Wide-bandgap semiconductors for next-generation photovoltaic and energy storage system solutions
Wide-bandgap (WBG) semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are enabling higher-efficiency, smaller-sized power conversion solutions for next-generation photovoltaics (PV) and energy storage systems (ESS). In this article we will highlight examples of such products offered by Infineon Technologies and Toshiba.
Strong growth in photovoltaic installations and power generation
The climate change target of net-zero greenhouse gas emissions by 2050 is driving many countries to adopt renewable energy production. A combination of improvements in photovoltaic panel efficiency and reliability, cost reductions, and government subsidies have driven significant growth in solar power generation. The cumulative installed photovoltaic capacity will exceed 1TW in 2022, with 240GW of new capacity added during the year. This accounts for two-thirds of new renewable energy capacity installed in 2022. The cumulative annual growth rate in the next few years is expected to be approximately 20%. It is estimated that wind and solar energy will account for approximately two-thirds of all energy production by 2050.
Trends in PV and ESS power conversion
For residential use, PV output is typically less than 20kW, with a maximum DC voltage of 600V and a typical single phase output of 110/230VAC. Currently, utility-scale photovoltaics are typically rated at 1,000V DC. Lower ohmic losses and the resulting increased end-to-end efficiency, plus fewer parallel strings to reduce installation costs, are driving the push to increase this voltage from 1,000V to 1,500V. The power output of grid-connected utility generators can range from a few hundred kilowatts to several megawatts. ESS systems scale accordingly, with typical residential wall-mounted units ranging from 3kW to 20kW, with battery voltages rising into the 450V range. Commercial and utility ESS installations range into the megawatt range.
The method of connecting the ESS can be AC coupling (in which the battery is connected to the grid through a combination of DC/DC and DC/AC conversion) or DC coupling (in which the DC photovoltaic power source is connected to the battery through a DC/DC converter). This hybrid inverter with built-in ESS function can avoid unnecessary power conversion. These converters must be bidirectional to capture and absorb battery energy based on supply and demand.
As electric vehicles become more popular, EV batteries can be used as energy sources in vehicle-to-home configurations. Here, the EV battery can be DC coupled to the photovoltaic hybrid inverter via a bidirectional DC/DC converter, or AC coupled to the grid via an onboard or offboard bidirectional DC/AC inverter.
A variety of converter designs are used here, each with its own advantages. CLLC and Dual Active Bridge (DAB) are common topologies for bidirectional isolated DC/DC converters, where a low voltage (such as a 48V battery or photovoltaic source) and a high voltage DC output (powering an inverter, such as 400V) Isolation is required. A non-isolated step-up DC/DC inverter can be used when low voltage is not available, such as the output of a larger string solar system that outputs 600V.
Transformerless bidirectional DC/AC converter designs – such as the High Efficiency Reliable Inverter Concept (HERIC) or the Multilevel Active Neutral Point Converter (ANPC) – due to increased efficiency and reduced system cost, size and weight and hence its increasing popularity. HERIC inverters can be used in single-phase string inverters with output powers up to several kilowatts, while ANPC can be used in central inverters with output powers from hundreds of kilowatts to several megawatts. Multilevel ANPC allows the use of lower voltage rated devices (Vbus ÷ (n – 1), where Vbus is the full bus voltage and n is the number of levels) and also reduces voltage transitions during conversion (dv/dt), Thereby reducing electromagnetic interference.
WBG devices for PV and ESS power conversion
WBG devices offer many advantages over traditional Si devices in PV and ESS bidirectional DC/DC and DC/AC converter designs. Higher switching frequency reduces system size and increases efficiency. Reduced losses can make cooling requirements simpler. GaN devices can also provide unique bidirectional capabilities for simpler AC switching topologies, reducing device count by a factor of 4.
Figure 1 shows the application space for SiC and GaN devices in solar and ESS applications. SiC is well suited to replace Si devices in higher power and higher voltage areas, with devices rated in the range of 1,200V to 3,300V. Fast-switching GaN devices with voltage ratings of 650V or less are ideal for individual photovoltaic microinverters and DC/DC converter applications.
