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Tuesday, March 21, 2023

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

Addressing “vary anxiousness” is important for engineers centered on electrical automobiles (EVs). Set by the vary and fueling expertise of inside combustion engine (ICE) automobiles, customers’ expectations are laborious to alter.

Battery capability is one consideration. It’s growing in each dimension and voltage as designers attempt to optimize vary by expanded vitality storage capability and incremental enhancements in effectivity. The dimensions and weight of auto electronics, significantly wiring harnesses, are additionally a goal for optimization. These elements have a big impression on automobile vary per cost; nonetheless, they’re a double-edged sword. An even bigger battery takes longer to cost; and parking at a charging station for 4 hours on a cross-country journey is a non-starter.

Increased DC-link voltage requires using totally different applied sciences for vitality conversion; and automobile modules should exhibit cutting-edge efficiency that’s protected as per ISO 26262 and dependable. Moreover, sure key efficiency indicator (KPI) aims like bettering vitality density (kW/l) and particular energy (kW/kg) make the design of techniques just like the OBC tougher.

Determine 1 The OBC is a key a part of an EV’s vitality worth chain. Supply: STMicroelectronics

The OBC structure

The OBC is a key a part of the vitality ‘worth chain’ (Determine 1). The dimensions of the battery drives the OBC’s output energy score; and its major function is to transform vitality from the ability grid into the direct present the battery administration system (BMS) makes use of to cost the battery pack. The OBC should achieve this whereas complying with stringent emissions necessities and assembly its KPIs.

Determine 2 Energy designers make use of totally different OBC architectures in EVs. Supply: STMicroelectronics

Designers make use of totally different architectures to realize their targets (Determine 2). They select among the many approaches primarily based on a number of aims, together with the character of the incoming energy (variety of phases), value/effectivity targets, and whether or not the design must assist automobile to grid (V2G) vitality transfers, requiring a bi-directional structure. Then again, module quantity and weight are largely dictated by discrete elements similar to capacitors, inductors, and transformers (Determine 3). These elements restrict energy-density efficiency.

Determine 3 The above instance exhibits a SiC-based OBC reference design. Supply: STMicroelectronics

The emergence of upper voltages in 800-V or increased EVs drives the usage of broad bandgap (WBG) semiconductor applied sciences in vitality conversion functions; particularly these linked to the DC-link bus, together with OBC, BMS and traction inverter. For the OBC, silicon carbide (SiC) or gallium nitride (GaN) are rising because the applied sciences of option to assist increased voltage and energy rankings.

SiC is right because it helps environment friendly operation at very excessive voltages and temperatures. It additionally reduces value and dimension because it requires a much less cumbersome and cheap cooling equipment. SiC and GaN assist increased switching frequencies than silicon and when mixed with sooner management loops, the WBG units can considerably shrink area necessities for the discrete elements proven in Determine 3. Subsequent, superior microcontroller architectures with enhanced digital-control capabilities allow assist for sooner switching and management loops, thereby delivering ranges of integration that assist obtain design aims like vitality density and value.

Shortcomings of standard MCUs

In fact, EV techniques current distinctive challenges that should be addressed by tailor-made options. That is clear within the alternative of the microcontroller. Typical automotive MCUs, similar to these designed for the powertrain in an ICE automobile weren’t designed for the important digital, analog, and system-level capabilities wanted to assist electrification design necessities. For instance, most conventional automotive MCUs can’t assist the excessive switching frequencies to reap the advantages of WBG applied sciences.

Determine 4 The standard MCUs weren’t designed to assist the upper switching frequencies of WBG transistors. Supply: STMicroelectronics

Many of those conventional automotive MCUs assist PWM switching frequencies of lower than 150 kHz and lack the PWM decision to benefit from the WBG applied sciences crucial in OBCs for energy issue correction (PFC) and DC-DC converter levels. For instance, some 200-MHz MCUs present timers/PWMs with an enter clock as little as 80 MHz. On this case, if the required PWM frequency is 150 kHz, the MCU will assist solely 9-bit PWM decision.

For the OBC, this functionality is unsuitable for silicon MOSFET-based implementations, not to mention WBG units. Whereas Determine 4 emphasizes the significance of switching frequency, PWM decision can be an necessary facet, because it largely determines the timing at which the switches are activated/deactivated primarily based on enter parameters measured by the analog-to-digital converters (ADCs).

To understand the complete potential of SiC/GaN units, the design should optimize the management loop. That requires sooner PWMs with excessive decision, exact dead-time management, sooner ADCs, and sooner computing to scale back control-loop timing. Moreover, ADC samples needs to be synchronized with PWM output management. So, the capabilities of the MCU have a serious impression on OBC weight, footprint, and value. Determine 5 exhibits a high-level block diagram of an OBC utilizing a standard MCU. This method employs an exterior DSP for the management loop and exterior comparators for cover.

