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A multiple-bus electric system for the next generation automobile
The single 12 V bus system used in automotive electric systems provides insufficient efficiency and has reached the limit of its power supply capabilities. A multi-bus system is proposed as the solution
Xiaoyan Yu & Eduardo M. A. Oliveira, Vicor Corporation
In a typical hybrid vehicle with a 12 V single bus system there are two energy sources. One is the high voltage DC battery (HV battery). The other source is the engine. The output from both sources supplies a single 12 V bus. The 12 V standard vehicle battery (LV battery) and all other loads are powered by this single 12 V bus.
There are several major issues inherent to this structure [1-4]:
A. The power limitation of the conventional 14 V alternator hinders increasing demand for higher power to improve driving safety and comfort.
B. Different loads achieve peak efficiency and performance at different input voltages. Thus a single bus system is fundamentally inefficient.
C. The 12 V bus actually varies from 9 V to 16 V typically, depending on the battery load and source conditions. This makes the load efficiency even worse.
D. It is difficult to enable auto start-stop features to improve power savings during vehicle idling. During the starting period, the battery voltage goes through a voltage “cranking deep” stage, and can go as low as 6 V. For the existing single 12 V bus system, even if the loads can tolerate this voltage cranking, it will limit equipment lifetime and reliability.
E. The 12 V lead acid automotive battery problem remains one of the most common issues in vehicles. It is always a big reliability concern because of the fact that it needs replacement or maintenance much more often than other standard equipment in the vehicle.
Intensive research has been done to replace the existing 12 V single bus system with a 42 V bus system [2-3], thus surpassing the power limitation of the conventional 14 V alternator, which solves issue A, above. However, issues B, C, D and E still remain unsolved. In addition, a complete 42 V bus system will require all the loads in the vehicle to be adapted to the new voltage.
A multi-bus system is proposed in this paper. It could be a simple but effective solution to the aforementioned issues inherent to the existing 12 V single bus structure. The main bus will be 42 V, to gain higher power levels and higher efficiency. 12 V remains a supported bus voltage, to support existing loads that are less power-intensive. Other bus voltages, including one high voltage bus, are also needed and will be discussed in the next section.
II. Automotive Loads Analysis
To increase the efficiency of the electric system inside the vehicle, a study of various automotive loads is necessary, since different loads achieve optimal efficiency and performance at different voltages. In addition, as the transition from gasoline fueled vehicles toward hybrid/electric vehicles progresses, most loads eventually should be able to be powered directly or indirectly through the HV battery instead of the alternator. The reason is simple: gasoline is much more expensive than electricity as a power source today.
The typical loads in the vehicle include:
1. Motors and actuators
Motors and actuators are the main loads for driving, thus it is preferred to keep them on the main 42 V bus. They achieve optimal efficiency and performance at 42 V .
As the motor starts, the 42 V main bus voltage is going to drop, thus it is beneficial to have an additional regulated converter to make a regulated 42 V bus. This regulated converter can be bypassed by a diode with low forward voltage drop to improve efficiency after the motor is started.
2. Active suspension
Active suspension helps to improve ride quality and driving safety as compared to passive suspension and semi-active suspension. It also consumes a lot more power than semi-active suspension. Currently, active suspension is only available in some high end luxury vehicles. Power consumption is a major barrier for active suspension to be used in mainstream vehicles in the foreseeable future. Even when active suspension is under standby operation, it consumes power, which could be a couple of hundred watts. When active suspension is in active operation, it can be the most power hungry component inside the vehicle .
Active suspension works most efficiently at a regulated voltage of about 350 V . Thus, it is more efficient to have a high voltage bus directly regulated from the HV battery bus.
3. DC electric compressor
The air conditioner in conventional gasoline-powered vehicles has a compressor powered directly from the engine. But this approach disables idle stop functions and also reduces gas mileage when powered by the alternator. Increasingly, hybrid vehicles are being designed with air conditioners that are powered by the batteries. The DC electric compressor input voltage ranges from 12 V to 610 V. The DC electric compressor is power-intensive equipment, thus to reduce the transmission loss, connection cable weight and volume, it would be beneficial to have a high input voltage. The DC electric compressor may also work efficiently with the 350 V bus voltage, and it can thus share one bus with active suspension.
In this paper, the DC electric compressor and active suspension will share the same 350 V high voltage bus.
