Signal and Power Isolation for Electric Vehicle Systems

By Ross Sabolcik, Vice President and General Manager, Power Products, Silicon Labs 

Automotive OEMs across the globe are announcing aggressive plans to launch new models of electric vehicles (EVs), hybrid electric vehicles (HEVs) and 48V mild hybrid electric vehicles (MHEV). Pure electric vehicles are experiencing double-digit growth rates. 48V MHEV systems are on the horizon bringing electrification to engine subsystems on standard internal combustion engines (ICE). The low cost of 48V mild-hybrid designs and their ability to retrofit existing drive trains should further accelerate the demand for power electronics for automotive applications.

As automotive designs move to electrification, high-wattage power electronics become critical components in the new electronic drivetrain and battery systems. These high-wattage electronics need to be communicated with and controlled by low-voltage digital controllers requiring electrical isolation of the controller from the power system. In these applications, galvanic isolation, usually semiconductor-based isolation, is required to allow the digital controllers to safely interface with the high-voltage systems of a modern EV.

 

EV System Overview

To be competitive with traditional ICE vehicles, the batteries used in EV/HEVs must possess very high energy storage density, near-zero self-leakage current and the ability to charge in minutes instead of hours. In addition, the battery management and associated power conversion system must be of minimal size and weight and “sip” battery current while delivering large amounts of high-efficiency power to the electric motor. Modern EV/HEV designs use modular components in the drive train and energy storage/conversion systems. EV/HEV battery management systems typically include five major circuit assemblies:

  • On-board charger (OBC): Energy storage is provided by Lithium-Ion batteries charged by an onboard charger
    consisting of an ac-to-dc converter with power factor correction and supervised by a battery management
    system. 
  • Battery management system (BMS): Battery cells are monitored and managed by the BMS to ensure high
    efficiency and safety. The BMS controls the charge, state of health, depth of discharge and conditioning of
    individual battery cells.
  • DC/DC converter: The dc/dc converter connects the high-voltage battery to the internal 12 V dc network, which
    also provides power to the accessories and bias to the local switching converters.
  • Main Inverter: The main inverter drives the electric motor and is also used for regenerative braking and returning
    unused energy to the battery.

Figure 1 illustrates these systems along with many of the other subsystems that need to be controlled or communicated within an EV. 

EV systems require robust, high-performance isolation to interface with digital controllers, allowing them to be protected from voltages that can be well over 300V. These subsystems, such as the OBC shown in Figure 2, are typically controlled over a CAN bus, which likewise needs to be isolated from other subsystems in the vehicle.

The low-voltage controllers in an EV need to send digital communication signals to other components that can be in a high-voltage subsystem over an often noisy connection due to the high currents and electrical switching. In addition, high-voltage power transistors need to be controlled by and isolated from the low-voltage controller, which also will need to measure the currents or voltages in other high-voltage sections of the system.

Other systems outside the EV, such as electric charging piles, have similar system requirements and needs for isolation. 

The following isolation components shown in Table 1 are frequently used to allow communication and control in EV systems. 

While different types of isolation technology have been deployed in electric vehicles, manufacturers are increasingly moving to modern, semiconductor-based isolation and away from older optocoupler-based solutions. These modern isolators have numerous advantages over optocouplers in demanding automotive applications including longer lifetimes, significantly improved stability over temperature and aging, faster-switching speeds and much higher noise immunity.

As automotive suppliers target wide bandgap power transistors such as gallium nitride (GaN) or silicon carbide (SiC) to meet ever-increasing power densities, the advantages of semiconductor-based isolation become critical. These GaN or SiC systems will often use higher switching speeds to reduce the size of the system magnetics, which can result in significantly higher electrical noise. Semiconductor isolation is ideal for dealing with these higher speeds and noisier environments.

Shrinking the size and increasing the power density of these systems will drive operating temperatures higher, which can stress optocouplers and reduce their performance. Semiconductor-based isolation has significantly better performance and reliability over these higher temperature ranges, making them an ideal choice for automotive EV designs. 

 

Onboard Charger (OBC) Overview

The OBC system (see simplified diagram in Figure 2) is responsible for converting a standard ac charging source into
a dc voltage used to charge the battery pack in the vehicle. In addition, the OBC performs other key functions such as
voltage monitoring and protection. 

The OBC system takes the ac input source, converts it to a high-voltage dc bus voltage through the full-wave rectifier and provides power factor correction (PFC). The resulting dc signal is chopped into a switched square wave that is used to drive a transformer to create the required output dc voltage. The chopping of the input signal is accomplished using isolated gate drivers such as Silicon Labs’ Si8239x device.

