emerging as the technologies of choice to support higher voltage and power ratings.

SiC is ideal as it supports efficient operation at very high voltages and temperatures. It also reduces cost and size since it requires a less bulky and inexpensive cooling apparatus. SiC and GaN support higher switching frequencies than silicon and when combined with faster control loops, the WBG devices can significantly shrink space requirements for the discrete components shown in Figure 3. Next, advanced microcontroller architectures with enhanced digital-control capabilities enable support for faster switching and control loops, thereby delivering levels of integration that help achieve design objectives like energy density and cost.

Shortcomings of conventional MCUs

Of course, EV systems present unique challenges that must be addressed by tailored solutions. This is clear in the choice of the microcontroller. Conventional automotive MCUs, such as those designed for the powertrain in an ICE vehicle weren’t designed for the essential digital, analog, and system-level capabilities needed to support electrification design requirements. For example, most traditional automotive MCUs can’t support the high switching frequencies to reap the benefits of WBG technologies.

Figure 4 The conventional MCUs weren’t designed to support the higher switching frequencies of WBG transistors. Source: STMicroelectronics

Many of these traditional automotive MCUs support PWM switching frequencies of less than 150 kHz and lack the PWM resolution to take advantage of the WBG technologies critical in OBCs for power factor correction (PFC) and DC-DC converter stages. For example, some 200-MHz MCUs provide timers/PWMs with an input clock as low as 80 MHz. In this case, if the required PWM frequency is 150 kHz, the MCU will support only 9-bit PWM resolution.

For the OBC, this capability is unsuitable for silicon MOSFET-based implementations, let alone WBG devices. While Figure 4 emphasizes the importance of switching frequency, PWM resolution is also an important aspect, as it largely determines the timing at which the switches are activated/deactivated based on input parameters measured by the analog-to-digital converters (ADCs).

To realize the full potential of SiC/GaN devices, the design must optimize the control loop. That requires faster PWMs with high resolution, precise dead-time control, faster ADCs, and faster computing to reduce control-loop timing. Furthermore, ADC samples should be synchronized with PWM output control. So, the capabilities of the MCU have a major impact on OBC weight, footprint, and cost. Figure 5 shows a high-level block diagram of an OBC using a traditional MCU. This system employs an external DSP for the control loop and external comparators for protection.

Figure 5 The block diagram shows a typical OBC system using a traditional MCU. Source: STMicroelectronics

In a typical PFC or DC-DC control loop, the MCU measures voltage and current. Next, the MCU and DSP run an algorithm on these measured values, and then control the duty cycle of PWMs. Control-loop timing depends upon:

  • Voltage/current sampling rate
  • Compute throughput
  • Reaction time

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

While silicon MOSFETs require longer dead times to minimize switching losses, SiC/GaN allow shorter dead times. Short dead time increases the power that can be transferred from input to output in a cycle. Most conventional MCUs cannot support these small dead-times.

OBCs must include protection against over-current, over-voltage and over-temperature conditions. Typically, analog comparators are used to detect these faults and control the output as fast as possible to avoid damage. These comparators need very fast response times. MCUs not purpose-built for these applications either may not have the comparators, or their response time is too high, making them unsuitable to implement protection in OBC. Even if external comparators are used to implement protection mechanism, they need digital-to-analog converters (DACs) to generate reference and most MCUs typically do not have any or enough external DACs. Moreover, using external comparators increases solution footprint size and cost.

Beyond control loop mechanisms

Beyond control-loop and protection mechanisms, other aspects should be carefully examined.

  • Over-the-air (OTA) firmware upgrade support
  • Functional safety (ISO 26262)
  • Security

Automotive design cycles are accelerating, and OEMs must continuously deliver new features to keep pace with competition; thus, vehicles are becoming ‘software defined’. This enables the monetization of firmware-enabled features. These aspects require support for firmware upgrades post sales; so, the MCU must support OTA updates.

Automotive designs also require functional safety. Though each OBC’s design requirement may differ, in most cases, systems must support ASIL-B through ASIL-D. Not all MCUs support lockstep cores while others prohibit the use of independent execution. The ability for the designer to choose lockstep or independent execution of the cores affords greater flexibility to support various safety integrity levels. This allows for designs to be optimized for cost and scalability.

And, with connected cars, there is a greater risk of cyberattacks. Therefore, the OBC may require Evita Lite or Evita Medium security to deal with such threats. This security is especially important for vehicles connected to the power grid.

To facilitate electrification, some MCU vendors offer devices that address these new requirements. An example is Stellar E1 (SR5E1), which integrates standard MCU and DSP functionality into a single device to offer a single-chip solution for OBCs. Figure 6 shows a very high-level block diagram of an OBC implementation.

Figure 6 The high-level block diagram shows a 3-phase bi-directional OBC using Stellar E1 MCU. Source: STMicroelectronics

The Stellar E1 is an AEC-Q100 qualified MCU, which includes 2x Arm Cortex-M7 cores, so one core can be used for a PFC loop and one for a DC-DC stage in a bi-directional OBC implementation. To support fast control loops, the Stellar E1 contains a CORDIC math accelerator. The MCU contains 12 high-resolution timers with 104-ps resolution to support greater than 1-MHz PWM switching frequency with precise dead-time control. Coupled with fast compute capability, the high-resolution timers replace an external DSP.

These devices also include on-chip fast comparators to implement protection. Additionally, they offer 2.5 MSPS 12-bit SAR ADCs delivering up to 5 MSPS in dual mode that can boost control-loop performance. Two MCUs cores in the device can run independently (for an ASIL-B system) or can run in lockstep mode if higher safety is needed.

The Stellar E1 microcontroller implements A/B swap-OTA firmware upgrades that facilitate field upgradeability. Moreover, a hardware security module (HSM) sub-system offers security up to EVITA medium to manage cybersecurity.

Purpose-built MCUs

Higher switching frequency delivers improved power density in OBCs, reducing weight, space, and cost. An MCU tailored for OBCs eliminates the need for external DSP/DSCs and includes peripherals capable of high-speed switching and diagnostics. OBCs demand fast control loops that involve complex calculations and tightly coupled feedback via various sensors; therefore, math accelerators and fast ADCs are essential.

Other features often needed include high-speed comparators as well as support for firmware upgrades, safety, and security. Here, purpose-built MCUs for e-mobility such as Stellar E1 can address key pain points for OBC system design.

 

This article was originally published on EDN.

John Johnson manages the Automotive Systems Marketing Group at STMicroelectronics.

Sachin Gupta is product marketing leader for automotive MCUs in the Automotive and Discreate Group (ADG) at STMicroelectronics.

 

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