48V/12V Dual Battery Automotive Systems Require Bi-Directional DC/DC Controllers : Page 3 of 7

August 16, 2017 //By Bruce Haug, Linear Technology
48V/12V Dual Battery Automotive Systems Require Bi-Directional DC/DC Controllers
Fuel economy requirements and ADAS functions have led to the introduction of a 48V supply bus for cars that coexists with the traditional 12V rail. Both systems are interconnected through bi-directional DC/DC controllers. This article describes the requirements and considerations to optimize the interplay.

Nevertheless, the implementation of an additional 48V supply network into vehicles translates into major design challenges for suppliers across the value chain.  In particular, providers of semiconductors and Electronic Control Units (ECUs) will be affected – they will need to adjust their operational range to the higher voltage and in part re-design their products. Correspondingly, the manufactures of DC/DC converters will need to develop and introduce specialized ICs to enable this high power transfer. Accordingly, Linear Technology has designed and developed a number of DC/DC converters which are able to facilitate this energy transfer with very high efficiency to both conserve energy while also minimizing the thermal design required due to their much lower power loss.

It is clear that there is a need for a bi-directional step-down and step-up DC/DC converter that goes between the 12V and 48V batteries. This DC/DC converter can be used to charge either battery and allows both batteries to supply current to the same load if required. Most of the early 48V/12V dual battery DC/DC converter designs use different power components to step-up and step-down the voltage. However, the recently released LTC3871 bi-directional DC/DC controller from Linear Technology uses the same external power components for the step-up conversion as it does for stepping down the voltage.


A Single Bi-Directional IC Solution

The LTC3871 is a 100V/30V bi-directional two phase synchronous buck or boost controller which provides bi-directional DC/DC control and battery charging between the 12V and 48V board nets. It operates in buck mode from the 48V bus to the 12V bus or in boost mode from 12V to 48V.  Either mode is configured on demand via an applied control signal. Up to 12 phases can be paralleled and clocked out-of-phase to minimize input and output filtering requirements for high current applications (up to 250A). Its advanced current-mode architecture provides excellent current matching between phases when paralleled. Up to 5kW can be supplied in buck mode or in boost mode with a 12-phase design.

When starting the car, or when additional power is required, the LTC3871 allows both batteries to supply energy simultaneously by converting energy from one board net to the other. Up to 97% efficiency can be achieved and the on-chip current programming loop regulates the maximum current that can be delivered to the load in either direction. Four control loops, two for current and two for voltage, enable control of voltage and current on either the 48V or 12V board nets.

The LTC3871 operates at a user selectable fixed frequency between 60kHz and 475kHz, and can be synchronized to an external clock over the same range. The user can select from continuous operation or pulse skipping during light loads. Additional features include overload and short-circuit protection, independent loop compensation for buck and boost modes, EXTVcc for increased efficiency, ±1% output voltage regulation accuracy over temperature, along with undervoltage and overvoltage lockout. The LTC3871 has been qualified to meet AEC-Q100 specifications and was designed for diagnostic coverage in ISO26262 Systems.

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