Introducing DC-DC converters to reduce weight and cost of automotive wiring machines

As electrical systems in modern vehicles proliferate, traditional 12 V electrical systems in EVs (electric vehicles)/HEVs (hybrid electric vehicles) creak under the stress.

The relatively low voltage in the 12V system results in high currents that require a lot of wiring, which is expensive, bulky, and difficult to run through the tight spaces of modern electric vehicles, affecting the efficiency of the vehicle. To address this problem, automakers are gradually introducing 48V systems that can provide higher power while reducing the weight and cost of wiring machines.

However, due to the number of legacy 12 V products used in vehicles, switching to a single 48 V system is not practical in the short term. The solution is to run 12 V and 48 V systems together, each with its own battery. Managing the power and charging of these different voltage systems can be complex if separate DC-DC converters are used for each system. The introduction of bidirectional DC-DC converters - which can act as a bridge between 12 and 48 V systems - simplifies design, reduces cost, and encourages adoption in lower-priced vehicles.

design:

In a typical electric vehicle architecture, low-power applications may be connected to the 12V side, while high-power applications (usually required for motors and heating elements) are connected to the 48V. At the heart of these mixed-voltage systems is a bidirectional DC-DC converter, bridging the two voltages. This important subsystem is both a step-down (“buck”) and step-up (“boost”) converter, allowing charging from another battery (Figure 1).

The bidirectional approach allows the same external components, including passive devices such as inductors and capacitors, to be used for both step-up and step-down conversions. As a result, size and weight are reduced, which improves vehicle efficiency/range and reduces manufacturing costs. Bidirectional DC-DC converters can also combine energy from both systems to provide as much power as possible when current consumption is greatest, such as when starting the vehicle.

Figure 1: Hybrid 48V/12V systems are typically segmented based on power requirements

Bidirectional DC-DC converters are divided into two types based on the galvanic isolation between the input and output: non-isolated and isolated bidirectional DC-DC converters. There is little need for galvanic isolation in automotive systems because all voltages are separated extra-low voltage (SELV), which is part of the reason for choosing 48V. Therefore, bidirectional DC-DCs are often non-isolated to avoid the weight and cost of transformers. Therefore, non-isolated multi-device interleaved bidirectional DC-DC converters (MDIBC) are a common solution.

The 12V supply comes from a sealed lead-acid battery, while the 48V supply can be a battery or a supercapacitor (SC), or usually a combination of both, capable of delivering peak current when needed.

Figure 2: Schematic of a Multi-Device Interleaved Bidirectional DC-DC Converter (MDIBC) The multiphase approach of the MDIBC shown in Figure 2 relies on interleaving the gate drive signals, which reduces the input ripple. In fact, acceptable input and output ripple levels can be achieved without increasing the value (and therefore, size and cost) of the passive components. The trend towards using wide bandgap (WBG) semiconductors is allowing for higher operating frequencies, thereby reducing the size of the passive components.

Unlike many traditional topologies, the MDIBC shares commonality with control circuitry, thermal management, and DC link capacitors, all of which increase overall reliability. The ability to allow power to flow in both directions means it can accommodate systems such as regenerative braking, returning power to the battery and improving the vehicle’s overall efficiency and range.