Electronics and Semiconductors

DC-DC converters for EVs

08 December 2023
Source: U.S. National Renewable Energy Laboratory

Think back to the days when semiconductors were not common, when the difficulty lay in increasing DC voltages for low-power initiatives. A series of vibrator circuits converted DC to AC, and then step-up transformers were used to boost the voltage. The rectifier circuit was the final frontier since it enabled the DC conversion. A motor-generator set was brought in for high-power conditions, with the motor's action spinning the generator to provide the required load voltage.

Although these strategies were successful, they were expensive and inefficient, yet they remained popular owing to an absence of other options. With the advent of power semiconductors and integrated circuits, the transformer-based DC power supply that uses high-frequency AC input to modulate voltage has emerged as the clear winner in electric vehicle (EV) applications. Rectifiers were used to convert the AC power back into DC, making dynamic voltage regulation possible.

Why EVs need DC-DC converters

To meet the increased power needs of EVs and hybrid vehicles, battery voltages have increased to the 300 V to 800 V range, although current flows and cable complexity have remained reasonable. However, not all electrical loads in a car work on these voltages, which is where the DC-DC converter comes in. This multipurpose component, sometimes referred to as an auxiliary power module (APM), may be found in any electric or hybrid vehicle that uses a high-voltage battery.

The DC-DC converters for EVs often fall into one of two categories — those that step down high voltages (buck converters) or bridge low voltage gaps by stepping up (boost converters). For instance, a vehicle's primary DC-DC converter is responsible for reducing the voltage from the battery to the standard 12 V, which is then used by a wide variety of systems, including headlights, pumps and window motors. The adaptability of this converter makes it ideal for meeting a wide variety of load requirements. In certain scenarios, DC-DC converters are called upon to handle bidirectional tasks, encompassing both voltage step-up and step-down functions, often encountered in mild hybrid setups.

Common types of DC-DC converters in EVs

DC/DC converters can be commonly classified into two main categories:

Converters without isolation

This kind of converter falls into use when there's a need to change the voltage by a relatively small factor (usually less than 4:1), and when there's no requirement for dielectric isolation between the input and output. Within this group, there are five primary types of converters: buck, boost, buck-boost, Cuk and charge-pump converters. The buck-boost and Cuk converters can perform both voltages decrease and increase tasks. On the other hand, the charge-pump converter is suitable for either boosting the voltage or inverting it, though it's typically used for lower power applications.

Converters with isolation

In this category of converters, a high-frequency transformer is commonly used. Situations where complete isolation is required between the output and input call for isolated converters. Numerous converter types exist within this group, including half-bridge, full-bridge, fly-back, forward and push-pull DC/DC converters. All these converters offer the advantage of being bidirectional and are capable of achieving high voltage decrease or increase ratios.

Factors to consider when designing a DC-DC converter for an EV

When selecting a DC-DC converter for an EV, there are several factors to consider. One important factor is the power transfer capability, as the converter should be able to provide reliable and rapid energy transfer. Efficiency is another crucial factor, as the converter should be able to efficiently convert and transfer power between the energy sources and the EV. The number of components and component count is also important, as a low component count can lead to a more compact and cost-effective converter. Additionally, the converter should be able to handle high-power and high-current levels, making it suitable for practical EV applications. Finally, the control system implementation and the choice of control strategy should be considered to ensure proper operation and performance of the converter.

Advantages of DC-DC converters for EVs

  • The adaptable DC-DC converters offer efficient energy conversion and can address the limitations of modern energy systems.
  • Many auxiliary systems and components in vehicles are designed to operate at standard automotive voltage levels (e.g., 12 V). Since EVs often use higher-voltage battery systems, DC-DC converters ensure compatibility by providing the necessary voltage levels for these components.
  • Bidirectional converters are suitable for low voltage applications and can transfer renewable energy efficiently, but they may not be appropriate for high voltage applications due to low gain.
  • Bidirectional converters also help in applications like regenerative braking, where energy generated during braking can be converted back into usable energy and stored in the main battery.
  • Isolated DC-DC converters provide an essential safety feature in EVs. They create a barrier between the high-voltage components, like the traction battery, and the low-voltage systems.

Conclusion

The DC-DC power electronics converter transforms DC voltages from one level to another, tailored to the specific demands of circuits. This fundamental component is used across a diverse range, from humble battery applications in EVs to the high-voltage realm of power transmission. Their ability to efficiently manage power, ensure compatibility, enhance safety and optimize vehicle components makes them a crucial factor in the successful operation and integration of diverse technologies within the EV ecosystem.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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