“When designing powertrains for hybrid electric vehicles (HEVs) and battery electric vehicles (BEVs), designers are under constant pressure to reduce costs while improving energy efficiency and reliability. While the switch to dual 12-volt and 48-volt power rails has helped improve energy efficiency by reducing the weight of chassis wiring, designers also need dedicated solutions to improve the management of the two power supplies so they can better support each other while simultaneously It also enables the vehicle to support bidirectional vehicle-to-grid (V2G) applications.
When designing powertrains for hybrid electric vehicles (HEVs) and battery electric vehicles (BEVs), designers are under constant pressure to reduce costs while improving energy efficiency and reliability. While the switch to dual 12-volt and 48-volt power rails has helped improve energy efficiency by reducing the weight of chassis wiring, designers also need dedicated solutions to improve the management of the two power supplies so they can better support each other while simultaneously It also enables the vehicle to support bidirectional vehicle-to-grid (V2G) applications.
This need has led to the development of bidirectional converters and bidirectional power factor correction (PFC) systems that designers can use to optimize the overall performance of dual 12V/48V electric vehicle (EV) designs, as well as connect to the grid for bidirectional power flow.
This article will describe and review the benefits of bidirectional power conversion for automotive systems and the associated standards. It then introduces solutions from vendors such as Texas Instruments, Analog Devices, and Infineon Technologies, and shows how to use them to implement bidirectional power converters.
What is bidirectional power conversion?
In HEVs with a 12V/48V dual voltage architecture, the bidirectional power supply links the 12V and 48V systems together so that either battery can be charged by the other. It also allows each cell to provide additional power to either rail under overload conditions (Figure 1). As a result, designers can use smaller batteries for each system, improving reliability, energy efficiency, and reducing costs.
In BEV, designers can use bidirectional PFC to support bidirectional battery charging as well as V2G operation. V2G systems support higher energy efficiency in several ways:
It can return energy to the grid during periods of high demand
It can reduce the charging rate of the battery as needed to help balance the load on the grid
It allows the use of vehicles to store energy from renewable sources
Dual voltage systems in HEVs are self-contained systems within the vehicle that improve fuel economy, while bidirectional chargers in V2G systems are designed for broader cost benefits beyond improved fuel economy and must be external interface.
The implementation of V2G requires the use of communication technologies and algorithms to sense grid conditions, as well as the ability to interface with EV charging infrastructure (Figure 2).
The resulting V2G infrastructure brings a number of economic benefits, including the ability to supply power to the grid during periods of peak demand (potentially generating income for vehicle owners), and to charge vehicle batteries during periods of lower demand (reduce vehicle charging costs) .
Standards related to bidirectional power conversion
The LV148/VDA320 specification defines the electrical requirements and test conditions for combining a 48-volt bus and a 12-volt bus in a dual-voltage automotive system (Figure 3). LV148 has been adopted by German car manufacturers Audi, BMW, Daimler, Porsche and Volkswagen for both conventional internal combustion engine vehicles and hybrid electric vehicles. At the time of writing, the standard ISO 21780 “Road vehicles – 48 V supply voltage – Electrical requirements and tests” is under development.
There are several communication protocols that can be applied to V2G systems, including:
ISO/IEC 15118: Defines a V2G communication interface for bidirectional charging/discharging of electric vehicles. The protocol uses the IEEE P1901.2 HomePlug Green PHY (HPGP) Wideband Power Line Communication (PLC) specification as the optimal protocol to ensure stable communication and high data rates. HPGP operates at frequencies from 2 MHz to 30 MHz, enabling the system to distinguish valid data on the connected line from noise from other nearby sources.
IEC 61850: Defines a communication protocol for intelligent Electronic equipment in substations that helps manage the flow of energy between renewable electricity sources and Electric Vehicle Supply Equipment (EVSE) such as chargers.
Bidirectional Multiphase DC-DC Converter for 12V/48V Systems
Given the high power levels of a typical 12V/48V bidirectional DC-DC converter, a polyphase topology is often required. The multi-phase design improves the overall conversion efficiency by enabling phase drop, thereby reducing the number of active phases as power requirements drop. The multiphase design also enables the use of smaller filter components at the output of each phase; the use of smaller inductors improves load transient performance. Finally, running the phases with proper interleaving reduces output ripple.
Texas Instruments’ LM5170-Q1 is a high-performance, multiphase, bidirectional current controller suitable for managing current transfer between the 48-volt and 12-volt parts of an automotive dual-battery system (Figure 5). It integrates basic analog functions to design high-power power converters with a minimal number of external components. Multiphase parallel operation can be achieved in two ways: by connecting two LM5170-Q1 controllers for three-phase or four-phase operation, or with multiple controllers synchronized to a phase-shifted clock for more phase operation.
