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Drive Trains Face Unique Challenges In Electric Vehicles

April 9, 2012
Silicon Labs' Ross Sabolcik considers the energy-conversion challenges in the Gen-III Prius and the Tesla Roadster and speculates about future vehicles.

I’m a car guy. I really enjoy classic car shows that revisit the automotive history. The variety of vehicles in the early days of the automobile industry is fascinating. From drive train technologies to methods used to control acceleration and steering, car concepts were wide open back then.

Recently I was watching a show about Jay Leno’s garage and his extensive car collection. It was cool to see how many drive train variants were explored for early automobiles. Gasoline, steam, and electric were all contenders to power the automobile in its earliest configurations. The Stanley Steamer, a steam-powered car, was particularly fascinating.

While steam may seem like an odd way to power a car to us today, steam locomotives were common throughout the 19th century, and the Stanley Steamer remained in production from 1902 until 1924. While there were many potential challengers, the internal combustion engine ultimately became the dominant drive train for the past 100 years.

The Next 100 Years?

This is an exciting time for the automotive industry. The methods for powering a car are once again being challenged, revisited, and redesigned. The excellent book by the Rocky Mountain Institute entitled Winning the Oil Endgame provides an analysis of how much of the energy in the gasoline consumed by an internal combustion engine is actually used to propel the car. Only an eighth of the fuel energy reaches the wheels, a sixteenth accelerates the car, and less than 1% ends up moving the driver.

Most major automakers are now widely deploying electric drive trains in a variety of configurations to improve those efficiency numbers. These drive trains offer some well-publicized benefits over internal combustion engines: high power-to-weight ratios, peak torque when the motor is stalled for excellent acceleration, regenerative braking technology, and fewer moving parts, which should result in higher efficiency and reliability.

The challenges associated with the battery technologies used in electric vehicles (EVs) and hybrid electric vehicles (HEVs) have been thoroughly explored. But another challenging aspect of EV/HEV design that merits close attention is the high-voltage drive system that must convert the battery voltage into a precisely controlled output to the electric motors.

Electric drive trains operate at very high voltage levels. Toyota’s third-generation Prius uses a battery pack with 28 nickel-metal-hydride (NiMH) 7.2-V batteries connected in series to create a dc voltage level of 201.6 V that is further boosted to more than 600 V. The Tesla Roadster uses 6831 lithium-ion (Li-ion) batteries in its battery pack with an operating voltage of 425 V dc.

The inverter stage in both vehicles must create a three-phase ac output to power the ac induction motors. The electric drive train circuitry must be able to handle these voltages. The other low-voltage electronics in the vehicle must be protected to prevent damage. Most importantly, the operator must be shielded from these potentially lethal voltages. Isolation is a key component to achieve these goals.

The motors are controlled via the use of high-voltage insulated gate bipolar transistors (IGBTs). In the Tesla Roadster, these IGBTs must be able to handle peak currents of 900 A to feed the ac induction motors. The IGBTs in turn need robust gate drivers to control their operation, and they’re often electrically isolated to provide protection and noise immunity.

The IGBT drivers need to provide a high gate drive current to turn on the transistors quickly and accurately, have low latency to ensure the highest efficiency, and provide high electrical isolation to protect the other drive electronics. The ability to reduce the latency and mismatch between drivers can help improve efficiency by minimizing the time that the transistors must be turned off to prevent shoot through.

In addition to controlling the high voltages and currents for the drive train, the electronics must be able to withstand the noise created by the electric drive train. The noise from the high-voltage drive systems can wreak havoc on the communication buses that interconnect the battery management system (BMS) and the numerous other subsystems. The BMS needs to be isolated to ensure robust communication performance. In some implementations the various sub-modules within the battery pack require isolation to reduce noise interference on the communication link.

Often times the entire vehicle controller area network (CAN) bus is electrically isolated to protect against interference from the electric drive train. The increased channel density and tight timing metrics of digital isolators are useful for reducing the size of the communication subsystems and ensuring accurate timing between bus lines.

The Next Drive Train While the battle for the next-generation, mass-production automotive drive train still hasn’t been decided, the future of the automobile certainly is once again up in the air. Looking 20 to 30 years ahead, it is fascinating to consider which car architecture will be parked in your garage and which will be resigned to a nostalgic car show parked next to the Stanley Steamer.

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