The main heating system inside a vehicle works usually by recycling the heat coming out of the engine, since the energy loss there can be used as heat inside the cabin. For electric vehicles (which have no engines) or hybrid vehicles (where engine use is minimized), super-insulated cabins and/or reversible AC-systems may be the solutions for the main heating system.
Separate from the main heating system, seats and windshields may require individual heating, most often provided by electric heaters. The heater works by supplying voltage to a resistor, and then spreading the heat. The heater is a pure resistive load, and its efficiency is barely related to the input voltage. The best choice may be to put this load onto the 42 V bus, to reduce the transmission loss.
The audio system inside a vehicle has a very wide power range, from less than 50 watts to over 1000 watts. This power requirement will continue to increase to meet rising demand for integrated entertainment and GPS capabilities. Increasing the supply voltage is one way to increase the available power level. The class AB amplifier is today’s standard in automotive audio applications , which has an efficiency of only approximately 50%. But increasing the input voltage would also increase the energy loss in the class AB amplifier, generating excessive heat and making it infeasible. The class D amplifier has become a hot topic in automotive audio applications, as it can achieve higher power levels with efficiencies as high as 95%. A tentative proposed input voltage bus for the audio system would be 42 V.
There are various types of light sources in existing vehicles, such as tungsten-halogen, HID and LED. The lights can be fit to the 42 V bus or 12 V bus, depending on the power level.
7. Power seats /power locks /power mirrors/power windows
This group of loads requires minimal average power, since the operating power is only required for a short amount of time during a driving cycle. Barely any efficiency improvement will be gained by optimizing the input voltage of this type of load, therefore it makes sense to keep them on the 12 V bus.
III. A Multiple-Bus Automobile Electric System
A multi-bus structure is proposed. There are four buses in this system:
1. Bus 1: Regulated 350 V
This is a high voltage regulated bus, to reduce transmission loss and increase load efficiency. High power load, suitable for high voltage needs such as active suspension and DC electric compressors, can be put onto this bus.
2. Bus 2: Unregulated 42 V
This is the main bus, and it is connected to the motors and actuators. It becomes unregulated because of the voltage swing resulting from the auto start-stop feature. Loads which are not sensitive to an unregulated input voltage can also be put onto this bus, for example, resistive load such as heaters and Halogen lights.
3. Bus 3: Regulated 42 V
Adding an additional regulation only DC-DC converter 3 to the unregulated bus 2 forms a regulated bus 3. A diode can be put in parallel with DC-DC converter 3 for better efficiency. During normal operation, the diode D1 is set to be forward biased such that it works like an ideal diode, and the DC-DC converter 3 is bypassed to achieve better efficiency. The voltage drop of Bus 2 at the startup of the engine will cause D1 to be turned off. Thus DC-DC converter 3 will regulate Bus 3. Loads with moderately significant power requirements such as HID or LED headlights can be put onto this bus.
4. Bus 4: 12 V
All legacy loads that are intermittent and have low average power will be put onto this bus. Power seats, power locks, power mirrors, and power windows are typical low average power loads. LED lighting is a high efficiency light source, and other than the headlights, most of the other LED lights have low power. These low power lights can be possibly put onto this 12 V bus.
42 V buses (Bus 2 and 3) enable the high power capability of the generator, higher power capability with higher efficiency to the motors and actuators, heaters, lights, and audio system, and the details have been discussed in the automotive loads analysis section. If the load benefits from a regulated input (as in the case of the audio system) then it will be put on the regulated version of this bus.
The 350 V bus (Bus 1) enables the higher power capability with higher efficiency for the active suspension and DC electric compressor.
The 12 V bus (Bus 4), which is stepped down to 12 V from Bus 3 instead of Bus 2, supplies a fixed 12 V (existing standard voltage) without interruptions to vehicle lights, power seats, power locks, power mirrors, power windows, and many other light power loads. A highly-efficient fixed-ratio converter 4 is used to step down the voltage from 42 V to 12 V.
The 12 V bus helps many vehicle loads to be directly used in this multi-bus system without adaptation.
All of the DC-DC converters and diodes in this structure are high efficiency and high power density in order to achieve an overall optimum system. The diode, such as D1 shown in Figure 3, can be an extremely low on-resistance MOSFET (less than 1 milliohm is achievable after paralleling) to ensure very low power dissipation, and act close to an ideal diode.
There may be a need for more than three different voltages in future vehicles. There are two ways to add more voltages efficiently:
1. Add more voltages by stepping from the Bus 1 or Bus 3 through the high efficiency fixed ratio DC-DC converters, just like the way Bus 4 is created.
