John Grabowski, EV/HEV Systems Applications Manager, ON Semiconductor
With the very recent reassertion by scientists that the current trajectory for reducing the rate of global temperature increases is simply not good enough, there is now even greater pressure to reduce vehicle emissions of CO2 gasses. The best way of achieving this is through a reduction of the average vehicle’s fuel consumption. The use of hybrid engineered vehicles, as opposed to pure internal combustion engine (ICE) variants, is one method of reducing fuel usage.
The powertrains of all vehicles must be able to operate over a very wide range of power and speed conditions typically referenced as a torque-speed range. The use of a hybridized powertrain allows system designers the freedom to optimize the two power sources at different points of the torque-speed range.
The electrical power source is able to provide a very large amount of torque, useful for acceleration, but it is only available for limited periods of time. The specific time is dependent on both the size of the battery and the torque output of the electrical machine itself. With the availability of the high torque producing power source, the ICE can be significantly downsized resulting in improved fuel efficiency. However, the addition of this hybrid power source is certainly not a straightforward engineering task and necessitates an approach that has a knock-on effect for many other vehicle systems.
To-date, electrification has been achieved by adding a high-voltage (in the region of 350V) battery and a high-performance electric machine directly coupled to the ICE powertrain. These ‘Full’ Hybrid vehicles (HEVs) have set the standard and have strong appeal from an enhanced efficiency standpoint. However, they tend to add a considerable amount of cost and weight to the vehicle.
Recently, 48V vehicle system architectures have received a significant level of attention. These systems can be thought of as a partial step towards a full HEV. Typically, they are referred to as ‘Mild’ Hybrid Electric Vehicles (MHEV). They are based around a relatively compact 48V battery, a high performance electric machine, and multiple 48V electrified sub-systems. The lower cost of 48V systems makes them attractive to automotive OEMs, and they look set to become part of most vehicle manufacturers’ ranges.
The MHEV Dual-Voltage Architecture
Currently, there are a large and increasing variety of 48V architectures. Most include a battery, a starter-generator, a voltage converter, and typically at least one 48V load. Since 48V vehicles still retain a 12V battery and multiple 12V loads, it is likely that these systems will exist as dual voltage architectures for the foreseeable future. See Figure 1.
With the use of a 48V dual-voltage architecture, many new vehicle electrical configurations are possible. Because the 48V system is fundamentally capable of higher power levels, the use of new higher powered sub-systems is possible. With the inception of the 48V system, the integration of 48V E-Compressor and 48V E-Roll stabilization systems, in 12V topologies, are now possible. In addition, the higher power availability will encourage the higher powered 12V loads to migrate to the 48V bus taking advantage of the increased efficiencies.
Initially, the 12V side of the dual voltage system will remain as is, without the 12V alternator. Since the only power generating source is the 48V alternator, the system will require a converter that will transfer the 48V generated power to the 12V battery. This converter needs to be compact, lightweight, and highly efficient. It is designed to be bi-directional, which enables the combined use of both batteries during periods of high current demand, required in situations such as cold starting. The bi-directional converter is able to convert power from either battery and transfer the power between them.
On the 48V side, the starter-generator is the primary component. It is responsible for generating all of the vehicle’s electrical power and start the vehicle. In addition, the starter-generator performs regenerative energy recovery during vehicle braking. In this mode, the machine acts as a generator providing negative torque to the powertrain which slows down the vehicle and recovers electrical charge for the battery. Starter-generators come in many configurations and power levels, each with very specific vehicle implementation goals.
48V MHEV DC-DC Converters
The bidirectional power converter is required to share charge between the two battery systems and typically has power ranges in the 1kW to 3kW range. The most popular topology utilized to maintain high efficiency over this large power range is the multi-stage buck boost converter. The buck topology permits power to flow from the higher voltage side down to the lower voltage side. The boost topology permits power flow in the opposite direction. A multi-stage design permits the sharing of many individual converter sub-circuits combined into a single high-power converter. This multi-stage design permits the shedding of some of the stages during light load conditions. The multi-stage shedding function is facilitated by the use of an output MOSFET’s that can be switched on or off to enable each particular stage. A typical converter design is shown in Figure 2. The highlighted boxes indicated areas where ON Semiconductor has significant product offerings.
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Figure 2. A Single Stage of a Multi-stage DC-DC Converter
The MHEV has a wide variety of conceivable 48V sub-systems, more than are possible with 12V only systems. The highest power load on the MHEV is the electrically controlled supercharger known as the E-Compressor. Since a supercharger needs to accelerate to extremely high speeds in fractions of a second, it requires large amounts of transient power. A typical supercharger drive consists of a low inertia three-phase motor driven by a three-phase power inverter. Peak power levels can reach upwards of 8kW, even though it has a relatively low average power. These wide power range profiles are a perfect match for 48V systems that generate large amounts of power for relatively short periods of time.
Many other vehicle sub-systems are also ideally suited to the 48V architectures for both single and three-phase configurations. A listing of potential 48V sub-systems can be seen in Figure 3.
Other 48V Systems
Many 48V sub-systems consist of low power three-phase motor drive systems. These motor drives are most conveniently realized by use of a module design. The ON Semiconductor line of Automotive Power Module (APM) three-phase modules are a perfect choice. These modules are a compact and efficient choice for the construction of a three-phase auxiliary motor drive system. The module has a six-device power bridge, internal current shunt, snubber capacitor and is constructed on an isolated ceramic substrate. A 48V inverter reference design has been built for evaluation of our 80V APM devices. The system configuration and picture is shown in Figure 3.
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Figure 3a. 48V Sub-systems Summary
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Figure 3b. An APM Based 48V Three-phase Auxiliary Motor Drive
It has the following features:
This three-phase motor drive system is in the final testing stage at the ON Semiconductor power laboratory in Ann Arbor, Michigan. ON Semiconductor will provide additional details when the testing is finished.
48V System Component Offerings
ON Semiconductor offers many components that are applicable to 48V electronic system design. A brief subset of these devices are shown in Table 1. We offer MOSFET’s in various discrete and modular packages. Our line of current sense amplifiers are capable of converting current shunt voltages, even in designs which have large common-mode voltage, into logic level signals. In addition, we are constantly developing new devices to expand our already world class portfolio. New device packages are also in the final stages of AEC qualification and will soon be released for production.
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Table 1. 48V MHEV Solutions Application Devices
48V Sub Sub-system Proliferation
One possible consequence of the 48V revolution, will be the generation of wide varieties of 48V peripherals. This proliferation will likely reduce the cost of 48V sub-systems and make them a more attractive addition to other high-voltage based vehicles. If these peripherals are included in Full- Hybrid or electric vehicles they will need to be powered by a 48V source, thus introducing the need for a triple voltage architecture. This architecture will generate a new solution component requirement for xEV vehicles, called the “the triple voltage converter”.