 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 module (BCM). The specific loads handled by the BCM also vary by manufacturer. Most of them cover things like power windows and door locks, windshield wipers, and lighting. The MCU-controlled BCM is made up of a mix of dc-dc converters, LDO regulators, LED drivers, and a variety of sensors and switches. Typical load voltages are in the 1.2- to 5-V range.
The BCM starts with the battery output that’s loaded with noise and transients. Some EMC filtering and battery-protection components prep the voltage for some regulators that generate the needed rail voltages. For small BCMs, an all-LDO architecture is used. The LDOs further clean and regulate the 12 V down to several lower-voltage buses. LDOs aren’t the most efficient of regulators, but with light loads they’re preferred. With loads greater than about 400 mA, the heat and inefficiency may make the LDO approach unacceptable.
Another approach is to use a combination buck switching regulator followed by LDOs. It’s more efficient. The buck regulator usually creates a 5-V bus; following LDOs make up any lower-voltage buses.
An alternative arrangement is to start with a buck converter followed by a boost converter and another buck regulator. The boost converter controls the voltage so that some loads can operate under cold-crank conditions and/or operate the engine start/stop feature implemented in many new vehicles. Under such conditions, the battery voltage drops drastically, so regulation is critical. Additional buck converters are also used to generate the other needed lower-voltage buses.
A good choice for the buck/boost approach is to use a SEPIC-type converter. The SEPIC is a combination buck/boost architecture that’s efficient and flexible. It can then be followed with additional converters or LDOs as needed (Fig. 3).
3. Here’s a simplified BCM automotive power supply using a SEPIC buck-boost converter.
EV Battery-Charging Solutions
EVs and HEVs both require battery-charging systems. Some of the chargers are internal to the vehicle while others are external, ei