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

In a typical boost controller, the synchronous diode or the body diode of the synchronous MOSFET conducts current from the input to the output. As a result, an output (VHIGH) short will drag the input (VLOW) down without a blocking diode or MOSFET to block the current. The LTC3871 uses an external low RDS(ON) P-channel MOSFET for input short-circuit protection when VHIGH is shorted to ground. In normal operation, the P-channel MOSFET is always on, with its gate-source voltage clamped to 15V maximum. When the UVHIGH pin voltage goes below its 1.2V threshold, the FAULT pin goes low 125μs later. At this point, the PGATE pin turns off the external P-channel MOSFET.

Conclusion

The LTC3871 brings a new level of performance, control and simplification to 48V/12V dual battery DC/DC automotive systems by allowing the same external power components to be use for step-down and step-up purposes. It operates on demand in buck mode from the 48V bus to the 12V bus or in boost mode from 12V to 48V.  Up to 12 phases can be paralleled for high power applications and when starting the car or when additional power is required, the LTC3871 allows both batteries to supply energy simultaneously to the same load. The additional 48V battery running a portion of a vehicle’s electrical system will play a central role in increasing available energy, while reducing wiring harness weight and losses. This additional energy capacity paves the way for new technologies, enabling cars to be safer and more efficient, all while lowering its CO2emissions.

About the author:

Bruce Haug is Senior Product Marketing Engineer at Linear Technology, now part of Analog Devices.

All images (C) Linear Technology.

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