Understanding trade-offs for DC-DC converters in EV charging

By Avnet Staff   |   June 24, 2024

When designing an EV charger, engineers are confronted with four primary considerations besides the required output power: cost, efficiency, reliability and bi-directional operation. Fast chargers are generally deployed as commercial chargers and rarely for residential use; so bidirectional operation has little bearing on the DC-DC converter's design, as its sole function is to supply power to the electric vehicle.

Cost is often the first consideration. The number of switches is one of the primary factors that determine the DC-DC converter's cost. Consequently, when cost reduction is the primary objective, a single LLC topology is likely to be selected. However, this topology places additional stress on components. Electrical stress could potentially affect reliability, which may increase operating and maintenance costs.

The semiconductor technology used dictates overall cost. When an aggressive cost target must be met, silicon IGBTs offer the most cost-effective solution. Designing for a higher selling price point supports the use of SiC MOSFETs. Increased switching efficiency is the primary benefit of using SiC, as it supports higher switching frequencies and higher breakdown voltages. Operating at higher frequencies can deliver considerable cost savings in the passive/inductive components as well as in the heat dissipation system (heatsinks and fans) application size and weight, but this should be weighed against the associated cost of the switches and related gate driving circuitry.

Due to SiC’s wide bandgap, the body diode of a SiC MOSFET exhibits a higher voltage drop than a silicon MOSFET, leading to a high conduction loss when operating as a rectifier. Therefore, it is customary to include an antiparallel SiC Schottky diode as a freewheeling diode (FWD) which in turn raises the component manufacturing cost.

Cost and efficiency are closely related. As mentioned previously, the increased SiC transistor cost is reflected as an overall application cost when considering only the immediate expense of replacing the power switches. A traditional three-level LLC converter configuration populated with Si transistors offers reduced energy cost in operation but has the greatest number of switches and would seem to be the costliest to manufacture, however using SiC transistors allows designers to reduce the configuration complexity by one order of magnitude, cutting the number of transistors in half.

Thermal management is a crucial factor for reliability. Although SiC technology exhibits greater efficiency and tolerance toward elevated temperatures, the thermal energy produced by IGBTs can be dissipated through the implementation of appropriately engineered heatsinks. Increased size and weight due to heatsinking are rarely significant concerns, given that off-board EV chargers typically do not have space limitations.

The complexity of the control logic (MCU or DSP) is proportional to the switching frequency. However, this circuitry has significantly less influence on the overall cost than the selection of switches.

Selecting the optimal converter topology, down to the components used, is an essential design activity when developing diverse EV charging systems. Advanced converter topologies are differentiated by the inductor and capacitor configuration, which has a big impact at a system level.

These topologies comprise LLC resonant converters, CLLC, CLLLC and three-level LLC converters. There are always trade-offs to be made and design considerations involved when meeting commercial objectives. Depending on the end-application’s requirements, these typically involve a balance between cost, efficiency and reliability. The same is true for DC EV chargers.

Characterizing EV chargers

An EV charger is categorized as either level 1, 2 or 3, related to the output power of the charger. A Level 1 charger can deliver up to 1.8 kW and is suitable for use in homes running from a standard 120 VAC wall socket.

Depending on the input power available, Level 2 can provide an output power of up to 19.2 kW at 80 A using a 208 VAC or 240 VAC power outlet. In Europe, the output power can reach 22 kW with a single-phase voltage of 230 VAC or a three-phase voltage of 400 VAC.

Level 3, also known as DC Fast Charging (DCFC), requires a three-phase power source, with a voltage of 400 VAC in Europe and 480 VAC in North America. Unlike levels 1 and 2, the output is already converted to DC. Level 3 chargers deliver 340kW or more.

What is the of DC-DC converter’s role in EV charging?

Level 1 and Level 2 chargers deliver AC, which is then rectified by the vehicle’s onboard converter (OBC) and fed to the battery. A DC fast charger removes the need for an OBC through its own two-stage conversion. The initial phase is an AC-DC converter, transforming three-phase AC electricity from the grid into DC. This intermediate DC voltage then feeds the second stage: a DC-DC converter.

The DC-DC converter adjusts the voltage to the vehicle’s needs, delivering a controlled transfer of energy to the battery. This dual-stage approach is intended to increase efficiency and minimize energy dissipation during the conversion and charging process.

Using off-board high-voltage conversion, DC fast charging removes the need for cumbersome and heavy transformers within the car. This also simplifies the charging process, and the result is faster charging. As electric vehicle designs evolve, more manufacturers will offer fast charging, increasing the need for off-board DC-DC converters.

DC-DC converter topologies for EVs

The future of EV fast charging depends on innovative high-power DC-DC converter topologies. But efficiency is becoming more important in relation to the limits of the power grid. While single-stage converters are simpler, their maximum power conversion efficiency is relatively low.

To address this, OEMs are focusing on multi-stage converter topologies with higher overall switching efficiency to maximize power transfer and reduce losses. In this article, we take a closer look at three DC-DC converter variants of the basic inductor-capacitor architecture: single LLC, CLLLC and 3-level LLC DC-DC conversion.

The LLC resonant converter topology is a DC-DC converter designed for high efficiency and soft-switching operation. The core of this converter is a resonant tank circuit on the secondary side, consisting of an inductor (Lm), a capacitor (Cr) and another inductor (Lr). These components determine the resonant frequency (Fr) of the tank circuit.

Using a full-bridge inverter on the primary side, the converter can achieve the necessary isolation with the help of a transformer. Operating at a switching frequency close to the resonant frequency enables the power switches to smoothly turn on and off under zero voltage conditions (soft-switching).

Utilizing the soft-switching technique results in decreased switching losses, which ultimately leads to improved efficiency. In addition, working near the resonant frequency can help minimize electromagnetic interference (EMI), which may result in the use of smaller magnetic components.

Nevertheless, the LLC topology requires a more complex design, and the range of output voltage is restricted in comparison to other converter types.

Comparing CLLC and CLLLC DC-DC converters for EV charging

The CLLC resonant converter topology incorporates the fundamental components of the LLC converter while introducing an additional capacitor (Cs) connected in series with the resonant capacitor (Cr). The capacitor (Cs) is used to capture the leaking inductance of the transformer and incorporate it into the resonant tank. This enables the use of a smaller and potentially less expensive resonant inductor (Lm). In addition, Cs allows power to flow in both directions.

The CLLLC resonant converter topology enhances the CLLC concept by incorporating an additional inductor (Ls) in series with the resonant inductor (Lm) on the secondary side. The inclusion of this extra inductor incorporates parasitic components into the resonant circuit and allows for a more precise control design, facilitating more refined optimization of the converter's performance.

The CLLC and CLLLC topologies retain the benefits of their predecessors, including high efficiency, reduced electromagnetic interference (EMI), and the potential for fewer magnetic components. Additionally, they have the capability for bidirectional power transmission. The more intricate resonant tank circuit, however, requires a more advanced control method in comparison to the LLC converter.

The Three-level LLC converter

This topology, also known as TL-LLC, is ideal for high-power designs with a wide output range and, therefore, is widely used in EV charging stations. This four-switch topology consists of two half-bridges in series, sharing a transformer and a resonant inductor.

Its main advantage is that the voltage stress on the switches is clamped to half of the input voltage, making it suitable for applications with high input voltages. Moreover, the complexity of the driving circuit is reduced, and low harmonic components are present in the output voltage.

Conclusion

Several trade-offs should be made when designing a DC-DC converter in an EV charger. These include cost, reliability, efficiency and bi-directional operation. The more suitable topology and components must therefore be chosen to match the specific requirements of the EV charger.

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