SiC-Based Solar Inverter

SiC-Based Solar Inverter Navitas Semiconductor and katek Group have announced that katek’s coolcept flex series of Steca solar inverters have embraced new Genesic power semiconductors for enhanced efficiency, size, weight and cost, as well as considerable market expansion. Through the acquisition of Genesic, Navitas marked a further expansion into the realm of high-power applications for wide-bandgap (wbg) technology, including electric vehicles, solar photovoltaics and data centers.

According to analysts, the installed capacity of solar power is projected to surpass that of natural gas in 2026 and coal in 2027, becoming the largest in the world, a threefold rise from 2022 to 2027. The global levelized cost of solar electricity is 40% less than that of coal and natural gas.

In an interview with Power Electronics News, Stephen Oliver, vice president of corporate marketing and investor relations at Navitas, noted that the acquisition of Genesic is an added value for engineering. By combining technical and market value, Navitas has been able to amplify the role of Genesic’s solutions.

1. Silicon carbide

Silicon carbide has a higher bandgap energy than silicon as a wbg semiconductor (3.2 eV, about 3× higher than that of silicon’s 1.1 eV). Higher breakdown voltages, higher efficiency and better thermal stability at high temperatures can be attained because it takes more energy to excite a valence electron in the semiconductor’s conductive band. A sic mosfet’s low drain-to-source on-resistance (RDS(on)), which is up to 300× to 400× lower than that of silicon devices at the same breakdown voltage, is its key benefit.

In order to minimize power losses, maximize efficiency and minimize component footprint, SiC-based power devices can deliver greater power levels. High electrothermal conductivity and exceptionally fast switching speeds are both features of SiC-based electronics. SiC devices are useful for switching designs, including power supplies, three-phase inverters, amplifiers and voltage converters, due to their low output capacitance and low RDS(on) (AC/DC and DC/DC). Significant cost savings and a reduction in the size of the magnetic components (transformers and inductors) utilized in many switching applications are also made possible by the use of SiC devices.

A crucial characteristic is thermal conductivity, which shows how easily heat produced by power losses in the semiconductor device can be extracted, keeping the operating temperature of the device from dangerously rising. It is more challenging to keep the working temperature low for semiconductor-based devices made of materials like silicon, which have a low thermal conductivity. Derating, a specific operating mode that introduces a partial performance decrease so as not to compromise the component at high temperatures, has been developed specifically for this purpose.

On the other hand, a high thermal conductivity means that the device may be appropriately cooled without causing any performance degradation. SiC is capable of operating at a temperature of at least 200°C, which is 50°C higher than the typical junction temperature of silicon MOS devices. For many SiC devices, this temperature can be as high as 400°C or higher. This property allows SiC power devices to operate efficiently even at high temperatures, avoiding performance derating and reduction of mean time to failure, with improvement in terms of quality and reliability.

According to Oliver, the “trench-assisted planar gate” SiC mosfet technology developed by GeneSiC is an added value with high-temperature and high-speed capabilities, resulting in lower operating temperature and a longer life.

“We are putting a lot of effort into thermal management,” he said. “We work efficiently without overheating the equipment. When switching quickly, the temperature remains low. We are constantly increasing the frequency to reduce the size, weight, and cost of magnetics, capacitors, and motor drives.”
According to Navitas, GeneSiC mosfets offer a high reported and 100% tested avalanche capability, a 30% longer short-circuit withstand time, and a steady threshold voltage for simple paralleling. Sixteen GeneSiC G3R75MT12J SiC mosfets are utilized in each 4.6-kW Steca coolcept fleX inverter. These devices, with ratings of 1,200 V and 75 mΩ, are utilized in a two-level converter with bidirectional boost converters and an H4 topology for AC voltage output.

GeneSiC products, except for DO-214 (SMD) packages, are made with silver (Ag) sintering, a die-attach and bonding technology that offers a void-free and strong bond with superior thermal and electrical conductivity. Ag sintering can decrease the junction temperature of an electronic device up to 100°C.

Genesic’s sic MOSFETs are built with maximum gate oxide fields that are much less than 4 MV/cm, which addresses the gate oxide reliability issue. The gate oxide–SiC interface’s quality is the second factor in terms of oxide dependability. A very low defect density is guaranteed by the gate oxidation procedure utilized for Genesic’s sic mosfet production, both within the gate oxide and at the gate oxide–sic contact.

The stability of the internal body diode of sic MOSFET transistors is another crucial consideration. The mosfet body diode in conventional H-bridge power-conversion circuits conducts the rated current during the freewheeling operation. Due to the operation of the body diode, sic MOSFETs from multiple top device vendors have seen considerable device characteristic degradation.

2. Solar technology

As Oliver pointed out, in solar, we have to divide the market into two parts. There is the residential sector, where there is a tendency toward microinverters, and the commercial sector, where higher-voltage ‘string’ inverters are required. Panels have gone from 150 W to about 350 W with an increase in solar cell efficiency up to almost 40% per the US’ National Renewable Energy Laboratory.

“For the microinverter, we did some work in collaboration with Enphase Energy,” Oliver explained. “They considered a switch from silicon to SiC, but for their power level and voltage, GaN was the optimum technology. They can run 10× faster and with ‘a significant’ reduction in system costs, so they opted for GaN. We estimate about a 25% cost reduction by switching from silicon to GaN.”

“In commercial solar, a chain or ‘string’ of panels stacks each panel’s DC output voltage, so higher voltage – 1,200 V – SiC is the best choice,” he continued. “It is the simplest, most efficient, and cheapest solution. So, when you go to high voltage, you go from old silicon IGBTs to SiC MOSFETs because you can increase the frequency and reduce the size. It’s like phone chargers: the frequency increases, but the box gets smaller. And the most important thing for solar is that the box also becomes lighter, and the weight during installation is a very important factor.”

Oliver pointed out that when the system becomes smaller and lighter, it becomes correspondingly cheaper. The transition from IGBT to SiC is underway. It will still take some time, but according to Oliver, whether it is automotive or solar technology, WBG technology will play an important role.

“The higher the voltage, the easier it is in terms of technique and topology to realize high power,” he said. “The same applies to wind turbines, and railway traction drives. GeneSiC silicon carbide runs up to 6,500 V – so high power applications are more easily addressed.”

Another thing to keep in mind is the sustainability and electrification of our world and the abandonment of gas engines. “In general, we tend to go a little slow, but wide-bandgap technology aims to make it efficient,” Oliver said. “From EVs to heat pumps, these are all possibilities for silicon carbide.”

“We have done some calculations and estimate that by 2050, which is the Paris Agreement target, switching from silicon to GaN and silicon carbide will reduce CO2 emissions by six gigatons per year, which is about 30% of the reduction target we need to achieve,” Oliver added. “The sustainability message of GaN and silicon carbide is therefore very strong.”

Very important will be to support the market’s demands with various equipment, retrofitting and renovating where possible, even in factories already working with silicon.

WBG power solutions increase electricity’s efficiency, dependability, and affordability, which accelerates the transition away from coal-fired power plants toward solar and wind energy sources and advances the use of clean, electricity-based energy in transportation, buildings, and industrial factories. According to the foremost authorities on carbon neutrality and climate finance, Natural Capital Partners, Navitas achieved CarbonNeutral certification in May 2022, making it the first semiconductor company in the world to do so.