Design Tips and Solutions for 25 kW DC Fast Electric Vehicle Chargers

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Convenient and efficient charging is crucial for the success of all battery powered electric vehicles (BEVs). The availability of charging stations and the speed of charging directly impact consumers' likelihood of choosing electric vehicles over fossil fuel vehicles. In this article, we will explore design tips for a 25 kW DC fast electric vehicle charger and relevant solutions offered by onsemi.

Direct Current Fast Charging Enhances Electric Vehicle Charging Efficiency

Direct Current (DC) fast charging technology is a critical method for modern electric vehicle (EV) charging, significantly reducing charging times and improving user convenience and efficiency.

Key technologies involved in DC fast charging begin with charging standards. Currently, there are various charging standards such as CHAdeMO, Combined Charging System (CCS), and Tesla Supercharger, among others. Different brands and models of EVs may support different charging standards, so it's essential to ensure compatibility between the charging equipment and the vehicle when selecting charging devices. 

Furthermore, DC fast charging typically delivers higher charging power compared to AC charging, allowing rapid energy transfer into the battery. The charging power level significantly impacts charging speed and efficiency. Therefore, it is crucial to choose the appropriate charging power based on the EV's requirements and the specifications of the charging equipment.

DC fast charging requires dedicated charging equipment, typically installed at charging stations or specific locations. During charging, it's important to pay attention to charging safety issues, including avoiding overheating, overcharging, or other safety risks. Generally, both the charging equipment and the EV are equipped with safety mechanisms. However, users should still be vigilant for any abnormalities during the charging process and take prompt action to address or stop charging if needed. Compared to slow charging, DC fast charging exerts a greater impact on the battery. Therefore, it is advisable to moderately control the frequency of DC fast charging to avoid excessive use, which can affect battery life and performance.

While DC fast charging technology effectively improves the charging speed and convenience of electric vehicles, it's important to consider charging standards, charging power, charging equipment, and charging safety issues during use to ensure a safe and reliable charging process.

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SiC Modules are Key Components in DC Fast Charging Technology

Silicon Carbide (SiC) modules are essential components in DC fast charging technology, comprising SiC MOSFETs and SiC diodes. The boost modules are used in DC-DC stages of solar inverters, utilizing SiC MOSFETs and diodes rating at 1200V.

SiC modules are power modules that use silicon carbide semiconductors as their switches, aiming to convert power efficiently and thereby improve system efficiency. The primary function of SiC modules is power conversion. Silicon carbide offers advantages over silicon due to lower resistance to move away from the source (leading to enhanced efficiency), enabling SiC devices to operate at higher switching frequencies. Systems based on SiC are more compact and lighter compared to silicon solutions, allowing for smaller designs. Therefore, SiC devices are an ideal solution for improving efficiency and enhancing thermal management.

To address the challenges faced by DC fast charging, onsemi continuously innovates in SiC technology and packaging solutions, aiming to simplify the design process of electric vehicle chargers. Leveraging a comprehensive portfolio of discrete power and analog solutions, protection devices, sensors, and connectivity products, onsemi offers high-quality components and customized systems tailored to customer needs. With over 20 years of accumulated system expertise, onsemi integrates all these technologies to provide comprehensive solutions for electric vehicle charging.

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Design Challenges in Fast Electric Vehicle Chargers

Designing a compact, efficient, and reliable fast electric vehicle (EV) charger is not an easy task. Apart from the actual power conversion circuitry, hardware protection technologies are crucial, requiring designers to analyze various ’what-if’ scenarios. Solutions will include snubbers formed by passive RC networks and blocking components. 

Excessive voltages and/or currents are always a concern, necessitating protection to ensure power semiconductors are not damaged. One technique involves adding a voltage comparator with defined thresholds and hysteresis. If overvoltage is detected, this comparator will block gate drivers.

Over current can be more challenging, although onsemi's NDC57000 gate driver features overcurrent desaturation protection (DESAT), thereby addressed with minimal impact on Bill of Materials (BOM) and product cost. Such hardware protections are especially critical during testing and debugging, particularly during bring-up phases when unpredictable switching is most likely. 

The NDC57000 can be used in the Power Factor Correction (PFC) stage to protect SiC Power Integrated Modules (PIMs), explaining testing methodology to evaluate the DESAT trip current threshold, which is a necessary functional testing. The DC link capacitors are used to provide the required peak tripping current and inject pulses into the gate to turn on the modules, allowing DESAT protection to trip. As a result of testing, theoretical values can be compared with practical values, and design adjustments can be made accordingly. 

