Author:
Jinchang Zhou, Product Line Manager, onsemi
Date
08/21/2024
PDF
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Figure 1: ANPC converters are easily built using modules
Electrification is prevalent across many sectors as manufacturers and consumers seek to find solutions that do not rely on fossil fuels as a source of energy. This is an important step for conservation as it limits the pollution that drives damaging global warming. As more electric vehicles (EVs) are deployed globally, the industry is looking to the next challenge – converting commercial and agricultural vehicles (CAVs) to electric propulsion.
However, this shift is putting the electrical grid under extreme pressure as the demand for electrical energy rises rapidly. Even though they are efficient, applications such as EVs, data centers, heat pumps and others require significant amounts of energy to operate.
New renewable sources of energy are coming to the fore, with solar power being one of the most popular forms alongside others such as wind or wave power. Only when an application uses energy from a renewable source can it be considered truly “clean.”
The solar energy market is long-established and relatively mature. According to a report by Fortune Business Insights, the market is currently estimated at USD 273 billion, growing to USD 436 billion by 2032. In 2023, the solar energy market accounted for over 40% in North America.
Power Conversion Challenges in Renewable Applications
Power generated by solar energy is rising rapidly. In 2022 (according to the IEA), there was 1300 TWh of electricity generated by this method, an increase of 26% over the previous year. Significantly, at this point, solar surpassed wind power to be the largest source of renewable electricity.
Solar photovoltaic (PV) panels generate direct currents (DC) while the grid requires alternating currents (AC), so central PV inverters are an integral part of large grid-tied installation. As all energy generated by the PV panels passes through, the inverter efficiency is one of the most important parameters. While the sun’s energy may be limitless, inefficient conversion would limit the energy reaching the grid. The associated wasted energy generates heat which poses a significant challenge as many solar installations are located in sunny climates / deserts where ambient temperatures are high.
Cost is an important consideration as it impacts the price of electricity to the consumer as well as the profitability of the electricity company. Given the high-powers required, many central inverters use multiple conversion modules in parallel, with the number required determined by the power rating of each individual module. As the power capacity of each module increases, the number of modules required decreases, thereby reducing cost.
While much progress has been made with EVs, less progress has been made with CAVs in moving to electric propulsion. However, though CAVs account for only 2% of the total vehicle quantity, they emit 28% of greenhouse gas from transportation. As these vehicles are larger, they consume more fuel per journey and create more emissions. There has been some progress, particularly in commercial passenger vehicles (such as buses), yet the vast majority of large trucks, construction machinery and agricultural vehicles (such as tractors) continue to be powered by diesel. Now things have started to change. To meet tight zero-emission regulation in global markets such as European Union, China and California, it is forecasted that the sales from electric trucks (battery electric and hybrid) will be increased from the current 5% of total sales to 40%-50% by 2030.
Electric commercial vehicles are simpler than their fossil fuel counterparts with less moving parts. This allows them to be smaller (for the same load carrying capacity) and increases reliability – reducing the costs associated with maintenance. Given the fact that battery cost has been significantly reduced, total cost of ownership of electric CAVs is reported lower than internal combustion engines (ICEs).
Similar to solar applications, efficiency is a key requirement for electric CAVs as each vehicle has a finite amount of energy in the battery to make a journey. The more efficient the conversion process within the inverter, the longer the range (or the smaller the battery needed for a given distance).
Given the future reliance on solar energy and electric CAVs, it goes without saying that reliability is essential.
Advanced Power Technology for Inverter Applications
One of the more common topologies used in high-power applications, such as three-phase solar PV inverters, is the three-level active neutral point clamped (ANPC) converter. This multilevel topology is specifically aimed at enhanced performance and efficiency.
Normal neutral point clamped (NPC) converters use diodes to connect the neutral point of the DC link capacitors to the output. In an ANPC configuration (figure 1), the clamping is performed by switches, allowing for better control that reduces switching losses and enhances efficiency. This reduces the need for thermal measures resulting in a smaller, lower cost solution.
The way the topology is arranged reduces the voltage stress on each individual switch, which increases reliability. Additionally, ANPC delivers an enhanced waveform that is beneficial for the grid.
By using a power module such as the QDual 3 IGBT modules from onsemi and paralleling multiple power modules, design engineers can create a high-performance three-level active neutral point clamped module with a system output power reaching 1.6 MW to 1.8 MW.
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Figure 2: QDual3 IGBT Modules
The QDual 3 modules integrate the latest 1200 Volts Field Stop 7 (FS7) IGBT and diode technologies, which offer the ultimate performance for high-power applications. FS7 technology has significantly improved conduction losses compared to previous generations.
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Figure 3: FS7 technology enhances key performance parameters
In FS7 IGBT process, a trench narrow mesa delivers low VCE(SAT) and high-power density while a proton implant multiple buffer ensures robust and soft switching characteristics (Figure 2). onsemi’s medium speed FS7 devices offer the lowest VCE(SAT) (1.65V) for motion control applications while their FS7 fast products have the lowest EOFF of just 57 µJ/A, making them ideal for high-power applications such as solar inverters and CAVs.
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Figure 4: FS7 IGBT deliver higher power density in a smaller size
The innovative FS7 technology shrinks the chip size in the latest QDual3 modules by 30% compared to the previous generation (Figure 3). This miniaturization, combined with advanced packaging enhancements, helps significantly increase the maximum current rating. In motor control applications with operating temperatures up to 150 degrees Celsius, the QDual3 delivers approximately 12% higher output power from 100 kW to 340 kW compared to other currently available products in the market.
With reliability being key in solar and CAV applications, the way the module is constructed and tested is of paramount importance. For example, onsemi modules use ultrasonic welding for terminal attachment instead of the wire bonding used on many competitive solutions. This enhances current carrying capability, provides a better thermal path and is more robust than wire bonding (Figure 4).
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Figure 5: Ultrasonic welding reduces temperatures and enhances reliability
The increased conductivity associated with this approach reduces electrical losses, thereby boosting efficiency. Additionally, this lowers the operating temperature that, along with the enhanced mechanical rigidity, increase overall reliability of the module.
Higher Power QDual3 Technology
Application specific QDual 3 Half-Bridge IGBT modules (NXH800H120L7QDSG for Central Solar Inverters, ESS, UPS and SNXH800H120L7QDSG for CAVs) are based upon their FS7 technology which reduces losses and enhances efficiency through improved VCE(SAT) and EOFF.
Currently, to complete a 1.725 MW inverter design using 600 A IGBT modules in an ANPC / INPC arrangement would require a total of 36 modules. However, as the new NXH800H120L7QDSG and SNXH800H120L7QDSG are rated for 800 A operation, the number of modules required reduces by 9. This represents a 25% saving in size, weight and cost – useful in solar applications and highly valuable in CAVs where the vehicle range will increase due to reduced weight and improved efficiency.
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Figure 6: The increased current capability allows systems to be built with fewer modules
The modules feature an isolated baseplate for thermal management and an integrated NTC thermistor. The solderable pins allow direct PCB mounting and are configured in an industry-standard layout that facilitates upgrading of existing designs to the latest QDual3 technology.
All QDual3 modules are subjected to onsemi’s stringent reliability testing which exceeds other devices on the market. Humidity testing is performed with a 960V bias for 2000 hours while others are tested at 80 V for 1000 hours. Vibration testing (which is essential for CAV applications) is performed at 30 G peak / 10 G RMS for 22 hours to meet AQG324. Other devices are tested at vibration levels as low as 5 G for durations as short as 1 hour.
