Using specialised power converters to bridge the dual 12v to 48v gap in automotive systems

Author : Rolf Horn | Applications Engineer | Digi-Key

01 October 2020

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As Rolf Horn, Applications Engineer at electronics component distributor, Digi-Key explains, with the additional electronics, motors & actuators increasingly being added to internal combustion engine (ICE) vehicles, the conventional closed-circuit 12-volt automotive electrical system – based on a lead-acid battery, charged by the alternator – is revealing its limitations.

This article was originally featured in Digi-Key Article Library and in the October 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.

For example, when using a 12-volt system, high power applications such as electric steering draw a high current, necessitating bulkier, heavier wiring looms. That extra weight becomes significant in a modern vehicle, which can have several kilometres of wiring.

An alternative approach employs higher voltage systems for the power-hungry applications to lower the current draw and allow for lighter wiring. Commercial implementations feature a conventional 12-volt network, supplemented by a 48-volt system based on lithium-ion (Li-ion) batteries. The 12-volt system is used for applications such as engine management, lighting, and seat and door adjustment, while the 48-volt system takes care of heavy-duty demands from functions such as electric steering, starting and HVAC.

These hybrid automotive electrical systems bring increased complexity and, with it, new design challenges. Key among these challenges is the management of the simultaneous charge and discharge of the two battery circuits, including bidirectional step-down (buck) and step-up (boost) between the batteries.

This article describes the evolution of dual 12/48-volt automotive electrical systems and explains the advantages of the new systems. It then explores how to use 12/48-volt bidirectional voltage regulators from Linear Technology and Texas Instruments to ease the design complexity of dual-voltage systems. It also considers the advantages of a future 48-volt-only decentralised vehicle power topology, and looks at a bus converter from Vicor suited for such a system.

The challenge of transitioning to 12/48-volt design

The transition to 12/48-volt systems is largely being encouraged by the need to drive high power consumption devices, while ensuring the vehicle still meets stringent economy and emissions regulations. For example, the switch from mechanical to electric drive for things like steering or superchargers dramatically reduces friction losses and boosts fuel economy. According to some auto manufacturers, a 48-volt electric system results in a 10-15% gain in fuel economy, with a proportional reduction in noxious emissions. The 12-volt side of the system continues to be needed because of the large volume of legacy 12-volt devices that will continue to be fitted into autos for years to come.

Figure 1. A bidirectional power supply can be used to manage power between the 12-volt & 48-volt circuits of automotive electrical systems (image source: Texas Instruments)

The 12/48-volt configuration consists of two separate branches: the traditional 12-volt bus uses a conventional lead-acid battery for legacy loads, while the 48-volt system, powered by a Li-ion battery, supports the heavier loads. While two separate charging circuits are needed to suit the respective batteries’ electrochemistry, there must be a mechanism that allows charge to move between them without danger of damage to either battery or any of the systems they power. There also needs to be a mechanism to provide extra power for the opposite voltage rail in an overload condition.

A newly proposed automotive standard – LV 148 – describes the combination of the 48-volt bus with the existing automotive 12-volt system. The 48-volt system incorporates an integrated starter generator (ISG) or belt start generator and the Li-ion battery. The system is able to deliver tens of kilowatts (kW) and is targeted at conventional cars, as well as hybrid electric and mild hybrid vehicles.

Designing a 12/48-volt system is challenging, because it requires careful management of the power transfer from the vehicle’s 48-volt rail to its 12-volt rail, and back. One option is to use a buck converter for the voltage step down, while power transfer in the opposite direction could be supplied with a boost converter. But designing in separate DC-to-DC converters takes up valuable board space and increases system cost and complexity.

An alternative approach is to use a single, bidirectional buck/boost DC-to-DC converter sited between the 12- and 48-volt batteries. Such a converter can be used to either charge the batteries or allow them to supply current to the vehicle’s various electrical loads (Figure 1)...

To read the full version of this article, visit the Digi-Key Article Library or read EPDT magazine's October 2020 digital issue.

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