Due to its characteristics, 48V has always been considered as the highest “safe voltage.” According to standard and certification agencies such as UL or IAC, 60V is the voltage level where extra protection on the cables (such as galvanic isolation or extra strength insulation) starts to be necessary. To minimize the costs and the protection grade, a voltage underneath 60V shall be selected. That is the reason why 48V voltage level has been chosen: it represents a good tradeoff between the need to minimize power losses and the duty to provide proper protection and insulation.

The future of 48V

There are several applications where 48V power architecture can provide a valuable and efficient solution. We can identify three major areas of interest: automotive, cloud and robotics. In the next paragraphs, we will examine what is driving 48V power supply in each of those fields and how it is possible to leverage some improvements from one area into another to create innovative solutions to the problems.

Automotive

Automotive electrification trends, outlined in Figure 1, show that today electrified vehicles already dominate the market because of the widespread of 12V start/stop feature.  As we go forward, that trend will continue in the next years with mild hybrids, full hybrids, and electric vehicles. In all electrified automobiles the 48V bus can be successfully adopted, and more functionalities will be added with autonomous drive and infotainment solutions.

Figure 1: electrification of automobiles

Regarding the automotive power nets shown in Figure 2, they are currently based on the legacy 12V bus which is still going to be there for quite some time. Figure 2 outlines, for instance, the 12V battery and the belt starter generator which is going to start the motor and maybe regenerate some power during the braking. The interesting thing is that all this design can still correctly work with a 48V bus. As time goes on all the automotive loads are going to migrate from the 12V bus into the 48V bus, and DC/DC converters will become bi-directional enabling sharing of power back and forth. As automobiles become fully electrified, 12V systems are going to be phased out.

Figure 2: 48V automotive applications

By adopting an automotive 48V bus, we can get higher power density and therefore smaller motors. Except for the power train (which requires a much higher voltage), several automotive applications can be supported by a 48V bus: starter generator, battery management system, auxiliary motors, DC/DC converters, and eFuses.

Cloud

Cloud relies on powerful and efficient servers which take care of all the relevant computational and networking activities. Figure 3 shows the block diagram of a typical server blade, which includes a 12V power bus, some protection devices, multiphase controllers, peripherals, points of load (POL) and a low dropout regulator (LDO) providing a clean power supply.

Figure 3: server blade block diagram

Peak power in systems like that is today around 400W, but in a few years, it is going to reach and exceed 1 kW. With such high rack power densities, a 48V bus demonstrates to be a better solution than 12V, since it involves lower transmission losses and helps to reduce the conductor size. A gradual approach can be chosen to introduce 48V bus in cloud applications. The upper side of Figure 4 shows a traditional power architecture with an UPS at the front end, an AC bus which goes around the racks and a series of voltage converters (AC to 48V, 48V to 12V and 12V to the point of load). That is indeed a good solution since much infrastructure has already been deployed around 12V. An alternative solution is shown in the lower side of Figure 7, which relies on the assumption AC/DC conversion is performed at the front end of the facility. A 400V DC bus powers the rack, where only DC/DC conversion is now be performed, and the UPS can be directly attached to the rack. The drawback of this solution is that it is harder to protect DC high voltage (the 400V bus) than AC high voltage.

Figure 4: cloud power architectures

A more innovative architecture is depicted in Figure 4, where the 48V bus goes directly to the point of load. That kind of solution is a quite challenging solution, due to requirements imposed by the core processor. In fact, the core processor requires a voltage of about 1V, but with very tight tolerance on that value. We have to convert the nominal 48V down to 1V, which means a duty cycle of only 2%. Also, high dynamic requirements impose a high efficiency across the loads. Figure 5 shows high efficiency and high density switched tank converter designed and made available for free by Google, demonstrating a great interest in enabling the ecosystem around 48V. The main idea would be to have DC/DC converters with 48V in input and 12V/6V or lower voltages in output. Other aspects should also be kept into account, such as:

• accelerator card: if it is plugged in the server board, a high priority shall be given to space occupancy, and that would maybe necessitate a different solution
• GaN components: it could represent another solution due to their high switching frequency and efficiency.

Figure 5: getting 48V to the POL

Today, on the market are already available pre-built power modules, such as the Flex Isolated DC bus converter able to convert 48V down to 1V. Other solutions are already planned for the next few years.

