Digital power control, battery management converge in EVs

Henry Ford knew, as he said back in 1923, that “saving even a few pounds of vehicle weight . . . could mean that they would also go faster and consume less fuel.” That perennial wisdom is the reason that lithium battery chemistries, with their higher specific energy (joules/kg), are leading the way toward the next generation of weight-efficient, plug-in electric vehicles. 

But images of exploding lithium-ion laptop batteries are still vivid in our memories and are amplified when we consider the far greater total energy of an electric vehicle battery. That concern and others have given rise to an evolution in highly intelligent battery management systems (BMSes) that communicate with high-power battery charging systems to address such concerns as safety, cost, battery longevity, vehicle range (aka range anxiety) and overnight charging—all painful concessions for the promise of lower carbon emissions and higher fuel economy. 

As automotive OEMs define their requirements for next-generation battery management and charging systems, semiconductor companies are advancing products that anticipate those needs. This article investigates the design requirements, architecture and challenges associated with the development of a high-power (>3-kW), off-line battery charger for automotive plug-in hybrid electric vehicles (PHEVs) and demonstrates why digital power architectures are being created for such applications.

EV design environment

Electric transportation is a broad term for vehicles that use a high-voltage battery and electric motor for propulsion. The advantage of such an approach over vehicles powered solely by an internal combustion engine (ICE) is that the electric motor is far more efficient than the ICE in producing torque, particularly during acceleration. Additionally, an electric vehicle can reclaim kinetic energy during braking that otherwise would be lost as heat.

Hybrid electric vehicles (HEVs) differ from emerging PHEVs in that they use a lower-capacity battery and electric motor to assist a primary ICE during acceleration. The result of that torque blending, coupled with the capacity for regenerative braking, is improved fuel utilization and reduced carbon emissions.

Reducing emissions, however, does not fully satisfy the requirements of newer legislation for zero vehicle emissions. Hence, the PHEV has emerged as a vehicle powered entirely from clean electric grid energy.¹

The so-called series electric vehicle, unlike a parallel HEV, does not blend torque from two sources; rather, all torque for propulsion comes from a larger electric motor, typically >80 kW. In some cases, a small, performance-optimized range-extending ICE is added to address the range limitations of the pure EV battery.² The ICE acts as a generator to power the motor and charge the battery. Adding a high-voltage battery and the electric motor, whether in a PHEV or HEV, radically alters the vehicle’s electrical, mechanical and safety systems. Consequently, sophisticated and highly intelligent power electronics and battery management systems are required.