The LM5170-Q1 includes a dual differential current sense amplifier and dedicated channel current monitor, achieving a typical current accuracy of 1%. The regulated 5A (A) half-bridge gate driver is capable of driving parallel MOSFET switches up to 500W per channel. The diode emulation mode of the synchronous rectifier avoids negative current flow, and also allows for improved light-load efficiency through discontinuous operation. Versatile protection features include cycle-by-cycle current limit, high- and low-voltage port overvoltage protection, MOSFET fault detection, and overtemperature protection. The controller is capable of automotive functional safety.
Texas Instruments offers the LM5170EVM-BIDIR evaluation module for engineers to evaluate the LM5170-Q1 in a 12V/48V dual battery system application. The two-phase circuits operate with 180° interleaving, and both share a maximum DC current of up to 60 A. The evaluation module also includes various jumpers to flexibly and easily configure the circuit for many different use cases, including functions controlled by microcontrollers (MCUs) and high-power unidirectional buck or boost converters.
Master/Slave Polyphase Architecture for Bidirectional Converters
Analog Devices offers the LT8708 Buck-Boost Switching Regulator Controller for 12V/48V Bidirectional Power Converters. The LT8708 is an 80V synchronous 4-switch buck-boost DC-DC controller with bidirectional capability that can support load currents up to approximately 30A. For higher current requirements, the LT8708 master controller can be combined with one or more slave chips. The use of a master/slave architecture can reduce the solution cost in multiphase designs because a single (more expensive) master IC can control multiple (lower cost) slave ICs.
When slave ICs are connected to the master IC, they scale up the power and current capability of the system. But it is important that the slave IC has the same conduction mode as the LT8708 so that it can conduct current and power in the same direction as the master IC. The master IC controls the overall current and voltage limits of the LT8708 multiphase system, and the slave ICs need to comply with these limits.
The slave IC can easily be connected in parallel with the LT8708 by connecting the four signals together (Figure 6). Two additional current limits (forward VIN current and reverse VIN current) are provided on each slave IC, which can be set independently.
Analog Devices’ DC2719A demo board uses the LT8708 combo associated slave IC (LT8708-1), delivering 40 A. The evaluation board can operate in both forward and reverse modes. The controller has integrated input voltage and output voltage regulators, as well as two sets of input and output current regulators for forward or reverse current control. Features are included to simplify bidirectional power conversion in battery/capacitor backup systems and other applications that may require regulation of VIN, VOUT, IIN and/or IOUT.
Bidirectional Power Factor Correction for Grid Interactive BEVs
For designers of grid-interactive BEVs, Infineon offers the EVAL3K3WTPPFCSICTOBO1 evaluation board, a 3300-watt bridgeless totem-pole power factor corrector with bidirectional power capability (Figure 7). This bridgeless totem pole PFC board is suitable for applications requiring high energy efficiency (~99%) and high power density (72 watts per cubic inch).
By using wide-bandgap semiconductors, it becomes feasible to employ totem-pole topologies in PFC applications with continuous conduction mode (CCM) operation. In this case, Infineon’s IMZA65R048M1 CoolSiC MOSFET in a TO-247 four-pin package can be used to increase the energy efficiency up to 99% at half load. The converter is designed to operate at high line voltages (176 Vrms minimum, 230 Vrms nominal) in the CCM with a switching frequency of 65 kilohertz (kHz).
This 3300 Watt Bridgeless Bidirectional (PFC/AC-DC and Inverter/AC-DC) Totem Pole is a system solution developed using Infineon Power Semiconductors and Infineon Drivers and Controllers. Infineon devices used in the design include:
64 milliohm (mΩ) 650 volt CoolSiC MOSFET (IMZA65R048M1) in TO-247 four-pin package as totem pole PFC high frequency switch
・ 17 mΩ 600 V CoolMOS C7 MOSFET (IPW60R017C7) in TO-247 package for totem pole PFC return path (low frequency bridge)
2EDF7275F Isolated Gate Driver (EiceDRIVER)
ICE5QSAG QR Flyback Controller and 950V CoolMOS P7 MOSFET (IPU95R3K7P7AKMA1) for Bias Auxiliary Supply
XMC1404Q048X0200AAXUMA1 Infineon microcontroller for PFC control implementation
The totem pole implemented on the EVAL3K3WTPPFCSICTOBO1 board operates in both rectifier (PFC) and Inverter modes under CCM and is fully digitally controlled using Infineon’s XMC1404Q048X0200AAXUMA1 microcontroller.
In response to designers’ need to improve energy efficiency, the dual-voltage 12V/48V architecture has emerged as the preferred topology for HEVs and BEVs. This requires efficient power management to optimize the use of this architecture. The advent of bidirectional DC-DC converters and battery chargers has enabled 12-volt and 48-volt systems to support each other in situations where one of them needs to be charged or under overload conditions.
Likewise, for BEVs, the bidirectional PFC stage supports bidirectional power flow between the battery and the utility grid. The economic benefits of the resulting V2G connectivity are not limited to improved fuel economy, but also include the ability to power the grid during peak demand periods and to charge car batteries when power demand is low.