The output voltage can be filtered to the final dc voltage using sync field-effect transistors (FETs) under the control of isolated gate drivers. The output voltage can be monitored to provide closed-loop feedback to the system controller using isolated analog sensors such as Silicon Labs’ Si892x device.

The entire system can be monitored and controlled through an isolated CAN bus. The CAN bus is isolated with digital isolators with integrated dc/dc power converters such as Silicon Labs’ Si86xx and Si88xx isolators. 

 

Battery Management Systems (BMS) and CAN Bus

The simplified BMS system (shown in Figure 3) highlights the importance of signal and power isolation for interfacing with one of the EV subsystems. In most EV subsystems, the CAN bus is isolated from the high voltages in that subsystem through digital isolation. Modern digital isolation requires a power supply on both sides of the isolator (the high-voltage domain and the low-voltage domain). This power supply can also be used to power other devices attached to the isolator such as a CAN bus transceiver. 

The high-voltage domain shown in Figure 3 is the side with the battery pack, and the low-voltage domain is the side with the CAN transceiver. This example focuses on the CAN bus interface, and it’s likely there is additional isolation between the microcontroller (MCU) and the battery pack itself. 

By using a total isolation solution that includes integrated dc-dc conversion, developers can reduce the size and complexity of their overall system designs. These isolated solutions with integrated power can be used in numerous subsystems in the vehicle that include a CAN bus transceiver. 

 

Traction Motor Systems

Delivering power to the wheels is the final stage of an EV, which requires several critical isolated components to be incorporated into the design. A traction motor drive system is required to take the high-voltage dc output of the battery and drive the traction motor. The traction motor in most electric vehicles will be an ac induction motor. To drive the motor, the traction motor controller must synthesize a variable ac waveform from the high-voltage dc rail from the battery pack.

These systems require isolated drivers between the motor controller and the power transistors. The isolation allows the low-voltage controller to safely switch the high-power transistors to create the ac waveform. In addition, there is likely an isolated CAN bus in the motor control system and some method to sense the current being driven to the motor for monitoring and controlling speed and torque. 

Figure 4 illustrates a simplified traction motor control system using a variety of digital isolation devices: 

Other Automotive Isolation Considerations

Automotive electronics are required to meet more stringent testing and quality standards than industrial devices. Most automotive customers require the more stringent AECQ-100 qualification, ISO/TS16949 audit compliance, extended operating temperatures ranges (-40 oC to +125 oC) and extremely low defect rates.

These enhanced requirements mean automotive electronics suppliers need to take additional steps to ensure their components can meet their customer’s needs. Additional quality controls at the wafer fab, device packaging and final assembly are required.

To provide a true automotive-grade device, these enhanced device parameters also must be supported by quality systems and documentation such as the Part Production Approval Process (PPAP), International Material Data Systems (IMDS) and China Automotive Material Data Systems (CAMDS).

The race to electrify automotive fleets is accelerating with more vehicles arriving from more manufacturers every year. This increase in the number and type of EVs creates opportunities for electronic suppliers to grow their device footprint in the vehicle’s power electronics systems. The high voltages and noisy environments in these drive systems require robust, high-performance galvanic isolation to ensure safe and reliable operation. Ever-increasing power densities from raising the wattages and shrinking the sizes of EV subsystems create demanding thermal and electrical noise conditions. Semiconductor-based isolation offers significant advantages over legacy optocoupler solutions, which make them an ideal choice in these demanding EV applications.

Automotive customers demand wider operating temperatures, higher quality, and more stringent documentation and systems than industrial customers. Electronics suppliers that can meet all of these demands are poised to ride the coming EV wave. 

Author Bio

Ross Sabolcik is the Vice President and General Manager of Silicon Labs’ power products, overseeing the company’s digital isolator, isolated gate driver, isolated FET driver, current sensor, and Power over Ethernet (PoE) products. These products are used in a wide range of industrial, green energy, automotive, and consumer electronics applications. Mr. Sabolcik joined Silicon Labs in 1999 and has served as director of marketing for the company’s sub-GHz wireless products; director of applications and systems engineering for the company’s broadcast products; director of applications engineering for wireline products; and as an applications engineering manager for wireline products. Prior to Silicon Labs, Mr. Sabolcik worked at National Instruments, managing product development for embedded software, as well as board-level hardware design for precision PC instrumentation. He holds a master’s degree in computer and systems engineering from Rensselaer Polytechnic Institute and a BSEE from Penn State University. 

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