TND6366/D
Rev. 3, August − 2023

Fast DC EV Charging:
Common Topologies and Power
Devices Used in the System© Semiconductor Components Industries, LLC, 2022

August, 2023 − Rev. 3

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Publication Order Number:
TND6366/D

Fast DC EV Charging: Common Topologies and Power
Devices Used in the System
The role of DC Fast charging (DCFC) is clear in removing barriers to EV adoption. The needfor shorter charging times has resulted in higher power EV fast charging approaches 400 kW
entering the market. In this paper, we will discuss an overview of typical power converter
topologies and power devices for the AC−DC and the DC−DC used in fast DC EV charging.

Figure 1. Diagram of Fast DC EV Charger Architecture

Active Rectification Three−Phase PFC Boost Topologies
Three−phase Power Factor Correction (PFC) systems (or also call Active Rectification orActive Front−End systems) are becoming of great interest, experiencing a sharp increase indemand in recent years. Three−phase Power Factor Correction (PFC) topologies are key for
efficiently powering the fast DC charging. By incorporating Silicon Carbide (SiC) power
semiconductors to your three−phase PFC topologies, one can address the normally conflicting
challenge of reducing power losses and increase power density.
The front−end three−phase PFC boost stage might be implemented in multiple topologies,
and several ones might fulfill the same electrical requirements. Figure 2 illustrates common PFC
architectures in fast DC EV charging applications. One of the first distinctions to be made among
them is bi−directionality. The T−Neutral Point Clamp (T−NPC) and I−NPC topologies are
suitable for bi−directional operation by replacing some of the diodes with switches. The
6−switch architecture is bi−directional per se.

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2Figure 2. Typical Three−Phase Power Factor Correction (PFC) Boost Topologiesfor Fast DC EV Charging. 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 powerdevices is the number of levels in the architecture. The 6−switch topology is a 2−levelarchitecture, normally implemented with 900 V or 1200 V switches for fast DC EV chargers.
Here SiC MOSFET modules with low RDSon (6 − 40 mW) are a preferred solution, especiallyfor higher power ranges above 15 kW per block.
Such integrations exhibit a superior power performance than discrete solutions, increasing
efficiency, simplifying the design, reducing overall system size and maximizing reliability. The
T−Neutral Point Clamp (T−NPC) is a 3−level topology that uses 1200 V rectifiers (replaced withswitches 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 these3−level topologies.

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

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

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

Figure 4. Full−Bridge LLC Converter
Full−bridge phase−shift Dual Active Bridge (DAB) Zero Voltage Transition (ZVT)
Phase−shifted full−bridge DAB with secondary synchronous rectification topologies are also
very typical. These operate with PWM and in general 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, which simplifies thecumbersome secondary side rectification and reduces the necessary breakdown voltage rating
on secondary switches or diodes. With ZVT achieved, these converters can provide stable highefficiency across a wide output voltage range. This is a convenient factor for chargers
supporting 800 V and 400 V battery voltage levels. The PWM operation of the DAB brings towww.onsemi.com
4benefits. Firstly, it tends to keep the Electromagnetic Interference (EMI) spectrum of theconverter tighter than in FM systems. Furthermore, the behavior of the system at low loads iseasier 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 alternatives andsuitable solution for fast DC EV charging.

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

Full−Bridge Phase−Shift Zero Voltage Transition (ZVT)
For uni−directional operation, the conventional full−bridge phase−shift ZVT (Figure 6)
remains still a utilized option with diminishing penetration. This topology operates similarly as
the DAB, but the inductor sitting in the secondary side introduces significant difference in therectification behavior. The inductor sets high reverse voltages on the diodes, which will be
proportional and inversely proportional to the duty cycle, and therefore, depending on theoperating conditions, reverse voltages on the diode in excess of two and three times the output
voltage might arise. Such a situation might be challenging to address in high output voltage
systems (like in EV chargers) and typically multiple secondary windings (featuring a lower
output voltage) are connected in series. Such a configuration is not so convenient, especially
if for given power and voltage ratings a different topology with a single output would deliver thesame or better performance.
SiC modules represent a very suitable and common 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.

Figure 6. Full−Bridge Phase−Shift ZVT Converters

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5Topology Variations
Multiple variants for the discussed topologies exist, bringing additional advantages andcompromises. Figure 7 shows a common alternative of the full bridge LLC converter used forfast DC EV charging. In the phase−shift, the switches are under half of the input voltage and600 V and 650 V break−down voltage devices are used. 650 V SiC MOSFETs, 650 V
SuperFET3 Fast Recovery (FR) MOSFETs and 650 V FS4 IGBTs will help address different
system requirements. Similarly, diodes and rectifiers for the primary side need blocking voltage
ratings of 650 V. This 3−level architectures allow for a unipolar switching, which contributes inreducing the peak current and current ripples, which will results into a smaller transformer. Oneof the main downsides of this topology is the additional complexity level that the control
algorithm requires, compared to 2−level version with fewer power switches. The DAB as wellas the can easily be connected in parallel or stacked both on the primary side and on thesecondary side to best suit the current and voltage needs of the fast DC EV chargers.

Figure 7. 3−Level Full Bridge LLC. This Variation is Stacked on the Primary Side
(only a 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 see in Figure 6and 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 uniqueperformance−to−cost solution. Due to their zero reverse recovery characteristic, these devices
significantly enhance rectification performance and efficiency compared to silicon−basedalternatives, drastically reducing losses and the complexity of the rectification stage.
Silicon−based diodes such as the HyperFast, UltraFast and Stealth could serve as analternative in very cost constraint projects at the expenses of performance and complexity.
Solutions with center−tap rectification (Figure 6) are not convenient for high voltage output
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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 thisvalue. Regular full−bridge phase−shift converters (inductor at the secondary side), asexplained, require higher breakdown voltage diodes in both rectification methods (full−bridgeor center−tap rectification). To overcome the need for 1200 V or 1700 V rated diodes inconventional full−bridge phase−shift converters, several outputs would be connected in series.

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