2. Create additional voltage from the existing buses. The “Yeaman topology” is one way to implement this [7-8]. By putting Bus V1 and V2 in series, a new bus V3 is able to supply a voltage of V1+V2 without introducing additional converters into the system.
This structure provides multiple fixed voltages (350 V, 42 V and 12 V) through Bus 1-4. It solves all of the aforementioned issues (A, B, C, D and E) inherent to the existing 12 V vehicle electric system in a simple and effective way:
➢ It enables the use of a 42 V alternator, thus eliminating the power limitation of the conventional 14 V alternator;
➢ It fits various loads to their maximum efficiency via multiple bus voltages;
➢ It increases the efficiency even further because the bus voltage variation has been minimized;
➢ It enables an auto start-stop feature because that route is separated and regulated.
➢ A super-capacitor C at 42 V bus can store 12.25 times as much energy as it could store at 12 V bus. This introduces the possibility of replacing lead acid batteries with much more reliable devices, such as super-capacitors.
Assuming that the converters used in this multi-bus structure are all highly efficient and modularized, additional benefits can be achieved:
➢ Light load efficiency: Modularized DC-DC converters can parallel in array, and part of the array can be disabled during light load conditions . This method can increase light load efficiency more effectively than conventional methods such as phase shading and pulse skipping.
➢ Redundancy: Modularized converters can easily achieve higher system reliability by providing N+1 redundancy.
The major challenge of this multiple-bus system is to design the DC-DC converters 1-4 with high efficiency and high power density modules.
1) Fixed ratio isolation-only converter
The Sine Amplitude Converter   could serve as the fixed-ratio isolation-only DC-DC converter. The Sine Amplitude Converter uses the natural resonant frequency of the primary side and switches at the zero crossing points of its resonant portion. S1, S2, S3 and S4 always turn on and off as the resonant current is zero. As S1 and S3 turn on (during t1 through t2), the primary side resonant current IP flows as a sine wave, until it reaches zero. Then S2 and S4 turn on and the primary side resonant current IP flows in the opposite direction, still with a sine wave shape, as shown during t2 through t3. The switching between S3 and S4 makes the secondary side current IS remains postive to supply the output.
Because the switching loss in this converter is approximately zero, this converter is capable of operating at a very high switching frequency, up to several MHz, which enables high power density. As a full zero-current-switching (ZCS) on secondary and partial ZCS on primary topology (the error is due to the magnitizing current, and zero-voltage-switching (ZVS) on the primary side has been used to make the switching loss negligable), existing Sine Amplitude Converter products are able to achieve 92-98% efficiency from 150 W to 1k W. They can be put in parallel to achieve even higher power levels.
2) Regulation-only converter
The ZVS buck-boost converter  could serve as a regulation-only DC-DC converter. During the first stage, S1 and S4 turn on, and the current flowing through the inductor IL increases at a speed proportional to Vin. During the second stage, S3 turns on and S4 turns off; IL could be flat or decrease/increase depending on the difference between input and output voltage. During the third stage, S2 turns on and S1 turns off; IL decreases at a speed proportional to Vout. During the fourth stage, S4 turns on and S3 turns off, and a slightly negative current goes through the inductor. During the transition, the zero-voltage-switching buck boost controller is used to enable zero voltage transitions and high efficiency.
Existing ZVS buck-boost converter products are able to achieve up to 97% efficiency from 120 W to 400 W, with a switching frequency of approximately 1MHz, which enables high power density. They can be put in parallel to achieve even higher power levels.
3) Regulated and isolated converter
The transformer-coupled ZVS buck-boost converter    could serve as the converter with both regulation and isolation. Figure 7 (a) is the schematic. During the first stage, S1 and S4 turn on, and the primary side current of the transformer IP increases at a speed proportional to Vin. During the second stage, S2, S3 and S5 turn on; the secondary side of the transformer takes over the energy stored in the first stage, and then the current IP decreases at a speed proportional to –Vout. During the third stage, S2 and S4 turn on, and a slightly negative current IP goes through the primary side. During the transition the zero-voltage buck-boost controller is used to enable zero voltage transitions and high efficiency.
Existing transformer-coupled ZVS buck-boost converter products have achieved 93-94% efficiency range from 450 W to 600 W with a switching frequency of up to approximately 1MHz, which enables high power density. They can be put in parallel to achieve even higher power levels.
