Digital Power Control and Battery Management Strategies in Electric Vehicles

As Henry Ford observed in 1923, “Saving even a few pounds on automobiles…means they go faster and use less fuel.” This timeless truth is exactly why the lithium-ion battery chemistry industry is leading the world toward the next generation of plug-in electric vehicles.

However, the memory of exploding lithium-ion laptop batteries is still fresh, and the impression is further magnified when considering the greater total energy of electric vehicle batteries. These concerns and other factors have led to the development of highly intelligent battery management systems (BMS). Such BMSs need to communicate with high-power battery charging systems to meet requirements such as safety, cost, battery life, vehicle range, and overnight charging, which is a compromise to achieve lower carbon emissions and higher fuel economy.

As automotive manufacturers define requirements for next-generation battery management and charging systems, semiconductor companies are advancing the development of products that are expected to meet these requirements. This article will discuss the design requirements, architectures, and challenges associated with the development of high-power (>3kW), offline battery chargers in plug-in hybrid electric vehicles (PHEVs), and provide examples of why digital power architectures were created for these applications.

Electric Vehicle Design Environment

Electric vehicles are a broad term for vehicles that use high-voltage batteries and electric motors for propulsion. The advantage of this technology over vehicles powered solely by an internal combustion engine (ICE) is that electric motors are much more efficient than ICEs at producing torque, especially during acceleration. In addition, electric vehicles can recover kinetic energy during braking, which other vehicles would only lose as heat.

Hybrid electric vehicles (HEVs) differ from the emerging PHEVs in that they use a lower capacity battery and an electric motor to assist the main ICE in acceleration. This hybrid torque coupled with regenerative braking capabilities can further improve fuel efficiency and reduce carbon emissions.

However, the reduction in emissions is not enough to fully meet the latest legal requirements for zero-emission vehicles. Therefore, the power of PHEV, an emerging type of vehicle, comes entirely from clean grid energy.

So-called series electric vehicles, unlike parallel HEVs, do not mix torque from two sources. All propulsion torque comes from a larger electric motor, typically greater than 80kW. In some cases, a small, performance-optimized range ICE is added to address the range limitations of pure electric vehicle batteries. The ICE acts as a generator to power the electric motor and charge the battery. Whether in a PHEV or HEV, the addition of a high-voltage battery and electric motor fundamentally changes the electrical, mechanical and safety systems of the vehicle. Therefore, complex and highly intelligent power electronics and battery management systems are ultimately required.

Battery Design Challenges

Engineers have perfected gasoline propulsion over the past 100 years or so. Now, OEMs and their suppliers are changing their ways, forming alliances, and concentrating on optimizing electric propulsion.

The high cost of electric propulsion systems is reflected in product development and component complexity, requiring complex and fault-tolerant automotive intelligence and power electronics systems to continuously manage tens of kilowatts of power.

In a traditional gasoline-powered car, measuring the fuel level is a simple task. Depending on the specific car, the fuel gauge may be just a bimetallic strip driven by a heating coil connected to a sending component. In an electric vehicle, the "tank" is a high-voltage battery composed of many battery cells (possibly 100 or more) connected in series/parallel. Accurate determination of the state of charge (SOC) requires accurate voltage measurement (within a few millivolts) of each cell.

This is the job of the battery management system. The BMS is a highly accurate system that reports detailed information about the voltage, current, and temperature of the battery cells to a central processor, which is then responsible for calculating the battery's SOC. Failure to accurately measure the battery can not only misreport the battery SOC, but also shorten the battery's service life or create unsafe, potentially catastrophic situations.

To avoid this, the industry has developed ICs that meet emerging standards such as ISO26262, which ensure reliable system operation through hardware built-in test functions and N+1 redundant protection for safety-critical functions such as overvoltage/undervoltage monitoring of battery cells. If a cell in a battery pack is forced into a deep discharge state or is overcharged, the cell may be permanently damaged and may enter thermal runaway - a self-destructive state. Therefore, secondary protection is required in addition to the primary battery monitoring system.

More advanced BMS are able to synchronize voltage and current measurements as a means of continuously measuring the battery impedance, an important indicator of the battery's state of health (SOH).

Figure 1: Battery management system for high battery count applications.

Figure 1 shows a typical battery cell configuration and BMS that is sufficient to measure battery SOC and SOH. Note that any single cell in a series stack will limit the capacity of the entire stack. In other words, if a cell reaches its maximum or minimum voltage before the others, the charge or discharge cycle must be interrupted. Cell balancing circuits (shown in green) are used to ensure that all cells are charged and discharged evenly and uniformly.

