In case you haven’t noticed, automotive electrical systems are undergoing a significant change from the old 12-V systems to a new hybrid 12- and 48-V system. The number of new electrical and electronic devices and subsystems added to new vehicles over the years has significantly increased the electrical load on the old, reliable lead-acid 12-V battery. And the breaking point has arrived.
In addition, hybrid electrical vehicles (HEVs) and full electrical vehicles (EVs) are gradually gaining acceptance. HEV/EVs need an even more aggressive electrical system. This article explores the new systems that more adequately deal with the modern electronic needs of the modern automobile.
- Taking charge of electric vehicles – both in the car and on the grid
- System Power Architectures in Body Control Modules
- Bridging 12V and 48V in dual-battery automotive systems
The Emerging 12/48-V Dual-Battery Electrical Systems
The end is near for the long-lived 12-V electrical system in vehicles. First appearing in 1955, the 12-V system replaced older 6-V systems. Now, 60+ years later, that system has reached its limits. With the continuous inclusion of new electronic systems over the years, the total electrical demands in a car’s traditional 12-V electrical system exceed its capability. For example, at low temperatures, the static-load components can account for all of the power generated by the alternator. And more new loads are still to come.
The solution to this problem has been known for years. In the early 2000s, a 36/42-V system was proposed though never adopted. Getting the whole auto industry to adopt a new system isn’t easy, but now there’s no choice. A dual 12- and 48-V system is the solution. The strategy being employed by automakers is to move power-hungry vehicle components and subsystems to a new 48-V rail powered by a lithium-ion battery.
The new electrical system looks like that in Figure 1. The loads are divided with the most power-hogging systems on the 48-V bus and lesser loads on the 12-V bus. The heaviest loads are from the starter, water and fuel pumps, HVAC, cooling fan, and body-stabilization systems. The availability of a 48-V source makes possible enhancements like electrical turbochargers that are far more effective than the exhaust-driven type. Besides providing much more power in general, up to about 10 kW, the 12/48-V system means lower currents that result in smaller lighter cables.
1. Shown is the architecture of a dual 12/48-V battery system in new vehicles.
In Figure 1, you will see the dc-dc converter, which is the key to making this dual system practical. The converter provides for a bidirectional-power-transfer capability that keeps both sources of power alive under all conditions. This arrangement will charge either battery from the other if it’s discharged and meet temporary overload conditions.
While a pair of dc-dc converters would work—one buck for 48 V to 12 V and a boost for the 12 V to 48 V—it’s possible to create a bidirectional buck-boost converter. One way to implement such a system is to use Texas Instruments’ LM5170-Q1, a multiphase bidirectional current controller IC. This buck-boost controller forms the core of a bidirectional converter for 12/48-V systems. It manages the current direction and average current with a pulse-width-modulation (PWM) control scheme under the direction of an MCU.
Figure 2 shows a simplified version of the bidirectional dc-dc converter using this IC. An evaluation board and reference design (TIDA-01168) using this chip are also available.
2. This bidirectional dc-dc converter for dual-battery voltage systems uses the LM5170-Q1. Some external components have been omitted for clarity.
Powering Vehicle Body-Control Modules
All automotive electrical systems divide up related electrical loads into segments. Examples are separate loads for engine, transmission, ADAS, infotainment, and brakes. The loads operate from the 12-V battery and vary from manufacturer to manufacturer.
One load that all systems implement is body electronics, which is handled by the body-control m