B. Protocols and Standards
To enable this multi-bus system, protocols and standards need to be researched and regulated in order to ensure that the various types of vehicle components are compatible with each other. As mentioned before, different types of loads work with the best efficiency at different input voltages. A standard for these voltages needs to be researched and identified. For the electric equipment controlled by the automobile central controller, such as the DC-DC converters, a digital controller interface is required. Thus a universal automotive protocol also needs to be developed, or selected from existing protocols.
V. Efficiency Improvement Achieved with this Structure
There are several levels of power loss savings achievable with this multi-bus system compared to a single-bus system.
1) Load voltage optimization for increased power efficiency
This is related to the load analysis, as mentioned before. Different loads achieve the best efficiency and performance at different input voltages. This is a major reason why this multi-bus system is proposed: to provide multiple voltages to achieve the best efficiency for various loads, yielding an overall high efficiency system.
An individual load may gain greater efficiency through optimization of the load voltage. One example is for the audio system: by increasing the input voltage of the audio system and using a class D amplifier, it typically gains up to 40% greater efficiency compared with the current vehicle-standard class AB amplifier.
Overall, at least a 20% efficiency increase could be expected via load voltage optimization.
2) Bus voltage variation power loss saving
As input voltage changes, the efficiency will also change. If the bus voltage has variation, the efficiency may vary too. The efficiency changes as the input voltage changes for different types of loads due to: (a) constant resistor load (b) constant current load (c) constant power load (d) constant voltage load. If the maximum efficiency is 99.9%, only constant voltage load will show a significant efficiency variance over line voltages. If the maximum efficiency is 99% or lower, the variance of efficiencies over line will be noticeable. If the maximum efficiency can only achieve 90%, the constant current load, constant power load, and constant voltage load can drop to 82%, 50%, and 50% efficiency respectively, at 9 V, 9 V, and 16 V respectively. One should note that the constant resistor load distribution efficiency is constant over all line voltages.
The loads with a switch mode power supply can be treated as constant power load. It is probably common for high power loads to be constant power load. The loads with a linear power supply can be treated as constant current load (for example, a LED driven by a linear regulator). The battery is a typical constant voltage load. The heater for seats/windows is a typical resistor load.
Notice that sometimes the bus voltage variance power loss and load voltage optimization power loss do not agree at a common optimum input voltage, thus one needs to look at them together. For example, if an LED light has a linear power supply, it looks like a constant current load in the system. As the input voltage increases, it looks like the efficiency is higher. However, the LED will still see the same voltage and make the same amount of light. The additional power is actually absorbed by the linear regulator, so the efficiency inside the LED light system is lower. If these two losses are added together, the minimum voltage to drive the linear power supply would achieve the highest efficiency overall.
At least another 10% overall efficiency increase (rough estimation) can be expected for the bus voltage variation power loss savings.
3) Idle power savings
With a multi-bus structure, the voltage “cranking deep” has been separated and regulated for loads sensitive to voltage variation. This enables the use of the auto start-stop feature, which yields a typical 10% increase in fuel economy .
4) Converter power loss savings
A best-in-class converter for hybrid and electric vehicles has a typical peak efficiency of 90%. During full load, this architecture is probably able to improve efficiency by approximately 3%. However, it saves much more power during light load.
Once a vehicle goes to steady state, it runs at light load. By disabling part of the converter array during light load, light load efficiency can be improved. Assuming a full array has 80% efficiency at 10% load, by simply turning off half of the converter modules at light load, a 10% improvement can be achieved.
5) Overall power loss savings
The overall system power loss is divided into three areas: power loss from sources, converters, and loads. On the source side, assuming 50% of the power is from the engine, then a 10% × 50% = 5% power loss savings can be expected. On the converter side, for converters that spend 90% of the time working at light load, a 3% × 10% + 10% × 90% = 9.3% power loss savings can be achieved. On the load side, at least 20% power loss savings can be achieved by optimizing the load voltage, and 10% power loss savings can be achieved by fixing the bus voltage, thus yielding 30% total power loss savings.
The single 12 V bus system that has been used in automotive electric systems for decades provides insufficient efficiency, and has reached its limit for providing power. The fundamental issues have been investigated and a multi-bus system is proposed as the solution for the next generation of automotive electric systems. Converters to fit this multi-bus system have also been researched. This multi-bus system would have much higher efficiency, smaller size, higher redundancy, and enable higher power levels from the alternator.
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12 July 2013