How wide bandgap semiconductors are improving renewable energy design

Engineers use the latest wide bandgap technologies to maximize offshore and onshore wind farms, solar panel output and the ocean’s tides.

Today’s sharper focus on energy generation and consumption places the spotlight solidly on renewable energy solutions. Challenges associated with climate change, such as more frequent extreme weather events, force society to reconsider its relationship with fossil fuels.

Change has been taking place. Over the past decade, the U.S. closed more than 500 coal-fired power plants, many of which were replaced by natural gas alternatives. Natural gas, when burned correctly, generates about half the CO2 of coal and does away with coal’s NO and SO2 emissions. However, scientists say it won’t deliver the CO2 reduction that is desperately needed to limit global temperature rise to 1.5°C – the most ambitious target of the Paris Climate Accord.

As a result, the industry has been focused on renewable energy sources to deliver net-zero emissions. Progress is being made, with the International Energy Agency (IEA) reporting that 28% of global electricity generation came from renewables in the first quarter of 2020, up 2% over the same period in 2019. While the COVID-19 pandemic disrupted supply chains and placed some projects on hold, renewable projects such as solar photovoltaic (PV) and wind continue to grow.

Of course, one of the challenges of renewables is the disconnect between generation and demand. Energy Storage Systems (ESS) are growing in importance to bridge this gap, from grid-tied solutions to power walls in private homes. Furthermore, as the number of electric vehicles (EV) on our roads increases, they are increasingly being seen as part of the energy mix. Chargers are quickly becoming bidirectional to enable EVs to bridge power grid dips or even power homes in the event of total power loss.

Wide bandgap: A critical component for renewables

Silicon technologies such as MOSFETs and IGBTs have been the staple switching solutions for power inverters for decades. Since their introduction, both device types have seen continual improvements in their designs to reduce their losses in switching converters, enabling the construction of ever more efficient power conversion topologies. This approach can be considered a success, with converters regularly attaining peak efficiencies of more than 95% and power factor correction (PFC) designs cracking 98% efficiency. While impressive, remember that just 1% efficiency loss of a small PV solution or bidirectional vehicle charger can easily range from 500 W to 3 kW. This energy must be dissipated and removed.

Wide bandgap (WBG) technology has been making its presence known over recent years, with the number of suppliers offering silicon carbide (SiC) MOSFETs and gallium nitride (GaN) transistors growing steadily. However, design engineers are naturally hesitant to make the move for various reasons.

Current devices remain more expensive than silicon alternatives, but this gap is reducing yearly. Perhaps the more significant hindrance to adoption is the learning curve involved. WBG switches function differently from their silicon counterparts and are not simply a drop-in replacement. Power converters need to be redesigned to operate at higher switching frequencies to attain the full benefit, leading to a host of new challenges in layout and fulfillment of EMC/EMI requirements.

A closer look at SiC vs. IGBT in wind energy generation

SiC MOSFETs are promoted as an excellent alternative to existing designs using IGBTs. IGBTs have been a mainstay of >1000 V applications for large PV inverters and wind turbines, offering medium-speed switching. However, the end application suffers due to thermal management challenges, pushing up the total size and weight. When operated under similar test conditions, SiC MOSFETs exhibit a lower turn-on switching loss than IGBTs at 25°C, which drops as temperature increases.

Turn-off losses are also lower, rising slightly at elevated operating temperature, improving over the well-known tail current caused by the accumulation of minority carriers in IGBTs. The on-resistance characteristics are also improved, increasing minimally as operating temperature rises. As a result, with lower turn-on, turn-off, and conduction losses, SiC MOSFETs can typically boast more than 60% lower losses than an equivalent IGBT device. Devices such as the Infineon CoolSiC Trench MOSFET FF6MR12KM1P are well suited as IGBT replacements in such cases.

Offshore wind turbines place a high demand on reliability to keep their operational costs under control over their lifetime but also provide limited space for equipment. They typically use voltage source converters (VSC) in a back-to-back configuration, consisting of a three-phase, two-level rectifier for the low voltage side with a three-phase, three-level neutral point clamped (NPC) inverter. For a 460 V, 240 kW design, the DC-link lies at around 760 V.

 

High voltage conversion topologies

Wind turbines typically use voltage source converters (VSC) in a back-to-back configuration.

In research undertaken at the Norwegian University of Science and Technology, the move from silicon IGBT to SiC MOSFET showed that at switching frequencies of around 5 kHz, the SiC solution offered around 1% improved efficiency. At 50 kHz, SiC was as efficient as using an IGBT switching at 10 kHz. This higher switching frequency can be useful in several ways, such as improving power density by reducing the size of passive components, which decreases overall volume and weight. Alternatively, switching at 10 kHz results in higher efficiency than using an IGBT, which simplifies the cooling requirements. In a separate analysis of wind turbine applications at The University of Nottingham, the move to SiC resulted in a one-third reduction in heatsink volume, enabling the cooling fan to be discarded. They also noted a drop in energy losses of more than 70%, leading to financial operation savings that would cover the added cost of SiC within two years.

