Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme

In the first part of this series of articles,[1]We describe the main system requirements for a fast charger for electric vehicles, outline the key stages of the development process for such a charger, and learn that onsemi’s team of application engineers is developing the described charger. Now, in part two, we’ll dig deeper into the gist of the design and go into more detail. In particular, we will review possible topologies, explore their advantages and trade-offs, and understand the backbone of the system, including a half-bridge SiC MOSFET module.

By: Karol Rendek, Stefan Kosterec, Dionisis Voglitsis and Rachit Kumar

In the first part of this series of articles,[1]We describe the main system requirements for a fast charger for electric vehicles, outline the key stages of the development process for such a charger, and learn that onsemi’s team of application engineers is developing the described charger. Now, in part two, we’ll dig deeper into the gist of the design and go into more detail. In particular, we will review possible topologies, explore their advantages and trade-offs, and understand the backbone of the system, including a half-bridge SiC MOSFET module.

As we know, EV fast chargers usually include a three-phase active rectifier front end to process AC-DC conversion from the grid and apply power factor correction (PFC), followed by a DC-DC stage to provide isolation and enable the output voltage Adapt to the needs of electric vehicle batteries (Figure 1).

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 1. A high-power fast DC charger with multiple power stages (left).
High-level architecture of a fast DC charging system for electric vehicles (right).

Given the challenging requirements presented and current market trends, the systems engineering team considered several alternatives to implement these two conversion stages. Finally, the conclusion is to utilize a 6-switch active rectifier at the AC-DC stage and a dual active bridge (DAB) at the DC-DC stage that relies on phase-shift modulation. Both architectures support bidirectional functionality and help benefit from 1200-V SiC module technology, which is the cornerstone of fast and ultrafast DC chargers. Next, we’ll dive into the two main power stages.

Active Rectifier Boost Stage (PFC)

The 3-phase 6-switch active rectifier stage helps achieve a power factor of 0.99 and a total harmonic distortion of less than 7%, which are common requirements for commercial DC charger systems. Compared with 3-level PFC topologies such as T-NPC or I-NPC, it provides an efficient bidirectional scheme with low component count. Overall, this two-level architecture achieves system requirements while delivering superior price/performance.[2]

The DC link will operate at a high voltage of 800 V to reduce peak currents, thereby maximizing energy efficiency and power density (Figure 2). For this, the two-stage architecture requires a V of 1200 VBDpower switch.

The switching frequency of the system is set to 70 kHz to keep the second harmonic below 150 kHz, which keeps conducted emissions under control and facilitates compliance with EN 55011 Class A (EU) and FCC Part 15 Class A (US) regulations ( for systems connected to the AC grid). Among other things, these codes set limits on the level of conducted emissions injected into the grid. This approach simplifies the complexity of the EMI filter, making an off-the-shelf solution ideal for the purpose of this project.

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 2. Three-Phase 6-Switch Topology with Power Factor Correction (PFC)
The active rectifier stage, also known as the PFC stage.

Dual Active Full Bridge (DC-DC)

The DC-DC stage of the DAB will contain two full bridges, a 25-kW isolation transformer, and an external leakage inductance on the primary side to achieve zero-voltage switching (ZVS) (Figure 3). Implementing the converter in a single transformer configuration facilitates bidirectional operation. Furthermore, the symmetry of the converter with a single transformer helps to maximize the operating range of the ZVS of the power switch for high energy efficiency.

This addresses a major challenge for the project, maximizing energy efficiency over a wide output voltage range (200 V to 1000 V), enabling a DC-DC peak target energy efficiency of 98%. The converter operates at 100 kHz, which is a compromise to keep switching losses and the core and AC losses of the magnetics to a reasonable level.

Additionally, the system will run flux balance control on the transformer, a technique that eliminates the bulky series capacitors required to work with the transformer in a DAB phase-shift configuration. In this fast charger converter, given the high root mean square (RMS) operating current of 50 A, the necessary voltage rating of a few hundred volts, and an estimated capacitance value of a few tenths of a microfarad, this capacitance would run under. With the current state of the art, all these requirements will result in a large size capacitor. Therefore, the flux balance control strategy helps to reduce the size, weight and cost of the system.

Overall, the DAB DC-DC converter provides a comprehensive solution for EV fast chargers, and it is becoming a typical solution for this new fast charger market. This topology can utilize phase-shift modulation to provide high power and energy efficiency over a wide output voltage range. Additionally, developers can leverage their expertise in traditional full-bridge phase-shifted ZVS converters because of the similarities between the two systems.

Another option is the CLLC resonant converter, a frequency modulation topology that typically provides the highest peak converter efficiency when operating within a limited output voltage range. This converter is a modification of the LLC, allowing bidirectional operation. However, controlling, optimizing, and tuning CLLCs for bidirectional functionality and achieving high output power over a wide output voltage range can become cumbersome, requiring a combination of frequency modulation and pulse width modulation.

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 3. Dual Active Bridge (DAB) DC-DC stage.The system contains two
Full bridge with an isolation transformer in the middle.

Operating Voltage and Power Modules

The DC link between the AC-DC and DC-DC stages will operate at high voltage (800 V) to reduce the current value to maximize energy efficiency and power density. The output voltage will swing between 200 V and 1000 V (as previously mentioned). Since the converter is based on a two-stage topology, a 1200-V breakdown voltage switch is required to operate at such voltage levels.

The NXH010P120MNF1 half-bridge SiC module (Figure 4) contains 1200 V, 10 mΩ SiC MOSFETs and is the backbone of the PFC stage and DC-DC converter. This module has an ultra-low RDS(ON)greatly reduces conduction losses, and minimized parasitic inductance reduces switching losses (compared to discrete alternatives).

