48V/12V DC/DC for automotive dual-rails offers bidirectional power flows

September 22, 2016 // By Graham Prophet
LTC3871 is a 100V/30V bidirectional two phase synchronous buck or boost controller, for 48V/12V automotive dual battery systems; it provides bidirectional DC/DC control and battery charging between the 12V and 48V boardnets, operating 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 with 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 current matching between phases when paralleled. Up to 3 kW can be supplied in buck mode or in boost mode with a 12-phase design.


Current 12V automotive systems, Linear notes, are reaching their 3 kW power limit due to the ever-increasing demand for more electrical devices. A newly proposed standard, LV148, combines a secondary 48V bus with the existing 12V system. The 48V rail includes a belt starter generator (BSG) or an integrated starter generator (ISG), a 48V lithium-ion battery and a bidirectional DC/DC converter for delivery of up to 10 kW of available energy from the 48V and 12V batteries combined. This technology is targeted for conventional internal combustion cars, as well as hybrid electric and mild hybrid vehicles.


When starting the car or when additional power is required, the LTC3871 allows both batteries to supply energy simultaneously by converting energy from one boardnet to the other. Typically, the 12V bus will continue to power the ignition, lighting, infotainment and audio systems. The 48V bus will supply active chassis systems, power steering, air conditioning compressors, electric superchargers/turbos and support regenerative braking. 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 boardnets.


The LTC3871 operates at a selectable fixed frequency between 60 kHz and 460 kHz, 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