Advanced Power Electronics Deliver Needed Voltage Boost to EVs

Texas InstrumentsBy Paul Pickering, Contributing Editor
[Sponsored by Texas Instruments]

The first “horseless carriages” had no use for electricity—they used kerosene lamps for lighting and a naked flame to ignite the mixture. Luckily for generations of electrical engineers, that situation didn’t last long: In the 1912 model year, Cadillac introduced the first vehicle with a battery, starter motor, and generator. Electricity and electronics have played an increasingly important role in automobile development ever since.

Those early systems used a 6-V bus voltage, but 12 V soon became the standard because it provided greater power with less current. An alternator-charged 12-V lead-acid battery has delivered system power for decades. As the number of electronic modules grew, the increasing load on the 12-V system led to unsuccessful proposals for a higher system voltage, including an abortive 42-V standard that dates back to the late 1990s.

Systems based on 12-V technology have been steadily proliferating. However, the rise of hybrid and pure electric vehicles in recent years has greatly expanded the opportunities for power-electronic designers.

Table - The rise in electronic content leads to increased fuel savings, but also elevates system bus voltage. (Source: TI “Driving the green revolution in transportation” PDF)

Table
The rise in electronic content leads to increased fuel savings, but also elevates system bus voltage. (Source: TI “Driving the green revolution in transportation” PDF)

Automotive Apps for High-Voltage Power Electronics

What are some the key applications for high-voltage power electronics? The table lists the main categories of vehicles with “new-generation” electric technology and the expected reduction in fuel consumption for each entry.

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The minimum electronically assisted change to a vehicle powered by an internal combustion engine (ICE) is start/stop—stopping the engine during prolonged idling, and then starting it up again when the driver presses the gas pedal. This modification reduces fuel consumption, but requires a more robust battery and starter motor.

From there, hybridization becomes more extensive, culminating in the pure electric vehicle. Each stage of technology replaces more mechanical functions with their electrical equivalents.

Increasing bus voltage is another characteristic feature. Start/stop and micro-hybrid vehicles can use the existing 12 V; as electric motors replace more mechanical or hydraulic functions, the electrical system consumes more power and bus voltage begins to increase. For example, the Tesla Model S high-performance EV uses a 100-kWh Li-ion battery pack with a nominal output of 375 V.

The HEV/EV includes numerous flavors of high-voltage converters

Figure 1
The HEV/EV includes numerous flavors of high-voltage converters. (Source: TI “How to Design Multi-kW DC/DC Converters for Electric Vehicles (EVs) – EV System Overview”)

Figure 1 summarizes the many applications for high-voltage power electronics in a hybrid or electric vehicle. Of course, the modules used depend on the type of vehicle. Not every vehicle will contain every option, but there are potentially three different dc buses: the primary HV bus, 48 V, and 12 V.

A wide variety of dc-dc converters converts between voltages. Depending on the application, options include both isolated and non-isolated topologies, and unidirectional and bidirectional power flows. At higher levels of electrification, inverters convert dc to ac for pumps, compressors, and the traction motor itself. And finally, onboard or offboard chargers convert single- or multi-phase ac power to dc as needed.

What applications target these different converters?

DC-DC Conversion

The most common class of power circuit is the dc-dc converter. Although full hybrid or electric vehicles have migrated to a 48-V system for most non-traction applications and no longer include a 12-V battery, that doesn’t mean 12-V systems have disappeared. It doesn’t make economic sense to redesign all of the legacy systems to operate from higher voltages, so such systems will employ a dual 12-V/48-V architecture: Traditional systems will use 12 V, and the new power-hungry applications will operate from the 48-V bus.

A multi-rail isolated dc-dc converter also provides bias voltage for the traction-motor power stage. Of course, multiple point-of-load buck converters and even linear regulators reside in numerous applications throughout the vehicle, too.

DC-AC Conversion

The primary application for dc-ac converters is to drive electric motors. The HV bus powers the drive system that provides traction to the wheels. Hybrid and most pure electric vehicles use brushless dc (BLDC) motors for this purpose; Tesla Motors, with its ac induction motor, is one of the few exceptions.