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 4. NXH010P120MNF1 SiC module with 2-PACK
Half-Bridge Topology and 1200-V, 10-mΩ SiC MOSFET,
Used to implement AC-DC and DC-DC converters.

The superior thermal conductivity of the power module package increases power density (relative to discrete SiC devices), reduces cooling requirements, and enables a small footprint and robust solution. SiC modules become an important element, enabling >98% energy efficiency in the AC-DC and DC-DC stages of compact and lightweight systems, respectively.

In addition, the module enables magnetics to shrink in size for higher switching frequencies, while the reduced cooling infrastructure requirements help lower the cost per watt of the overall system. The use of fan-based active cooling on the SiC modules should be sufficient to effectively reduce losses in the system in a 25kW DC charging pile power stage for electric vehicles. Capacitors and magnetics are chosen to minimize their cooling requirements while meeting technical specifications.

Control Modes and Strategies

Digital control will run the system, relying on the powerful Universal Control Board (UCB),[3]It uses a Zynq-7000 SoC FPGA and an ARM-based chip. Such a versatile control unit facilitates the testing and easy operation of multiple control methods in the digital domain – such as single-phase shifting, expanding-phase shifting and dual-phase shifting, as well as flux balancing on DAB transformers – and Handles all onboard and external communications. Two UCB units will be used, one for the PFC stage and the other for the DC-DC.

driver

Gate drivers are also critical to overall system performance and energy efficiency. To take full advantage of SiC technology, SiC MOSFETs must be driven efficiently and ensure fast switching. Unlike silicon-based devices, SiC MOSFETs typically operate in the linear region (rather than saturation). After choosing the appropriate VGSAn important aspect to consider is that, unlike silicon-based devices, when VGSWhen increased, SiC MOSFETs exhibit R even at relatively high voltagesDS(ON)significant improvement.[4]

To ensure the lowest RDS(ON)and greatly reduce the conduction loss, it is recommended to use a V of +20 V during conductionGS. For turn-off, -5 V is recommended, which reduces losses during the “turn-off” transition and improves robustness against accidental turn-on.

Additionally, high drive current is necessary to achieve high dV/dt suitable for SiC MOSFETs, which also helps minimize switching losses. With this in mind, the NCD57000 5-kV galvanically isolated high-current driver was chosen for the PFC and dc-dc stages.

The single-channel chip ensures fast switching transitions, source/sink currents +4-A and -6-A, and is durable, showing high common-mode transient immunity (CMTI). Due to the discrete outputs, the gate resistances for turn-on and turn-off are independent (Figure 5), allowing separate optimization of turn-on and turn-off dV/dt values ​​and reduced losses.

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 5. Isolation with DESAT Protection and Discrete Outputs
Simplified application schematic of a gate driver.

In addition, the on-chip DESAT function is very beneficial to ensure the fast overcurrent protection required for SiC transistors, which are characterized by shorter short-circuit withstand times than IGBTs. The lower drive system will replicate the upper drive system, which is a proven good practice in high power applications for fast switching systems.

Isolation and symmetry of the circuit (upper and lower) help prevent problems from different sources (EMI, noise, transients, etc.), resulting in a more robust system. The +20-V and -5-V isolated bias supplies will be provided by the SECO-LVDCDC3064-SiC-GEVB with industry standard pinout.

key bill of material

Table 1 outlines the key semiconductor components and functional blocks that will be used in the design.

Table 1. Key semiconductor components used in 25-kW electric vehicle DC charging piles
Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme

integrate everything

Figure 6 shows how all the system components described above fit together in a practical design to provide a complete solution. Figure 7 gives you a good idea of ​​what the actual hardware looks like.

The PFC stage sits on top of the DC-DC stage, forming a compact and comprehensive structure. The overall dimensions of these modules add up to a maximum of 380 x 345 x (200 to 270) mm (L x W x H), and the height varies with the packaged inductive device. Ultimately, these 25-kilowatt units can be stacked together to achieve higher power levels in an ultra-fast DC charger for electric vehicles.

Introduction to the next part

In subsequent parts of this article series, we will discuss the development of a three-phase PFC stage and DAB phase-shifting converter in further detail, including simulation and other system considerations. The test results will be shown at the end.

Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 6. High-level block diagram of a 25 kW electric vehicle DC charging pile
Development of a 25 kW Fast DC Charger Based on Silicon Carbide: An Overview of the Scheme
Figure 7. 3D models of the actual PFC (left) and dc-dc (right) stages.
The SiC modules are located under each heat sink. In these models,
The gate drive power supply, Universal Controller Board (UCB) and
Passive block.Other views of these components can be found in the following
seen in the online video.

references

1. “Developing A 25-kW SiC-Based Fast DC Charger (Part 1): The EV Application” by Oriol Filló, Karol Rendek, Stefan Kosterec, Daniel Pruna, Dionisis Voglitsis, Rachit Kumar and Ali Husain, How2Power Today, April 2021 .
2. “Demystifying Three-Phase PFC Topologies” by Didier Balocco, How2Power Today, February 2021.
3. SECO-TE0716-GEVB product page.
4. ON semiconductor Gen 1 1200 V SiC MOSFETs & Modules: Characteristics and Driving Recommendations,” application note AND90103/D.
5. NXH010P120MNF1: SiC Module product page.
6. NCD57000 product page.
7. SECO-LVDCDC3064-SIC-GEVB product page.
8. NCD98011 product page.
9. NCID9211 product page.
10. NCS21xR product page.
11. SECO-HVDCDC1362-15W15V-GEVB product page.

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