DC Fast EV Charging: Common System Topologies and Power Devices

by  Didier Balocco  - 08-31-2022 

The role of DC Fast charging (DCFC) is evident in removing barriers to EV adoption. The need for shorter charging times has resulted in higher power EV fast-charging approaches 400 kW entering the market. This blog will overview typical power converter topologies and power devices for the AC−DC and the DC−DC used in DCFC.  Diagram of Fast DC EV Charger Architecture

Figure 1. Diagram of Fast DC EV Charger Architecture

 

Active Rectification Three−Phase Power Factor Correction Boost Topologies

Three-phase Power Factor Correction (PFC) systems (also called Active Rectification or Active Front-End systems) are becoming of great interest, experiencing a sharp increase in demand in recent years. PFC topologies are essential for efficiently powering DCFC. Incorporating Silicon Carbide (SiC) power semiconductors into your PFC topologies can address the challenge of reducing power losses while increasing power density.

The front−end PFC boost stage might be implemented in multiple topologies, and several might fulfill the same electrical requirements. Figure 2 illustrates common PFC architectures in DCFC applications. One of the first distinctions to be made among them is bi−directionality. The T−Type Neutral Point Clamp (T−NPC) and I−Type Neutral Point Clamp (I−NPC) topologies are suitable for bi−directional operation by replacing some diodes with switches. The 6−switch architecture is bi−directional per se.

Typical PFC Boost Topologies for DCFC

Figure 2. Typical PFC Boost Topologies for DCFC. T−NPC (top−left), 6−switch (top−right) and I−NPC (bottom)

An additional important factor that will influence the design and the voltage rating of the power devices is the number of levels 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 with low RDSon (6 – 40 mΩ) are a preferred solution, especially for higher power ranges above 15 kW per block.

Such integrations exhibit a more superior power performance than discrete solutions, increasing efficiency, simplifying the design, reducing overall system size, and maximizing reliability. The T−NPC is a 3−level topology that uses 1200 V rectifiers (replaced with switches in a bi−directional format), with 650 V switches back−to−back on the neutral path. The I−NPC is a 3−level architecture and might be fully implemented with 650 V switches. The 650 V SiC MOSFETs or IGBTs with co−pack diode represent excellent alternative solutions for these 3−level topologies.

 F1−2 PACK SiC MOSFET Module Half−Bridge

Figure 3. F1−2 PACK SiC MOSFET Module Half−Bridge. 1200 V, 10 mΩ

 

DC−DC Topologies

When looking into the DC−DC conversion stages, three main isolated topologies are employed: the full−bridge LLC resonant converter (LLC converter), the full−bridge phase−shift Dual Active Bridge (DAB) Zero Voltage Transition (ZVT) converter (DAB-ZVT converter), and the full−bridge phase−shift Zero Voltage Transition converter (ZVT converter) (Figures 4, 5 and 6).

 

Full−Bridge LLC Resonant Converter

The LLC converter enables Zero Voltage Switching (ZVS) on the primary side and also − at the resonant frequency and below − Zero Current Switching (ZCS) on the secondary side, resulting in a very high peak efficiency around the resonant frequency. As a pure frequency modulated (FM) system, the LLC converter efficiency degrades when the system’s operating point shifts away from the resonant frequency, which might be the case when wide output voltage operation is required. Yet, advanced hybrid modulation schemes enable pulse with modulation (PWM) in combination with FM, limiting the max frequency runaway and the high losses. Still, these hybrid implementations add complexity to the already sometimes cumbersome LLC converter control algorithms. Furthermore, the current sharing and synchronization of LLC converters in parallel are not trivial. When operation around relatively tight voltage ranges and/or development skills to implement advanced control strategies that combine FM and PWM are available, the LLC converter is a difficult design to beat. Not only could it deliver the highest efficiency, it could be a very well−rounded solution from all perspectives. The LLC converter can also be implemented in a bi−directional format as a CLLC resonant converter, another sophisticated topology.  Full−Bridge LLC Converter

Figure 4. Full−Bridge LLC Converter

 

