Fast DC Charging for Electric Vehicles: Common System Topologies and Power Devices

The role of direct current fast charging (hereinafter referred to as "DCFC") in removing barriers to electric vehicle adoption is clear. The demand for shorter charging times has driven nearly 400 kilowatts of high-power electric vehicle fast charging into the market. This blog will give an overview of typical power converter topologies and AC-DC and DC-DC power devices used in DCFC.

Figure 1. Electric vehicle DC fast charging architecture diagram

Active rectification three-phase PFC boost topology

Three-phase power factor correction (PFC) systems (also known as active rectification or active front-end systems) are gaining increasing attention, with demand growing dramatically in recent years. PFC topology is critical to power DCFC efficiently. Incorporating silicon carbide (SiC) power semiconductors into your PFC topology can solve challenges, reduce power losses and increase power density.

The front-end PFC boost stage can be implemented in a variety of topologies, and several topologies can meet the same power requirements. Figure 2 shows a common PFC architecture in DCFC applications. One of the first differences between them is bidirectionality. T-neutral point clamped (T-NPC) and I-NPC topologies are suitable for bidirectional operation by replacing some diodes with switches. The structure of 6 switches is a two-way perse.

Figure 2. Typical PFC boost topology for DCFC. T-NPC (top left), 6-switch (top right), and I-NPC (bottom)

Another important factor that affects the design and power device voltage rating is the number of stages in the architecture. The 6-switch topology is a 2-level architecture typically implemented with 900 V or 1200 V switches for fast DC EV chargers. Here SiC MOSFET-modules are the preferred solution in the low RDS on (6-40 mQ) region, especially for the high power range above 15 kW per block.

This integration demonstrates superior power performance over discrete solutions, improves energy efficiency, simplifies design, reduces overall system size, and maximizes reliability. T-Neutral Point Clamp (T-NPC) is a 3-level topology using a 1200 V rectifier (replaced with switches in bidirectional form) with 650 V switches back-to-back in the neutral path. I-NPC is a 3-level architecture and may be implemented entirely with 650 V switches. 650 V SiC MOSFETs or IGBTs with co-packaged diodes represent excellent alternatives to these 3-level topologies.

Figure 3.F1-2 PACK SiC MOSFET half-bridge module. 1200 V, 10 mΩ

DC-DC topology

When studying the DC-DC conversion stage, three isolation topologies are mainly used: full-bridge LLC resonant converter (LLC converter), full-bridge phase-shifted dual active bridge (DAB) zero-voltage transition (ZVT) converter ( DAB-ZVT converter) and full-bridge phase-shifted zero-voltage transition converter (ZVT converter) (Figures 4, 5 and 6).

Full Bridge LLC Resonance

The LLC converter implements zero-voltage switching (ZVS) on the primary side and zero-current switching (ZCS) on the secondary side at and below the resonant frequency, resulting in very high peak efficiency near the resonant frequency. As a pure frequency modulation (FM) system, the energy efficiency of the LLC decreases when the system operating point deviates from the resonant frequency, which may be the case when wide output voltage operation is required. However, advanced hybrid modulation schemes enable today's pulse modulation (PWM) to be combined with frequency modulation, limiting maximum frequency runaway and high losses. However, these hybrid implementations still add complexity to the already sometimes cumbersome LLC control algorithm. In addition, current sharing and synchronization of parallel LLCs converters is not an easy task. In general, LLC is a hard-to-beat design when it is possible to operate over a relatively small voltage range, and/or when the development skills are available to implement advanced control strategies combining FM and PWM. Not only does it offer maximum energy efficiency, it is also a very comprehensive solution from every angle. LLC can be implemented in bidirectional form as CLLC, which is another complex topology.