The EV traction-motor drive circuit includes both low-voltage microcontrollers and high-voltage power transistors. (Source: TI “Driving the future of HEV/EV with high-voltage solutions” PDF, p. 5)

Figure 2
The EV traction-motor drive circuit includes both low-voltage microcontrollers and high-voltage power transistors. (Source: TI “Driving the future of HEV/EV with high-voltage solutions” PDF, p. 5)

Figure 2 shows a diagram of a typical EV traction-motor drive circuit. A microcontroller implements a high-power three-phase inverter by generating the PWM signals that regulate the traction-motor speed and torque. Other lower-power inverters use the HV or 48-V bus to drive BLDC motors for compressors, water pumps, cooling fans, and other accessories.

AC-DC Conversion

The primary application for an ac-dc converter involves charging the vehicle battery from an offboard source. SAE 1772 governs the allowable power levels and communication protocols for vehicle charging in the U.S.; Europe and Japan have similar standards. J1772 Level 1 charging uses a single-phase 120-V ac source, such as a standard residential outlet, to supply up to 1.9 kW; Level 2 employs split-phase 240 V ac with up to 19.4 kW. Level 3, the highest power, is still being finalized, but some manufacturers, notably Tesla, already offer their own Level 3 “supercharging stations” with a proprietary format.

A typical onboard charger (Fig. 3) comprises a front-end ac-dc stage with an active power-factor-correction (PFC) circuit and a dc-dc stage with a regulated voltage, based on the battery specification, to charge the battery.

Maximizing the efficiency of the onboard charger (OBC) reduces battery-charging time, which is a critical concern for EV owners. (Source: TI “Driving the future of HEV/EV with high-voltage solutions” PDF, p. 3)

Figure 3
Maximizing the efficiency of the onboard charger (OBC) reduces battery-charging time, which is a critical concern for EV owners. (Source: TI “Driving the future of HEV/EV with high-voltage solutions” PDF, p. 3)

Since a vehicle charger handles multi-kilowatt power levels, it’s important to pick topologies that reduce losses and maximize efficiency. An interleaved boost converter is a popular choice for the PFC, because it combines high power density, low ripple current, low losses, and small inductor sizes for efficient filtering.

A phase-shifted full-bridge (PSFB) converter, such as TI’s UCC28951-Q1, is often used for the secondary dc-dc stage. The PSFB uses four primary-side switches and the control loop is quite complex, but it is well-suited to high-power applications with a large variation in output voltage. The inductor-inductor-capacitor (LLC) PSFB topology can maximize efficiency with its zero-current switching.

Meeting the HEV/EV Challenge

What challenges do these higher voltage and power levels present to car designers and electronics suppliers?

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Safety is the top priority. Vehicle users must be protected from potentially lethal voltages and currents.  One critical safety strategy is to separate high voltages and currents from other circuits with an isolation barrier at the die, package, or board level. Automotive designs typically include an enhanced level of protection, called reinforced isolation, that provides twice the isolation required for simple functioning.

Isolated circuits appear in multiple applications. For example, Fig. 2 includes several isolated dc-dc converters, and the three-phase traction inverter requires six galvanically isolated gate drivers.

HV operation and kilowatt power levels bring risks to other electronic circuits as well. Potential hazards include overvoltage, overcurrent, power spikes, and electromagnetic interference (EMI).

In addition to ensuring safety and coping with EMI, automotive designs must also survive the challenging automotive environment. This is especially harsh for equipment intended for underhood applications. Temperatures can range from −40 to +150°C; mechanical stresses include high levels of shock and vibration, as well as exposure to salt spray, gasoline, lubricants, and solvents.

Since improvements in efficiency translate into reduced power consumption, longer battery life, and, hence, increased vehicle range, power designers are always searching for new topologies and new materials. Switching power transistors manufactured from silicon carbide (SiC) and gallium nitride (GaN) are replacing silicon MOSFETs and IGBTs because they allow higher temperature operation and more efficient switching.

These new transistors often come packaged in stacked power modules with low-inductance interconnects that can reduce parasitics, which also improves switching speed and increasing power density.

Conclusion

The rise in HEV and EV vehicles has dramatically increased the quantity of high-power, high-voltage circuitry. Designers are developing a range of process and product improvements to meet the challenges posed by the automotive environment. Find out more about TI’s solutions for hybrid and electric powertrains here.

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From a Department of Energy study, this map shows the impact of 400 strategically placed (about 70 miles apart) fast-charging stations along the U.S. Interstate highway system. The red areas would be reachable within a 70-mile radius from the Interstates. (Courtesy of DoE)