Full−Bridge Phase−Shift Dual Active Bridge Zero Voltage Transition Converter

DAB-ZVT converters with secondary synchronous rectification topologies are also very typical. These operate with PWM and generally require a simpler control than LLC converters. The DAB can be considered an evolution of the conventional full−bridge phase−shift ZVT converter, but with the leakage inductor on the primary side, simplifying the cumbersome secondary side rectification and reducing the necessary breakdown voltage rating on secondary switches or diodes. With ZVT, these DAB-ZVT converters can provide stable high efficiency across a wide output voltage range—a convenient factor for chargers supporting 800 V and 400 V battery voltage levels. The PWM operation of the DAB brings two benefits. First, it tends to keep the converter’s Electromagnetic Interference (EMI) spectrum tighter than in FM systems. Second, the system’s behavior at low loads is easier to address with a fixed switching frequency. Implemented with synchronous rectification, the DAB is a bi−directional native topology and is one of the most versatile and suitable alternative solutions for DCFC.  Full−Bridge Phase−Shift DAB-ZVT Converters

Figure 5. Full−Bridge Phase−Shift DAB-ZVT Converters

 

Full−Bridge Phase−Shift Zero Voltage Transition Converters

For uni−directional operation, the conventional full−bridge phase−shift ZVT converter (Figure 6) remains a utilized option with diminishing penetration. This topology operates similarly to the DAB, but the inductor on the secondary side introduces a significant difference in the rectification behavior. The inductor sets high reverse voltages on the diodes, which will be inversely proportional to the duty cycle. Therefore, reverse voltages on the diode over two and three times the output voltage might arise depending on the operating conditions. Such a situation might be challenging in high output voltage systems (like EV chargers). Multiple secondary windings (featuring a lower output voltage) are typically connected in series. Such a configuration is not so convenient, primarily if a different topology with a single output for given power and voltage ratings would deliver the same or better performance.

SiC modules represent a suitable and standard solution for the full−bridge in the DC−DC power conversion stages mentioned above, starting at 15 kW. The higher frequencies enabled help shrink the transformer and inductor sizes and, therefore, the complete solution form factor. Full−Bridge Phase−Shift ZVT Converters  Topology Variations

Figure 6. Full−Bridge Phase−Shift ZVT Converters

Topology Variations

Multiple variants for the discussed topologies exist, bringing additional advantages and compromises. Figure 7 shows a common alternative by stacking two stages of the full bridge LLC converter used for DCFC. On the stacked side, the switches are under half the voltage; 600 V and 650 V breakdown voltage devices are used. 650 V SiC MOSFETs, 650 V SuperFET3 Fast Recovery (FR) MOSFETs, and 650V FS4 IGBTs will help address different system requirements. Similarly, diodes and rectifiers for the primary side need blocking voltage ratings of 650 V. These 3−level architectures allow for unipolar switching, which reduces the peak current and current ripples, resulting in a smaller transformer. One of the main downsides of this topology is the additional complexity level that the control algorithm requires, compared to the 2−level version with fewer power switches. The DAB can easily be connected in parallel or stacked both on the primary and secondary sides to best suit the current and voltage needs of the fast DC EV Charger.

3−Level Full Bridge LLC converter—stacked on the primary side

Figure 7. 3−Level Full Bridge LLC converter—stacked on the primary side (only half of the input voltage is applied to each transformer) and connected in parallel on the secondary side.

Secondary Side Rectification

Regarding the secondary rectification stage, multiple solutions are possible, as seen in Figure 8, and all could be used with different topologies. For 400 V and 800 V battery levels and full−bridge rectification, the 650 V and 1200 V SiC Schottky diodes typically bring a unique performance−to−cost solution. These devices significantly enhance rectification performance and efficiency compared to silicon−based alternatives due to their zero reverse recovery characteristic, drastically reducing losses and the complexity of the rectification stage. Silicon−based diodes such as the HyperFast, UltraFast, and Stealth could serve as an alternative in very cost constraint projects at the expense of performance and complexity. Solutions with center−tap rectification (Figure 6) are not convenient for high voltage output rectification stages. Unlike in full−bridge rectification, where diodes withstand a reverse voltage equal to the output voltage, in center−tapped configurations, the diodes withstand two times this value. Regular full−bridge phase−shift converters (inductor at the secondary side) require higher breakdown voltage diodes in both rectification methods (full−bridge or center−tap rectification). Several outputs would be connected in series to overcome the need for 1200 V or 1700 V rated diodes in conventional full−bridge phase−shift converters.

Find out more about Three-Phase Active Front End /Power Factor Correction (PFC) Topologies here.

3-phase PFC / AFE topologies play a critical role

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