Determine 5 The block diagram exhibits a typical OBC system utilizing a standard MCU. Supply: STMicroelectronics

In a typical PFC or DC-DC management loop, the MCU measures voltage and present. Subsequent, the MCU and DSP run an algorithm on these measured values, after which management the responsibility cycle of PWMs. Management-loop timing relies upon upon:

  • Voltage/present sampling charge
  • Compute throughput
  • Response time

Controlling/monitoring voltage/present within the OBC requires excessive ADC pattern charges coupled with good CPU throughput (DMIPS) augmented with math accelerators. These decide algorithm execution time. The variety of PWM channels and related decision determines the pace and accuracy of output management and the extent of integration of converter levels attainable within the machine. For instance, parallel output levels are used to extend output energy; and this configuration requires sampling present and voltage on each levels concurrently. This requires 4 ADC situations; so, not solely are the variety of channels necessary but additionally the variety of situations.

Whereas silicon MOSFETs require longer useless occasions to reduce switching losses, SiC/GaN enable shorter useless occasions. Brief useless time will increase the ability that may be transferred from enter to output in a cycle. Most standard MCUs can’t assist these small dead-times.

OBCs should embody safety towards over-current, over-voltage and over-temperature situations. Usually, analog comparators are used to detect these faults and management the output as quick as attainable to keep away from injury. These comparators want very quick response occasions. MCUs not purpose-built for these functions both might not have the comparators, or their response time is just too excessive, making them unsuitable to implement safety in OBC. Even when exterior comparators are used to implement safety mechanism, they want digital-to-analog converters (DACs) to generate reference and most MCUs usually do not need any or sufficient exterior DACs. Furthermore, utilizing exterior comparators will increase resolution footprint dimension and value.

Past management loop mechanisms

Past control-loop and safety mechanisms, different elements needs to be rigorously examined.

  • Over-the-air (OTA) firmware improve assist
  • Purposeful security (ISO 26262)
  • Safety

Automotive design cycles are accelerating, and OEMs should repeatedly ship new options to maintain tempo with competitors; thus, automobiles have gotten ‘software program outlined’. This allows the monetization of firmware-enabled options. These elements require assist for firmware upgrades publish gross sales; so, the MCU should assist OTA updates.

Automotive designs additionally require practical security. Although every OBC’s design requirement might differ, generally, techniques should assist ASIL-B by ASIL-D. Not all MCUs assist lockstep cores whereas others prohibit the usage of unbiased execution. The flexibility for the designer to decide on lockstep or unbiased execution of the cores affords larger flexibility to assist varied security integrity ranges. This enables for designs to be optimized for value and scalability.

And, with linked automobiles, there’s a larger danger of cyberattacks. Subsequently, the OBC might require Evita Lite or Evita Medium safety to take care of such threats. This safety is particularly necessary for automobiles linked to the ability grid.

To facilitate electrification, some MCU distributors supply units that deal with these new necessities. An instance is Stellar E1 (SR5E1), which integrates normal MCU and DSP performance right into a single machine to supply a single-chip resolution for OBCs. Determine 6 exhibits a really high-level block diagram of an OBC implementation.

Determine 6 The high-level block diagram exhibits a 3-phase bi-directional OBC utilizing Stellar E1 MCU. Supply: STMicroelectronics

The Stellar E1 is an AEC-Q100 certified MCU, which incorporates 2x Arm Cortex-M7 cores, so one core can be utilized for a PFC loop and one for a DC-DC stage in a bi-directional OBC implementation. To assist quick management loops, the Stellar E1 accommodates a CORDIC math accelerator. The MCU accommodates 12 high-resolution timers with 104-ps decision to assist larger than 1-MHz PWM switching frequency with exact dead-time management. Coupled with quick compute functionality, the high-resolution timers change an exterior DSP.

These units additionally embody on-chip quick comparators to implement safety. Moreover, they provide 2.5 MSPS 12-bit SAR ADCs delivering as much as 5 MSPS in twin mode that may enhance control-loop efficiency. Two MCUs cores within the machine can run independently (for an ASIL-B system) or can run in lockstep mode if increased security is required.

The Stellar E1 microcontroller implements A/B swap-OTA firmware upgrades that facilitate subject upgradeability. Furthermore, a {hardware} safety module (HSM) sub-system affords safety as much as EVITA medium to handle cybersecurity.

Function-built MCUs

Increased switching frequency delivers improved energy density in OBCs, lowering weight, area, and value. An MCU tailor-made for OBCs eliminates the necessity for exterior DSP/DSCs and contains peripherals able to high-speed switching and diagnostics. OBCs demand quick management loops that contain advanced calculations and tightly coupled suggestions through varied sensors; due to this fact, math accelerators and quick ADCs are important.

Different options usually wanted embody high-speed comparators in addition to assist for firmware upgrades, security, and safety. Right here, purpose-built MCUs for e-mobility similar to Stellar E1 can deal with key ache factors for OBC system design.

John Johnson manages the Automotive Programs Advertising and marketing Group at STMicroelectronics.

Sachin Gupta is product advertising and marketing chief for automotive MCUs within the Automotive and Discreate Group (ADG) at STMicroelectronics.

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