“The electrification of the automotive industry is developing at an ever-increasing rate, mainly driven by the government’s promulgation of carbon dioxide (CO2) emission reduction standards. The European Union has set a target of only 95g/km of new car emissions by 2020. Other countries such as China are enacting similar regulations. To meet these standards, automakers are developing light-duty hybrid electric vehicles that use secondary high-voltage batteries in addition to standard 12V car batteries.
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Author: Garrett Roecker
The electrification of the automotive industry is developing at an ever-increasing rate, mainly driven by the government’s promulgation of carbon dioxide (CO2) emission reduction standards. The European Union has set a target of only 95g/km of new car emissions by 2020. Other countries such as China are enacting similar regulations. To meet these standards, automakers are developing light-duty hybrid electric vehicles that use secondary high-voltage batteries in addition to standard 12V car batteries.
German automakers have begun to define and build systems based on 48V batteries. The 48V battery can provide more power at a lower current than the traditional 12V battery, while saving the weight of the wiring harness without affecting performance. In this development process, the LV148 standard has become the main starting point for dual-battery automotive systems. The top-level block diagram of the dual battery system is shown in Figure 1.
Figure 1: Block diagram of a dual-battery car system
What are the challenges of the proposed system? How to overcome obstacles? Many OEM system requirements state that energy must be transferred from the 48V rail to the 12V rail and vice versa. If the battery is discharged, two-way power transmission is required to charge the battery and provide additional power to the opposite voltage rail under overload conditions. In order to charge the battery without damaging the battery, the controller must be able to control the charging current very accurately. In most automotive applications, the maximum power transmission is not small, usually in the range of 2kW to 3kW. The voltage variation on the two rails can be large. According to the LV 148 specification, the 48V power rail is usually between 36V and 52V, while the 12V power rail can be in the range of 6V to 16V. The protection circuit must also be present for any fault conditions that may damage the system. With these requirements, it is obvious that the DC/DC converter required to bridge the 48V and 12V voltage rails is not a simple design project.
Realizing that the voltage ranges of the 48V power rail and the 12V power rail never overlap greatly reduces design complexity. For power transmission from the 48V power rail to the 12V power rail, a buck converter can be used, and a boost converter can be used to achieve power transmission in the direction of the 12V to 48V power rail. Due to kilowatt-level power requirements, each converter should use synchronous MOSFETs instead of freewheeling diodes to improve system efficiency.
Buck and boost topologies are well known in power electronics, but designing two separate converters will take up valuable board space and increase system complexity and cost. A closer look at these two topologies shows that the power chains of the buck and boost converters are very similar. The two topologies consist of at least two power MOSFETs, an Inductor, and a certain amount of output capacitance. The controller is the difference between topologies. In the buck topology, the controlled switch is a high-side MOSFET; in the boost topology, it is a low-side MOSFET. By simply changing the controlled switch, assuming you have selected the correct controller, you can change the direction of current flow in the inductor while using the same power train components. Figure 2 shows the evolution from two converter solutions to a single converter solution.
Figure 2: The evolution of a single-controller bidirectional converter
Although synchronous switching is necessary for high-current designs, it is not effective for all obstacles. At a power of 2kW, the 12V power rail will conduct approximately 166A. A quick look at these content, you will find that you will need multi-phase operation to achieve this design in actual operation. By using a multi-phase architecture, the physical size of components can be reduced and thermal management becomes easier. In order to more easily parallel each power phase, the control scheme in buck or boost mode operation should be current mode control. Multiphase operation also allows interleaved switching of each phase. Not switching each phase at each time can reduce output ripple, which in turn helps reduce electromagnetic interference (EMI).
In all systems, you must design protective circuits for operator safety. Common protection functions, such as undervoltage lockout (UVLO) and overvoltage protection (OVP), ensure that the battery will not be overcharged or overcharged. The peak inductor current limit helps prevent excessive stress on each power phase and saturates the inductor. In a dual-battery car setup, a circuit breaker is also needed to disconnect any electrical connections between the 48V and 12V rails. Monitoring circuits can also help expand safety functions. For example, during energy transfer, monitoring the current in each channel can indicate if or when a fault condition occurs.
Digitally controlled DC/DC converters are a possible solution, but this method has several major disadvantages. First, a large number of discrete components are required: current sense amplifiers, power MOSFET gate drivers, protection circuits, and monitoring circuits for each phase. Each component will take up valuable space on the printed circuit board (PCB). Second, a high-end microcontroller is needed to implement the converter’s current and voltage control loops. Third, the microcontroller also introduces a delay in the protection circuit, which can cause catastrophic damage at high power levels. Fourth, the design cycle of digital control can be on the order of several years. You must have an in-depth understanding of switching power supplies and digital control. That being said, there are some additional advantages. From a system level perspective, digital control can be more flexible, allowing dynamic changes in control program parameters and regulating voltage. Sharing information with other subsystems can improve overall system performance.
TI’s LM5170-Q1 synchronous two-phase bidirectional buck/boost controller solves many of these challenges. Integrated current sense amplifiers, high current gate drivers, and system protection features (including integrated circuit breakers and channel current monitoring) eliminate many discrete components required in digital solutions. Stacking multiple controllers in parallel can deliver kilowatts of power, while LM5170-Q1’s proprietary average current mode control scheme optimizes the control of current rechargeable batteries. Read the blog post “Selecting a Bidirectional Converter Control Scheme” to understand how TI’s average current mode control method compares with conventional control schemes. Bridging 48V and 12V batteries is complicated, but it is possible if you carefully consider the steps.
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