Batteries & Fuel Cells
New IC eases the design of 48 V/12 V dual battery automotive systems
By Tony Armstrong, product marketing director for Analog Devices’ Power by Linear product group
T
he future of 48 V/12 V battery systems in automobiles is just around the corner. Most of the major automobile
manufacturers across the globe have been working on proving out their systems for the past few years and it is evident that their implementation will be relatively near term. This is a necessary and crucial step in the long and arduous journey to the fully autonomous passenger vehicle that does not require a human at the controls and has true autonomous driving. Nevertheless, this doesn’t mean the 12 V battery is going away; there are far too many legacy systems in the installed vehicle base for this to occur. This means that autonomous cars will have both a 12 V battery and a 48 V battery. This fact means that the vehicles’ internal
systems will either run off the 48 V lithium- ion (Li-Ion) battery or the 12 V sealed lead- acid (SLA) battery—but not both. Nevertheless, in addition to having two separate charging circuits for these individual batteries due to their respective chemistries, there must also be a mechanism that allows charge to move between them without causing any damage to the batteries or any of the systems within the vehicle. Moreover, having two batteries also allows for redundancy should one of them fail during operation. While this certainly complicates the
design of the various electrical subsystems within the vehicles, there are some advantages to be gained. According to some auto manufacturers, a 48 V-based electric system results in a 10 to 15 per cent gain in fuel economy for internal combustion engine vehicles, thereby reducing CO2 emissions. Moreover, future vehicles that use a dual 48 V/12 V system will enable engineers to integrate electrical booster technology that operates independently of the engine load, thereby improving acceleration performance. Globally, fuel economy regulations have been tightening, while autonomous driving capability with connectivity continues to proliferate in new automobiles. Accordingly, the 12 V automobile electric system has reached its usable power limit. Simultaneously, as if these changes are not already enough, there has been a significant increase in automotive electronic systems. These changes, coupled with related demands for power, have created a new spectrum of engineering opportunities. Clearly, the 12 V lead-acid battery automotive system with its 3 kW power limit must be supplemented.
www.cieonline.co.uk Furthermore, there are new automobile
standards that impact how these systems need to work. A newly proposed automotive standard, known as LV 148, combines a secondary 48 V bus with the existing automotive 12 V system. The 48 V rail includes an integrated starter generator (ISG) or belt start generator, a 48 V Li-Ion battery, and a bidirectional dc-to-dc converter, which can deliver tens of kilowatts of available energy from the 48 V and 12 V batteries. This technology is targeted at conventional, internal combustion automobiles, as well as hybrid electric and mild hybrid vehicles, as auto manufacturers strive to meet increasingly stringent CO2 emission targets. This new standard requires the 12 V bus
to continuously power the ignition, lighting, infotainment, and audio systems. Whereas, the 48 V bus will power active chassis systems, air conditioning compressors, adjustable suspensions, electric superchargers, turbos, and even regenerative braking. The implementation of an additional 48
V supply network into vehicles is not without significant impact. Electronic control units (ECUs) will be affected and will need to adjust their operational range to the higher voltage. This will necessitate that manufacturers of dc-to-dc converters
used to charge either of the batteries while simultaneously allowing both batteries to supply current to the same load if required in the system. From a legacy perspective, these initial 48 V/12 V, dual battery, dc-to-dc converter designs used different power components to step- up and step-down the voltage. However, ADI’s PbL group recently
introduced the LT8228, a bidirectional dc- to-dc controller that uses the same external power components for step-up conversion as it does for step-down conversion. The LT8228, as shown in Figure 1, is a
100 V bidirectional constant-current or constant-voltage synchronous buck or boost controller with independent compensation networks. The direction of
Furthermore, the LT8228 offers a
bidirectional input and output current limit as well as independent current monitoring. Masterless, fault-tolerant current sharing allows any LT8228 in parallel to be added or subtracted while maintaining current sharing accuracy. Internal and external fault diagnostics and reporting are available via the fault and report pins. The LT8228 uses a 38-lead TSSOP package. The LT8228 is a 100 V, bidirectional,
peak current-mode synchronous controller with protection MOSFETs. The controller provides a step-down output voltage, V2, from an input voltage, V1, when in buck mode or a step-up output voltage, V1, from an input voltage, V2, when in boost mode. The input and output voltage can be set as high as 100 V. The mode of operation is externally controlled through the DRXN pin or automatically selected. In addition, the LT8228 has protection MOSFETs for the V1 and V2 terminals. In applications such as battery backup systems, the bidirectional feature allows the battery to be charged from either a higher or lower voltage supply. When the supply is unavailable, the battery boosts or bucks power back to the supply. The LT8228 brings a new level of
Figure 1. LT8228 configured in a simplified, bidirectional battery backup system
will also need to introduce specialised ICs to enable this high power transfer. Accordingly, Analog Devices’ Power by
Linear (PbL) Group has designed and developed a few dc-to-dc converters that can enable this energy transfer with very high efficiency to conserve energy while simultaneously minimizing the thermal design aspects. The need for a bidirectional step-down
and step-up dc-to-dc converter that goes between the 12 V and 48 V batteries is clearly required. Such converters could be
the power flow is automatically determined by the LT8228 or is externally controlled. The input and output protection MOSFETs protect against negative voltages, control inrush currents, and provide isolation between terminals under fault conditions such as switching MOSFET shorts. In step-down mode, the protection MOSFETs at the V1 terminal prevents reverse current. In step-up mode, the same MOSFETs regulate the output inrush current and protect themselves with an adjustable timer circuit breaker.
performance, control, and simplification to 48 V/12 V, dual battery, dc-to-dc automotive systems by allowing the same external power components to be used for step-down and step-up purposes. It operates on demand in buck mode from the 48 V bus to the 12 V bus or in boost mode from 12 V to 48 V. When starting the car or when additional power is required, the LT8228 allows both batteries to supply energy simultaneously to the same load, giving a power conversion which will be required for the fully autonomous vehicles of the near future.
analog.com Components in Electronics April 2020 15
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