Addressing “vary nervousness” is important for engineers centered on electrical automobiles (EVs). Set by the vary and fueling expertise of inside combustion engine (ICE) automobiles, shoppers’ expectations are arduous to alter.
Battery capability is one consideration. It’s growing in each measurement and voltage as designers try to optimize vary via expanded vitality storage capability and incremental enhancements in effectivity. The dimensions and weight of car electronics, significantly wiring harnesses, are additionally a goal for optimization. These elements have a big affect on automobile vary per cost; nonetheless, they’re a double-edged sword. A much bigger battery takes longer to cost; and parking at a charging station for 4 hours on a cross-country journey is a non-starter.
Greater 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) goals like bettering vitality density (kW/l) and particular energy (kW/kg) make the design of programs 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 ranking; and its major position is to transform vitality from the facility grid into the direct present the battery administration system (BMS) makes use of to cost the battery pack. The OBC should accomplish that 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 objectives (Determine 2). They select among the many approaches primarily based on a number of goals, together with the character of the incoming energy (variety of phases), value/effectivity targets, and whether or not the design must help automobile to grid (V2G) vitality transfers, requiring a bi-directional structure. Then again, module quantity and weight are largely dictated by discrete parts comparable to capacitors, inductors, and transformers (Determine 3). These parts restrict energy-density efficiency.
Determine 3 The above instance reveals a SiC-based OBC reference design. Supply: STMicroelectronics
The emergence of upper voltages in 800-V or increased EVs drives the usage of large bandgap (WBG) semiconductor applied sciences in vitality conversion functions; particularly these related 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 help 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 measurement because it requires a much less cumbersome and cheap cooling equipment. SiC and GaN help increased switching frequencies than silicon and when mixed with sooner management loops, the WBG units can considerably shrink area necessities for the discrete parts proven in Determine 3. Subsequent, superior microcontroller architectures with enhanced digital-control capabilities allow help for sooner switching and management loops, thereby delivering ranges of integration that assist obtain design goals like vitality density and value.
Shortcomings of typical MCUs
In fact, EV programs current distinctive challenges that should be addressed by tailor-made options. That is clear within the selection of the microcontroller. Typical automotive MCUs, comparable to these designed for the powertrain in an ICE automobile weren’t designed for the important digital, analog, and system-level capabilities wanted to help electrification design necessities. For instance, most conventional automotive MCUs can’t help the excessive switching frequencies to reap the advantages of WBG applied sciences.
Determine 4 The traditional MCUs weren’t designed to help the upper switching frequencies of WBG transistors. Supply: STMicroelectronics
Many of those conventional automotive MCUs help PWM switching frequencies of lower than 150 kHz and lack the PWM decision to reap the benefits of the WBG applied sciences vital in OBCs for energy issue correction (PFC) and DC-DC converter phases. 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 help 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 also be an necessary side, 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 cut back control-loop timing. Moreover, ADC samples must be synchronized with PWM output management. So, the capabilities of the MCU have a serious affect on OBC weight, footprint, and value. Determine 5 reveals a high-level block diagram of an OBC utilizing a standard MCU. This technique employs an exterior DSP for the management loop and exterior comparators for cover.
Determine 5 The block diagram reveals 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 fee
- 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 phases doable within the machine. For instance, parallel output phases are used to extend output energy; and this configuration requires sampling present and voltage on each phases concurrently. This requires 4 ADC situations; so, not solely are the variety of channels necessary but in addition the variety of situations.
Whereas silicon MOSFETs require longer lifeless instances to attenuate switching losses, SiC/GaN enable shorter lifeless instances. Brief lifeless time will increase the facility that may be transferred from enter to output in a cycle. Most typical MCUs can not help these small dead-times.
OBCs should embody safety towards over-current, over-voltage and over-temperature circumstances. Usually, analog comparators are used to detect these faults and management the output as quick as doable to keep away from injury. These comparators want very quick response instances. MCUs not purpose-built for these functions both could not have the comparators, or their response time is simply 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 should not have any or sufficient exterior DACs. Furthermore, utilizing exterior comparators will increase resolution footprint measurement and value.
Past management loop mechanisms
Past control-loop and safety mechanisms, different points must be fastidiously examined.
- Over-the-air (OTA) firmware improve help
- Practical security (ISO 26262)
- Safety
Automotive design cycles are accelerating, and OEMs should constantly ship new options to maintain tempo with competitors; thus, automobiles have gotten ‘software program outlined’. This allows the monetization of firmware-enabled options. These points require help for firmware upgrades publish gross sales; so, the MCU should help OTA updates.
Automotive designs additionally require practical security. Although every OBC’s design requirement could differ, typically, programs should help ASIL-B via ASIL-D. Not all MCUs help lockstep cores whereas others prohibit the usage of unbiased execution. The power for the designer to decide on lockstep or unbiased execution of the cores affords larger flexibility to help varied security integrity ranges. This enables for designs to be optimized for value and scalability.
And, with related automobiles, there’s a larger threat of cyberattacks. Subsequently, the OBC could require Evita Lite or Evita Medium safety to take care of such threats. This safety is particularly necessary for automobiles related to the facility grid.
To facilitate electrification, some MCU distributors provide units that handle these new necessities. An instance is Stellar E1 (SR5E1), which integrates commonplace MCU and DSP performance right into a single machine to supply a single-chip resolution for OBCs. Determine 6 reveals a really high-level block diagram of an OBC implementation.
Determine 6 The high-level block diagram reveals 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 help quick management loops, the Stellar E1 comprises a CORDIC math accelerator. The MCU comprises 12 high-resolution timers with 104-ps decision to help larger than 1-MHz PWM switching frequency with exact dead-time management. Coupled with quick compute functionality, the high-resolution timers substitute 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
Greater 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 complicated 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 help for firmware upgrades, security, and safety. Right here, purpose-built MCUs for e-mobility comparable to Stellar E1 can handle key ache factors for OBC system design.
John Johnson manages the Automotive Programs Advertising Group at STMicroelectronics.
Sachin Gupta is product advertising chief for automotive MCUs within the Automotive and Discreate Group (ADG) at STMicroelectronics.
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