Figure 1: SiC and GaN application space in solar and ESS systems. (Image source: Infineon Technologies)
As shown in Figure 2, CoolSiC and CoolGaN devices produced by Infineon have multiple advantages over traditional Si devices. SiC improves RDS(on) characteristics as a function of temperature. GaN HEMTs have lower output and gate charges (Qoss and Qg, respectively), resulting in the lowest switching losses. GaN's lower Qoss can be used to reduce dead time and reduce RMS coefficients in soft-switching CLLC topologies.
Figure 2: SiC and GaN on-resistance, output charge as a function of drain voltage, and gate charge as a function of gate voltage compared to superjunction Si MOSFET devices with similar voltage and RDS(on) ratings Comparison. (Image source: Infineon Technologies)
These advantages translate directly into increased converter efficiency. For example, when discharging a 56V ESS battery to the 400V bus, a DC/DC CLLC converter operating above the resonant frequency can achieve much lower reverse recovery losses on the secondary-side high-voltage switch that is performing synchronous rectification. The Miller gate charge (Qgd) on the primary FET of the DAB converter will be much lower, resulting in much lower Eoff losses on the primary side FET.
The body diode's ability to withstand surge currents well above the device's nominal current rating may be another key advantage of SiC MOSFETs' ability to achieve bidirectional power conversion. SiC-based converters can operate at several times the switching frequency of similarly rated Si IGBTs while still having smaller or comparable losses, thus saving magnetics in the system and thus reducing size and cost. Infineon's discrete CoolSiC device products are rated from 650V to 2,000V and are suitable for hard switching and resonant switching topologies.
At PCIM 2023, Toshiba launched a 2,200V-rated SiC MOSFET with embedded Schottky barrier diode (SBD). The device is packaged as a dual SiC MOSFET module. This voltage rating allows the use of two-level inverters in 1,500V photovoltaic applications. They have fewer switching stages than three-level topologies and are therefore smaller and lighter. The 2,200V-rated device follows the same RDS(on) vs. Vds relationship as previous SiC MOSFETs rated at 1,700V and 3,300V.
SiC MOSFETs may be affected by the growth of base surface defects during the forward conduction phase of their built-in body diodes. This results in increased RDS(on) and reduced reliability. Unipolar SBD conduction effectively avoids this bipolar conduction phase, thereby improving reliability. Due to the lower forward conduction voltage of SBD, using SBD can also reduce dead time losses in the converter.
One of the problems in photovoltaic applications is cosmic ray degradation and failure. One study showed that the single-event burnout failure rate caused by ground neutron exposure is a function of the VDS/VAVAL ratio, where VDC is the rated voltage of the device and VAVAL is its avalanche onset voltage. For devices with a VDS/VAVAL of 0.5 or less, the real-time failure rate of 1 part per billion is much lower (
Figure 3: Comparison of neutron irradiation failure rates for Toshiba’s 2,200V SiC MOSFETs (shown in green) versus existing 1,700V devices. (Image source: Ogata et al., 2023)
High-temperature reverse-bias testing of these 2,200V MOSFETs at full rated VDS at 150°C showed that key DC parameters—such as threshold voltage Vth, RDS(on), and drain leakage—stressed at 2,000 hours. Less than 5% change occurred within the interval.
They compare the switching of three-level inverters using 1,200V and 1,700V Si IGBTs (operating frequency 3.9kHz) to two-level inverters using these 2,200V SiC MOSFETs (operating frequency 7.8kHz) Performance was compared. This is done at 1,200V, 200A and 125°C. Even at higher frequencies, the power consumption of the 2,200V converter is significantly reduced by 37%.
Both residential and utility-scale photovoltaics can benefit from the use of WBG power converters and ESS solutions. Microinverters and single-phase string inverters can benefit from GaN’s low-loss, high-switching performance, significantly increasing power density and simplifying converter topologies. High-power hybrid inverters and three-phase central inverters for utilities can also benefit from SiC devices, which have advantages over silicon-based devices in terms of power loss, high-temperature performance and power density.
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