Robotics

Another area where 48V power could be applied is industrial automation: we are referring to collaborative robots, delivery drones, and similar devices that are battery operated. Collaborative robots, for instance, can be easily powered by a 48V battery, a solution which is both safe and efficient. We know that robots involve many motor solutions in 12V and 24V. For the sake of simplicity, consider the size of a robot arm. The size of the joint is going to be determined by the motor inside of it. To minimize the space occupancy, the power density shall be increased. Again, 48V proves to be the right power solution, well within the safety voltage range.

Applications and solutions

We can summarize the considerations mentioned above in a diagram which shows how power solutions work across applications (Figure 6). The first column refers to AC/DC conversion: in automotive that solution is not needed since in electrified cars we already have a 400V or 800V DC battery. In server and industrial automation applications, as we have seen before, we need it. Next column is related to high voltage DC/DC. In cloud systems, high DC power distribution might be used or not; however, in automotive it is needed to convert EV battery voltage down to 48V.

Then we have mid voltage DC/DC which extends across all the applications. Multiphase buck converters will be needed in automotive to fulfill autonomy and infotainment demand for more processing power. In cloud systems, they can be used to convert 48V bus down to the core voltage. In industrial automation, they might be used to power all the sensors and processing units involved in robots. Moving on to the point of load, we encounter a similar situation: we have sensors, peripherals and maybe computing devices in automotive, cloud server and industrial applications. Another essential power solution is the motor drive. We know there are several kinds of motors inside an automobile: fans, pumps, turbochargers, compressors and more. In cloud systems, server motor drives are typically needed for cooling systems.

In industrial automation motors are obviously used to provide motion and to support other robot functionalities, such as gripping, lifting, traction, actuation and so on. The next column is related to eFuse: in cloud server, that solution allows hot swap of devices providing protection on the bus. In industrial automation, we can think about a scenario where there are modular robots, and we can plug new modules while the system is live. We will see further automotive applications of eFuse in the next paragraph. The last column is straightforward: wherever there is a battery, we need a battery management solution.

Figure 6: 48V applications and solutions

eFuse: features and functionality

Hot swap controllers are based on eFuses, which are integrated protection devices used to limit current and voltage to safe level during the operations. The eFuse is, therefore, a protection device that allows, while a bus is live, to plug something in or take it out, protecting both the bus and the hot-swapped device. A typical eFuse features several protections such as over current, over voltage and reverse current, has diagnostic features and can be reset either automatically or remotely from a controller.

Figure 7: an eFuse overview

Figure 7 outlines a typical situation, without eFuse, where inrush current and high voltage spike are generated when a device is plugged in. Figure 8 shows how a much more gradual rise in load current and load voltage can be achieved with eFuse.

Figure 8: load current and voltage when eFuse is applied

Automotive applications can benefit from eFuse technology, as well. If we look at a traditional automobile wiring diagram (Figure 9), we can see a junction box where everything needs to run through and some long heavy gauge cables that bring DC voltage to the loads. The junction box contains fuses, and each time one of them burns we need to open the box and replace it with a new one.

Figure 9: an automotive traditional wiring diagram

This scenario can be improved using eFuses, as shown in Figure 10. Here we have one 48V circuit that goes to all corners of the car, and on each of them, we have an eFuse protecting the cable and an eFuse protecting the load. A number of relays and switches can be reduced, and cables can be lower gauge since they are shorter. Total wiring weight and costs can be reduced, as well. Maybe in the future, it will be possible to reset eFuses directly from a smartphone. A solution based on eFuse is indeed a significant improvement, making cars more like an electronic device and less like a big mechanical device.

Figure 10: eFuse-based wiring diagram

ON Semiconductor 48V solutions

ON Semiconductor offers a wide range of 48V Mosfet solutions with a variety of break down voltages, technology, and package suitable for high-frequency switching or motor drive applications. ON Semiconductor is also renowned for its industry-leading FETs, like those used in the FDMF8811 100V power stage (Figure 11), available in a compact PQFN package.

Figure 11: Fet-based 100V power stage

ON Semiconductor has devices with even further integration, such as the buck converter shown in Figure 12. With an input voltage up to 65V, it features a variety of output currents and voltages. It can provide an output voltage of 48V or any other value down to 4.5V.

Figure 12: FAN65xxx buck converter

Conclusion

The requirements of many markets are coming together to develop a 48V ecosystem, and there is much opportunity in many applications to deploy innovative solutions able to solve existing problems.

https://www.powerelectronicsnews.com/problems-solutions/the-emerging-48v-ecosystem%EF%BB%BF

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