Wide bandgap technology is key for residential string inverters

Feature articles | February 26, 2024

By NicolasR

Power Management Materials & processes Energy Harvesting

Solar power is currently one of the fastest-growing electricity sources in the United States with residential solar installations growing exponentially in recent years, says Damijan Zupancic, Application Marketing Manager Solar & ESS at Infineon Technologies.

Current projections suggest approximately 100 million households will have rooftop solar photovoltaic (PV) cells installed by 2030 and wideband technologies will be key to this rollout.

There are a few key components that make up residential solar systems. These include solar cells and panels, an inverter, and energy storage devices (generally batteries). The inverter is an important part of the system that can have major impacts on the system performance as well as the overall cost of a system. However, recent advancements in technology and manufacturing processes have brought down the costs of inverters and have resulted in more reliable and dependable devices.

This growth is driven by demand for renewable energy resources, declining prices of solar cell arrays, and various incentives and investments from the government. In addition, the adoption of solar technologies is becoming more widespread throughout the country. Traditionally, most solar technology adoption and installations were led by the state of California. However, additional states such as Texas, New York, Florida, and Illinois have all seen major growth in the previous years.

Solar inverters have been around since the early days of solar panels in the 1950s and 1960s. During these times the inverter mainly consisted of relays that would convert the direct current (DC) electricity into alternating current (AC) electricity. It wasn’t until the 1970s and 1980s when semiconductor technologies were making great advancements that transistor based inverters were introduced. Transistor based inverters offered greater efficiencies, more reliable designs, and also more compact devices. It was then in the 1990s string inverters were introduced. The string inverter is one of the two inverter topologies commonly found in residential solar systems today with the other being the microinverter. Where as the microinverter mounts directly on the back of a solar panel to convert the current from DC to AC, the string inverter connects all the solar panels together in series and then converts the current from DC to AC at a single point.

In the early 2000s, insulated-gate bipolar transistors (IGBT) offered the best performance for solar inverter applications. The IGBT is basically a bipolar transistor with a metal oxide semiconductor gate structure. This allows the device to be controlled like a MOSFET while being able to handle higher currents than MOSFET devices at that time. With these devices, inverters were capable of being designed that offered lower switching and conduction losses with less generation of audible noise and harmonics. One example of these devices is Infineon’s 600V/1200V TRENCHSTOP IGBTs.

Eventually, advancements in MOSFET devices allowed an alternative option for solar inverter applications. Devices such as OptiMOS N-channel power MOSFETs offer high efficiency and power density at a competitive price. More recently, wide bandgap (WBG) devices are offering competitive advantages making them more attractive for current and future solar inverter applications. WBG technologies such as silicon carbide (SiC) have shown to offer significantly improved power conversion efficiency which helps saves energy while also reducing the size and weight of inverter devices. The size and weight of a system is important to keep in mind since it can ease the burdens of installation and maintenance. The evolution of transistor devices in solar inverter applications is best displayed below.

To demonstrate the increase in efficiency SiC can offer over Si, the graph below is presented. In the plot, the red traces represent the Si devices and green traces represent the SiC devices. As can be seen, the SiC devices not only offer higher efficiency but also offer larger improvements at higher switching frequencies. The 1.3% improvement at 70KHz switching frequency results in a 58.5W difference in power losses. The lower power losses results in significantly lower operating temperatures of the components which can help reduce thermal management solutions needed.

Overall, WBG semiconductor devices such as SiC and GaN currently are the most optimal choice for next generation solar inverter designs. They can provide numerous benefits at higher voltages such as greater power efficiency, smaller size, lighter weight, and lower cost. Wide bandgap devices also allow for higher operating switching frequency designs.

As a result, designs with wide bandgap devices can use lower inductance and exhibit an overall higher power density when compared to alternative technology options. Due to the numerous advantages WBG devices offer, they will continue to be ideal choices for innovative solar inverter designs moving into the future.

www.infineon.com