Battery Charger Basics

Electric vehicle chargers are classified based on output power/input voltage. Class 1 chargers are usually integrated on a circuit board, with an input voltage of 95V to 265V AC and a charging capacity between 1.5kW and 3.3kW. Dedicated Class 2 and Class 3 chargers work on 240V/480V wiring systems and can complete charging at a much faster rate, but are limited to the constraints of the vehicle battery and connectors. For example, SAE J1772 is currently the only approved electric vehicle connector standard in North America, with a power limit of less than 16.8kW.

Unlike batteries used in portable electronic devices, automotive-grade batteries can accommodate much larger charging currents without affecting battery life or approaching thermal runaway. The charger rating (C) is defined as the current flowing into the battery, proportional to the battery capacity measured in ampere-hours (Ah). For example, a 1C charger charges a 1Ah battery at 1A.

While traditional lithium-ion batteries may be limited to 1C, some car batteries can be charged at currents well above this limit, reducing recharge time. In fact, high-power Class 3 chargers operating at 480V/three-phase can charge an EV battery in about the same time as it takes to fill up a tank of gas.

Note that EV battery capacity is generally expressed in kilowatt-hours, which can be loosely related to the battery's ampere-hour rating by dividing the kilowatt-hour rating by the nominal battery flat voltage. For example, charging a 24 KWh battery from 10% to full takes 8 hours on a 3.3kW charger integrated into a Nissan LEAF EV.

It is also important to note that the depth of discharge of electric vehicle batteries affects the life of the battery cells, so such batteries usually need to retain at least 10% of the battery capacity at the beginning of a charging cycle.

Charger Architecture Design

The onboard charger must meet stringent industrial and government regulations for electromagnetic compatibility, power factor, and UL/IEC safety standards. Like all other lithium chemistry industries, electric vehicle propulsion battery chargers use a constant current, constant voltage (CC/CV) charging algorithm, where the battery is first charged by a programmable current source until it reaches the voltage set point, then it switches to the voltage regulation phase while monitoring the battery current as an indication of the completion of the charging cycle.

The charging current (power) is negotiated between the BMS, hybrid control module (HCM), and EV service equipment, depending on the input voltage used, temperature, and battery SOC/SOH, as well as other system considerations monitored by the HCM. The safety and fault tolerance of this control algorithm cannot be compromised.

A suitable power supply architecture involves interleaved power factor correction (PFC) followed by a phase-shifted full-bridge circuit, as shown in Figure 2. The control feedback parameters are digitized by a microcontroller. This microcontroller is able to digitally close multiple control loops and precisely modulate the high-voltage MOSFET switches. The centralized and highly intelligent control mechanism can address many issues that are not easily solved by analog technology.

Figure 2: Digital control interface for interleaved PFC and phase-shifted bridges.

More advanced microcontrollers integrate coprocessors (control law accelerators (CLA)) to accelerate the calculation of the control loop transfer function and multiple high-resolution pulse width modulators (PWMs) to control the power switches within 150ps. This architecture can dynamically adapt to line and load changes, record system operating parameter data, and implement forward-looking error-free algorithms while intelligently connecting all other automotive subsystems through a ground-isolated control area network.

Recent developments in digital power make this approach more feasible, cost-effective, scalable, and better suited for high-power multi-phase applications in electric vehicles.

A large, scalable, modular software library for digital compensation and every possible power topology can be integrated by an experienced software designer; test reports comparing digital and analog power solutions are also available. For example, consider the two-phase interleaved PFC function shown in Figure 2. The PFC boost switch is controlled by PWM1 implementing multi-mode PFC to generate a compatible voltage for the battery charger.

The adaptability of this topology is evident in Figure 3, where the digital compensation and phase management blocks are variable under software control. Using digital technology also makes the system less susceptible to noise and temperature, while intelligently synchronizing the power stage circuits to minimize interference and optimize filter design.

Figure 3: Software modular programming of high-power PFC approach.

Figure 3 illustrates the complete code modularity of the boost PFC. Similar code construction can implement a phase-shifted bridge with zero voltage switching, minimizing converter switching losses while improving efficiency. The cascade topology can achieve charger efficiencies of more than 95%, maximize system fault tolerance, and minimize system cost.