Figure 4. Full-bridge LLC converter

Full-Bridge Phase-Shifted Dual Active Bridge (DAB) Zero Voltage Transition (ZVT) Converter

DAB-ZVT converters with secondary synchronous rectification topology are also very typical. These work with PWM and generally require simpler control than LLC converters. DAB can be thought of as an evolution of the traditional full-bridge phase-shifted ZVT converter, but with the leakage inductor on the primary side, which simplifies tedious secondary-side rectification and reduces the necessary breakdown voltage rating of the secondary switch or diode. Due to the ZVT implementation, these converters can provide stable high energy efficiency over a wide output voltage range. This is a convenience factor for chargers that support 800 V and 400 V battery voltage levels. DAB's PWM operation brings benefits. First, it tends to make the converter's electromagnetic interference (EMI) spectrum tighter than in an FM system. Furthermore, with a fixed switching frequency, system behavior at low loads is easier to resolve. With synchronous rectification, DAB is a bidirectional native topology and is one of the most versatile alternatives and suitable solutions for fast electric vehicle chargers.  

Figure 5. Full-bridge phase-shifted DAB ZVT converter

Full-bridge phase-shifted ZVT converter

For unidirectional operation, traditional full-bridge phase-shifted ZVT (Figure 6) is still an available option, but is becoming less and less widely available. This topology works similarly to DAB, but the inductor located on the secondary side brings a significant difference in rectification. The inductor sets a high reverse voltage across the diode, which will be proportional and inversely proportional to the duty cycle, so depending on operating conditions, the reverse voltage across the diode may exceed the output voltage by two to three times. This situation can be challenging in systems with high output voltages (such as electric vehicle chargers), where often multiple secondary windings (with lower output voltages) are connected in series. Such a configuration is not that convenient, especially if the power and voltage ratings are taken into account and a different topology with a single output will provide the same or better performance.

SiC-modules represent a very suitable and common solution for full bridges in the above-mentioned DC-DC power conversion stages with powers above 15 kW. Higher frequencies help reduce the size of the transformer and inductor, thereby reducing the overall solution size.

Figure 6. Full-bridge phase-shifted ZVT converter

Variants of topology

Several variations of the discussed topologies exist, bringing additional advantages and trade-offs. Figure 16 shows a common alternative to a full-bridge LLC converter for fast EV charging. In phase shifting, the switching is below half the input voltage, and devices with cut-off voltages of 600 V and 650 V are used. 650 V SiC MOSFETs, 650 V SuperFET 3 fast recovery (FR) MOSFETs and 650 V FS4 IGBTs will help address different system requirements. Likewise, the diodes and rectifiers used for the primary terminal require a blocking voltage rating of 650 V. These 3-level architecture allows for single-pole switching, which helps reduce peak current and current ripple, which results in smaller transformers. One of the main disadvantages of this topology is that the control algorithm requires an additional level of complexity compared to the Level 2 version with fewer power switches. Dual active bridges as well as dual active bridges can be easily paralleled or stacked on the primary and secondary sides to best match the current and voltage needs of fast EV chargers.

Figure 7.3-Stage Full-Bridge LLC Converter - This variant is stacked on the primary side (only half the input voltage is applied to each transformer) and in parallel on the secondary side

Secondary side rectification

Regarding secondary-side rectification, as shown in Figure 8, there are several solutions possible, and they can all use different topologies. For cell levels and full-bridge rectification at 400 V and 800 V, SiC Schottky diodes at 650 V and 1200 V are often unique cost-effective solutions. Due to their zero reverse recovery characteristics, these devices greatly enhance rectification performance and energy efficiency, significantly reducing losses and rectification stage complexity compared to silicon-based alternatives. Silicon-based diodes, such as Hyperfast, UltraFast and Stealth, can be used as alternatives for very limited cost projects, but at the expense of performance and added complexity. Solutions using center-tapped rectification (Figure 6) are not convenient for high-voltage output rectification stages. Unlike full-bridge rectification, in which the diode's standard reverse voltage is equal to the output voltage, in a center-tapped configuration the diode is subjected to twice this value. Conventional full-bridge phase-shifted converters (inductor on the secondary side), as explained, require higher breakdown voltage diodes in both rectification methods (full-bridge or center-tapped rectification). To overcome the need for 1200 V or 1700 V rated diodes in conventional full-bridge phase-shift converters, several outputs will be connected in series.