Motor Control Adding 48V to 12V Means Major Benefits
Motor Control Adding 48V to 12V Means Major Benefits
Hybrid Electric Vehicle (HEV) and all-Electric vehicle (EV) designs are highly visible these days, but there is also a less-apparent but significant trend in the automotive industry: The addition of the 48V Direct Current (DC) bus to powertrains. This inclusion of 48V is not just for EVs and HEVs; it is also being put in place for more common internal combustion cars, even as the EV and HEV share of the market grows.
Engineers and car mechanics who were brought up on the ubiquitous 12V battery and components for cars (a standard that is over 100 years old) do not need to worry, because the 12V system is not going away soon. However, in the foreseeable future, 12V will likely coexist with the newer 48V system.
So what’s unique about the 48V system? The answer comes back, as it usually does in power situations, to Ohm’s law and basic physics related to Voltage drop (V = IR), Power (P = IV), and power loss (P = I2R). Delivering power requires a combination of current and voltage, though voltage drop (loss) increases linearly with a current and losses increase in the square of the current. In short: Delivering more power at a given voltage takes more current, and more current leads to higher voltage drop and unrecoverable power losses.
This is not a new de
velopment, as it was also known by the earliest electrical-energy pioneers; that’s why they migrated to ever-higher voltages as they sought to power street lights, homes, and industry. But for cars, the 12.6V (nominal) voltage available from rechargeable, lead-acid batteries was a good compromise with consideration to size, energy capacity (ampere (amp) hours), cost, longevity, and safety as well as other factors.
Today’s cars, however, need much more power than those of even just a few cars ago. In addition to basic functions such as a starter, lights, radio, power windows, electrically assisted power steering, networks, and infotainment, many other safety and convenience features now exist that are often grouped under the Advanced Driver Assistance Systems (ADAS) heading. ADAS requirements have led to the need for a battery that can deliver more power and, thus, more amps at 12V to power the various core functions and these newer peripherals. However, keeping IR drop and I2R power losses at an acceptable level at 12V requires thicker cables, more copper, and largerconnectors, which in turn means more costs, weight, and crowding in an already tightly constrained vehicle.
Is 48 the New 12?
After much analysis, the industry decided to use 48V as the “other” higher voltage. Unlike the lead-acid chemistry of the traditional 12V battery, this 48V battery is lithium-based for higher energy density by weight and volume. The resulting topology is known in the insets as a “mild hybrid”.
In this approach, the 48V battery following guidelines of the proposed automotive standard LV148-serves as a supplement (and combines) with the 12V lead-acid battery, which has a 3kW power rating (Figure 3). Both the 12V battery and the 48V battery will be used along with their distribution cables, and each battery will serve the load when that particular battery is the most appropriate fit for the need. For example, the 12V bus will provide power to the ignition, lighting, and infotainment while the48V bus will be responsible for the chassis-control systems, air conditioning, active/adjustable suspension, storing regenerative-braking energy, and driving electric superchargers and turbos.
What It Takes
The use of 48V takes more than a new battery structure. In addition to thinner cables and correspondingly smaller connectors, it requires new guidelines on the allowable bend radius (thinner cables allow for tighter bends and eases cable-run issues), consideration of cable insulation, and other “mechanical” changes. These are the obvious, beneficial, and easy-to-grasp changes. However, since the 12V and 48V buses are not isolated from each other, new functional blocks are necessary to co-manage them.
Other, less-obvious changes exist as well. Circuit-protection components tailored for spikes of up to 30V and 40V on the 12V line require higher voltage ratings to work alongside the 48V, as a lower-rated voltage component would trip in the 48V world. Even basic fuses- which are inherently current-sensitive devices-also have a maximum rating for the voltage across their terminals, so they need to be physically and electrically sized for lower currents (an easy task) but higher voltages (slightly more difficult). Similarly, the standard component that protects against a revevrse-battery connection and that is essential at each subcircuit, that is, in case an accidental misconnection of the primary cables occurs, also needs to be scaled to handle the reversal of higher voltage ratings.
However, the real challenge begins with battery management. In most systems, this involves the measurement and management of multiple batteries with the same nominal voltage, but in the 12V/48V design, there are two very different voltages. The preferred situation from an overall power-management perspective is to allow the 48V and 12V batteries to share and exchange energy when it makes sense.
Integrated circuits (ICs) like the LTC3871 from Analog Devices are designed for this situation (Figure 5). This bidirectional, buck/boost switching regulator (controller) allows both batteries to supply energy to the load simultaneously by converting energy from one battery to the other. It regulates switches in buck mode from VHIGH-to-VLOW and boost mode from VLOW-to-VHIGH depending on a processor-sourced control signal. At the same time, its accurate current-programming loop regulates the maximum current that can be delivered in either direction.
The 48V battery itself is special. In principle, its construct could consist of a simple, series connection of four, 12V, lead-acid batteries, but this would take up too much space and require heavy-gauge inter-battery cables. Instead, the 48V battery is a Li-ion unit specially designed to deliver that voltage, yet with a volume roughly the same as the 12V battery.
Bosch offers a battery design for automotive 48V systems (Figure 6).It measures just 309 mm x 175 mm x 90 mm, so it can be placed under a car seat or in the trunk. It is a passively cooled component that doesn’t need a fan (which would result in a higher cost and lower reliability) and makes no noise. The 7kg unit can deliver up to 13kW of power as it returns the energy it has accumulated.
Conclusion
The 12V battery has served the automotive industry well, as its long and successful track record clearly demonstrates. Nonetheless, this single, lower-voltage bus can no longer meet the rapidly increasing power needs of today’s vehicles due to its IR loss, power dissipation, and other issues.
The solution uses a 48V supplemental battery, which is no longer a “coming soon” prediction, as it is already in use in a large number of car propulsion sources ranging from internal combustion to mild and full hybrid to all-electric vehicles. The use of the 48V solution will expand rapidly, as it is the only viable solution to vexing power problems.
Enabling this new architecture requires new cables, connectors, active (power-management ICs) and passive (protection device) components, along with 48V Li-ion-based batteries. Perhaps, in a decade or two, the 12V battery will be a historical footnote only seen at antique car shows, but for the foreseeable future, both the 12V and 48V batteries and their buses will coexist and interact to provide essential power with high efficiency and low dissipation.
Reprinted with Permission by Mouser Electronics
Website: www.mouser.com