Switching out IGBTs for GaN in solar inverters

As with SiC MOSFETs, GaN HEMT (high electron-mobility transistors) can offer significant benefits over their silicon counterparts. The drain-source charge QOSS is much lower, leading to significantly lower losses during switching, as is the on-resistance. Also, they support significantly higher switching frequencies. As with SiC, this means design engineers can reduce the size of passive components in their designs.

Small-scale PV for homes and commercial buildings are becoming more popular. PVs are enabling consumers and business users to charge their EVs and cover some — occasionally all — of their electricity consumption needs. Single-phase applications are suited to string inverters, where photovoltaic panels are connected in series to generate a DC voltage. While such panels have improved over the years, their efficiency lies around 20%. Thus, it is critical to ensure the highest possible efficiency in converting panel voltage to line output or DC supply to charge a local energy storage system (ESS).

The string configuration is well suited to a 650 V GaN-based design. Each string is linked to a DC/DC boost circuit. These are controlled by a microcontroller or system-on-chip (SoC) that also handles maximum power point tracking (MPPT). The output feeds the DC link bulk capacitors from which a DC/DC converter can feed an ESS battery pack or a DC/AC inverter to provide single-phase AC for local use or the grid.

IGBT-based PV solutions for string inverters can achieve a peak efficiency of 98%, operating at switching speeds of 15 kHz to 30 kHz. Due to the frequency of operation, the magnetics are relatively large, heavy and expensive. Furthermore, anti-parallel diodes are needed with the IGBTs, which, in total, all contribute to the space requirement, bill-of-materials (BOM) and cost.

Moving to Nexperia’s GaN FET technology, such as the GAN041-650WSB allows the switching frequency to be pushed to between 100 kHz and 300 kHz. At this operating point, the output filters become much smaller, while the fidelity of the output signal leads to less harmonic distortion. These improvements trickle through the design to provide a smaller, lighter design with at least double the power density. Furthermore, efficiency can break through the 99% barrier, while BOM cost also drops.

 

Wide bandgap in solar inverter strings

Single-phase string inverters become more efficient, smaller and lighter when moving to GaN switches. (Source: Nexperia)

Gate drivers for WBG SiC and GaN switching converters

Another key difference between silicon and WBG switches is the implementation of the gate driver. Gate drivers are designed to push the gate of silicon MOSFETs as high as possible in the shortest time to provide fast switching and move the switch quickly between its lowest and highest resistance. WBG doesn’t change this design goal, but the voltages involved do change.

The gate of a GaN transistor looks like a diode with a forward voltage of around 3 V in parallel with the switch’s gate capacitance. Thus, while only a low voltage is needed to keep the transistor conducting, a slightly higher voltage is needed to turn it on. When turning it off again, a negative voltage is required in hard-switched applications. Suppliers now provide dedicated gate drivers, such as Infineon’s isolated 1EDF5673K. The device uses an RC-coupled gate driver circuit to deliver the required voltages for turn-on and turn-off.

 

Understanding WBG gate drivers

Isolated gate drivers are available designed for control of GaN HEMT. The RC-coupled design ensures the correct voltages are generated for hard-switching applications. (Source: Infineon)

SiC gate drivers typically function like their silicon counterparts, albeit at slightly higher turn-on voltages. However, the higher switching speeds result in new challenges, such as noise and EMI, and overvoltage due to parasitic inductance. So, while switching as quickly as possible seems beneficial, pure analog control of the gate may not always be the best approach. Microchip's 2ASC-12A1HP AgileSwitch is a digital gate driver that steps the gate voltage between turn-on and turn-off, reducing overshoot, ringing, and turn-off energy.

WBG: The core of next-generation renewable designs

While silicon devices have served us admirably for half a century, it is clear WBG technologies such as SiC and GaN are needed for next-generation renewable designs. From solar and wind to ESS, the power levels involved require the last percentage point of efficiency improvement. This will help increase power density and reduce energy losses that today’s designers are extracting with cooling concepts. Thanks to higher switching frequencies, magnetics can also be smaller, leading to more compact solutions that are lighter, easier to install and take up less space. Admittedly, WBG is not a simple drop-in replacement for existing switches. Still, engineers enjoy challenges and, as long as basic electrical principles are followed, the move should be both painless and beneficial.

Follow Avnet Silica on LinkedIn

• IEA Global Energy Review 2020: Renewables
• Journal of Physics conference series article: SiC MOSFETs for Offshore Wind Applications
• onsemi's Wide Band Gap portfolio