Using 48 Volt DC-to-DC Regulators in Parallel to Increase Power for Autonomous Vehicles

The move from 12 volt to 48 volt supply rails in automobiles is being driven by the insatiable demand for power as the number of electrical systems increases, including active suspension, steering, climate control, powered seats and windows, and advanced infotainment. It turns out that 48 volt power is the sweet spot for efficiently supplying lots of energy while eliminating the expensive components and heavy wiring that would otherwise be needed to meet stringent safety and performance standards for systems based on 12 volts.

In addition, the adoption of 48 volt systems will accelerate as the internal combustion engine (ICE) fleet gets replaced by electric vehicles (EVs). While EVs use 400 or even 800 volt systems to spin the wheels, manufacturers—notably Tesla, with its CyberTruck—have taken the opportunity that comes from designing a completely new vehicle to incorporate 48 volts as standard for the rest of the vehicle’s electrical systems (Figure 1).

Figure 1: Tomorrow’s vehicles will increasingly use 48 volts for systems such as the driver cockpit. (Image source: Vicor)

More power with lower resistive losses

But why move away from mature and proven 12 volt technology? After all, it did a pretty good job for decades, and it still has an enormous component supply chain with proven products. The simple answer is that while the modest vehicles of yesteryear required electricity for interior lights, the radio, and a cigarette lighter (yes, cars back then had built-in cigarette lighters), electric steering, GPS, and heated seats were the stuff of the car designers’ dreams. There was just no need for any extra power beyond the standard 12 volts. Moreover, lead-acid batteries provide around 2 volts per cell, so a standard six-cell battery supplied the requisite 12 volts with very basic (and cheap) voltage regulation.

But yesterday’s car designers’ fantasy is now today’s reality, and all types of vehicles, whether conventional, soft hybrid, hybrid EV (HEV), or EV, all demand more electrical power. The simple equation power (P) (in watts) = volts x current (in amperes, A) tells us that we can get more “umph” by either upping the volts or the current (or perhaps a bit of both). But we also know that P = A² x resistance (in ohms (Ω)). This tells us that achieving a power gain by simply increasing current multiplies resistive losses and adds to the many thermal management challenges.

It's better to raise the voltage to get more power from the same current. For example, imagine we need 75 watts from a 12 volt supply. This requires a current of 75/12 = 6.25 A. But if we raise the supply to 48 volts, the same power can be provided by a current of 75/48 = 1.6 A. A reduction in current of 75% means car designers can use lighter wiring, thereby saving on weight and pushing down costs (Figure 2).

Figure 2: By moving to 48 volt power for key systems, car designers can reduce the weight and cost of wiring harnesses. (Image source: Vicor)

48 volt power beyond automotive

Cars are not the only wheeled systems that can benefit from 48 volt buses. For example, warehouse robots, wheelchairs, forklifts, and last-mile autonomous delivery vehicles are taking advantage of 48 volt power (Figure 3). Unlike in the automotive example, these vehicles primarily use the 48 volt supply to power the traction motors; there’s little need for heated seats or infotainment in a warehouse robot.

Figure 3: Last-mile autonomous delivery vehicles get to their destination quicker and more efficiently thanks to 48 volt power for the traction motors. (Image source: Vicor)

For these product categories, 12 volt lead-acid batteries are being replaced by 48 volt lithium-ion (Li-ion) batteries made up of multiple cells. As engineers, we know that such batteries offer a nominal 48 volts, but as they discharge, their voltage drops. That means we’ll need good regulation to keep the voltage nice and stable as the battery gives up its energy.

Vicor offers an isolated (to 3000 volts DC (V_DC)), regulated DC-DC converter line that’s a good choice for this type of application. For example, from a 14 to 72 volt input, the DCM2322T72S53A0T60 converter offers a 48 volt regulated output at up to 2.1 A for a power output of 100 watts. The converter features a high-frequency, zero-voltage switching (ZVS) topology that reaches an efficiency of 90.1% to keep things cool and extend battery life while providing a high power density of 401 watts per cubic inch (in.³).

Boosting the power

The DCM2322T72S53A0T60 is an efficient regulator, but it might be that the application demands more than 100 watts for certain operational modes, such as when an electric wheelchair is climbing a hill. One option might be to specify a 48 volt DC-DC regulator with greater power output. For example, the DCM2322TA5N53A2M60 provides a maximum of 120 watts at 48 volts from a 43 to 154 volt supply. The trade-off when using a single, higher-power regulator is that there’s typically some loss in peak efficiency, and thermal management becomes more challenging.

Alternatively, the DCM2322T72S53A0T60 can easily be deployed as an array of modules to increase power output. Arrays of up to eight units have been qualified for a capacity of up to 800 watts. The good news is that the maximum power of the array is the sum of the maximum power from the individual modules. No derating is enforced by the array topology. Better yet, each module in the array can operate from a different input voltage supply, if necessary, while still providing up to 100 watts at 48 volts.

There is one design trade-off when using the DCM2322 modules in array mode: a decoupling network is needed to facilitate the parallel operation. In practice, this means that each DCM2322 simply requires an output capacitor before the output is fed to the common bus. Each DCM2322 also requires a separate input filter, even if all the modules share the same input voltage source. The job of the filters is to limit the ripple current reflected from each module, as well as to help suppress the generation of beat frequency currents that can result when multiple powertrain input stages are permitted to directly interact (Figure 4).

Figure 4: DCM2322 DC-DC regulated voltage converters can be arrayed to provide up to 800 watts; the schematic shows the input filters and decoupling networks needed for efficient and quiet operation. (Image source: Vicor)

Conclusion

Moving to 48 volt power from 12 volts brings key system advantages for applications such as warehouse robots, electric wheelchairs, and autonomous last-mile delivery vehicles. Such advantages include keeping the current down to enable the use of lighter and lower-cost wiring harnesses.

But as the sophistication of these end-products increases, so too do their power demands. You can scale your power system accordingly using multiple high-power-density, regulated DC-to-DC voltage converters from Vicor in parallel. That way, you can allow 48 volt systems to supply up to 800 watts or more.