For the main dual active bridge (DAB) DC-DC converter, the NDC57000 can also be used, relying on voltage drops to monitor current levels. However, this method is sensitive to device characteristics, and while some information is included in datasheets, prototype validation is still necessary.

Another approach is to simulate before making prototypes to set parameters more accurately. This allows for non-destructive simulation and understanding of primary and secondary short-circuit effects. The discrete enhancement of DESAT protection offers a wide operating voltage range solution for DC-DC stage designers with output voltage spans ranging from 200-1000V.

One significant advantage of SiC technology is its ability to operate at high frequencies. However, this means fast dv/dt slew rates, which can affect the physical layout of a 25 kW fast charger. Layout optimization is essential to minimize parasitic inductance, especially in power supply traces. Additionally, snubber circuits are needed at multiple points to minimize overshoot and ringing that could cause damage and EMI issues.

System-level control is another crucial area. In a 25 kW fast charger, multiple closed-loop controllers are equipped within the PFC and DAB to control parameters like active flux balancing in the transformer and primary-to-secondary phase shift to regulate output voltage and current. One challenge here is selecting the gain for each loop to ensure overall system stability. 

Due to the high-power equipment required for testing, designers often build a loop-back arrangement on the bench with two PFC stages and one DAB to allow safe testing under controlled conditions. Loop-back testing is also applicable during the burn-in stage of mass production, where energy is recovered from tested devices, saving significant manufacturing costs to achieve the mission of low-carbon emissions worldwide.

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Highly Efficient and Reliable IGBT Driver

The NCD57000 from onsemi is a high-current single-channel IGBT driver with internal galvanic isolation, designed specifically for high system efficiency and reliability in high-power applications. Its features include complementary inputs, open-drain FAULT and Ready outputs, active Miller clamp, negative gate voltage, accurate UVLO, DESAT protection, DESAT soft turn-off, support for high current outputs at IGBT Miller Plateau voltages (+4/-6 A), short propagation delay with accurate matching, high transient and electromagnetic immunity, and 5 kV galvanic isolation capability with independent high and low (OUTH and OUTL) driver outputs for ease of system design. 

The NCD57000 accommodates 5V and 3.3V signals on the input side and wide bias voltage range on the driver side, including negative voltage capability. It provides >5 kVrms (UL1577 rating) galvanic isolation and >1200 Viorm (working voltage) functionality. The NCD57000 is packaged in a wide-body SOIC-16 package with 8 mm creepage distance between input and output to meet reinforced safety insulation requirements.

To accelerate customer product development, onsemi also offers a reference design kit for the NCD57000. The SEC-25KW-SIC-PIM-GEVK is a 25 kW fast DC electric vehicle charger reference design kit based on SiC power integration modules. This complete SiC solution consists of PFC and DC-DC stages featuring multiple 1200V, 10 mohm half-bridge SiC modules NXH010P120MNF1 with ultra-low RDS(ON) and minimized parasitic inductance, significantly reducing conduction and switching losses. Leveraging a powerful Universal Controller Board (UCB) with Zynq®-7000 SoC FPGA and ARM®-based processor, this system delivers up to 25 kW of power at output voltages ranging from 200V to 1000V, achieving 96% all-time efficiency for charging 400V or 800V electric vehicle batteries. 

The SEC-25KW-SIC-PIM-GEVK also features the NCD57000 high-current driver with galvanic isolation and the SECO-HVDCDC1362-40W-GEVB auxiliary power solution, providing stable voltage rails for low voltage components, inrush control, integrated protection for over-voltage, and multiple communication interfaces.

The SEC-25KW-SIC-PIM-GEVK supports three-phase PFC and DAB for bidirectional power conversion of 400V/800V batteries, compliant with EN55011 Class A and IEC 61851 standards. It integrates SiC modules NXH010P120MNF1 with half-bridge, 1200V, 10 mohm SiC M1 MOSFETs, along with isolated high-current, high-efficiency gate drivers like the NCD57000.

Conclusion

DC fast charging technology represents a significant breakthrough in the field of EV charging, offering a more convenient and efficient way to charge electric vehicles. With continuous advancements and applications in technology, DC fast charging has become one of the mainstream methods for charging modern electric vehicles. This article discusses the design techniques for a 25 kW DC fast charger and the related solutions introduced by onsemi, aimed at accelerating the development of DC fast charging products and gaining a competitive edge in the market.

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