The diversity of architectural designs inevitably leads to a wide variety of solar panel installation configurations. To maximize solar energy conversion efficiency while preserving the aesthetic appeal of a building, a diverse range of inverters is required to achieve optimal solar energy conversion. String inverters have become the most popular choice in the international market. Based on a modular concept, each photovoltaic (PV) string (ranging from 1 kW to 5 kW) is connected to an individual inverter, which features Maximum Power Point Tracking (MPPT) on the DC side and grid-parallel connection on the AC side. Many large-scale PV power plants utilize string inverters. Their advantages include immunity to performance variations between modules in different strings and reduced impact from shading; furthermore, they mitigate mismatches between the optimal operating points of PV modules and the inverters, thereby increasing total power generation. These technical strengths not only lower system costs but also enhance overall system reliability. Additionally, the "master-slave" configuration allows multiple PV strings to be linked together; if the energy from a single string is insufficient to operate an individual inverter, the system can combine strings to ensure that one or more inverters remain operational, resulting in higher energy output. The latest approach involves multiple inverters forming a "team" rather than relying on a "master-slave" setup, which further improves system reliability. Driven by the growing demand for renewable energy, the market for solar (PV) inverters is expanding rapidly. These inverters require exceptionally high efficiency and reliability. This analysis examines the power circuits employed in such inverters and recommends optimal components for switching and rectification. There are three distinct general architectures available for PV inverters. Sunlight strikes solar modules connected in series, with each module containing a series of individual solar cells. The DC voltage generated by these solar modules is typically in the range of several hundred volts-a value that varies based on the irradiance conditions of the module array, the temperature of the cells, and the number of modules connected in the series string. The primary function of this type of inverter is to convert the input DC voltage into a stable value. This is achieved via a boost converter, which requires a boost switch and a boost diode. In the first configuration, the boost stage is followed by an isolated full-bridge converter, where the full-bridge transformer provides electrical isolation. A second full-bridge converter on the output side converts the DC voltage from the first stage into an AC voltage. The output is then filtered before being connected to the AC grid via a dual-contact relay, which ensures safe disconnection during faults and isolation from the grid at night. The second configuration is a non-isolated design, in which the AC voltage is generated directly from the DC output of the boost stage. The third configuration employs an innovative topology that integrates the boost and AC-generation functions into a single, specialized circuit. Although the conversion efficiency of solar panels is quite low, it is crucial for the inverter to achieve an efficiency as close to 100% as possible. In Germany, a 3 kW array of solar modules installed on a south-facing roof is expected to generate 2,550 kWh annually. Increasing the inverter's efficiency from 95% to 96% would yield an additional 25 kWh per year-an amount that would otherwise require the installation of extra solar modules, at a cost comparable to that of an additional inverter. Since improving efficiency from 95% to 96% does not double the cost of the inverter, investing in higher-efficiency units is a logical choice. For emerging designs, maximizing inverter efficiency in the most cost-effective manner is a key design criterion, alongside reliability and cost. Higher efficiency reduces temperature fluctuations over the load cycle, thereby enhancing reliability; thus, these criteria are inherently linked. The use of modular components also contributes to improved reliability. All the topologies shown in Figure 1 require fast-switching power devices, while the boost and full-bridge stages also necessitate the use of fast-switching diodes. Furthermore, switches optimized specifically for low-frequency (100 Hz) switching are also useful for these topologies. For any given silicon technology, switches optimized for fast switching exhibit higher conduction losses than those optimized for low-frequency applications.
The boost stage is typically designed as a continuous-conduction-mode (CCM) converter. The choice between 600V and 1200V devices depends on the number of solar modules in the array connected to the inverter. Power switches generally consist of either MOSFETs or IGBTs; in general, MOSFETs can operate at higher switching frequencies than IGBTs. Additionally, the impact of the body diode must always be considered-though this is not an issue in the boost stage, as the body diode does not conduct during normal operation. The conduction loss of a MOSFET is determined by its on-resistance ($R_{DS(ON)}$), which, for a given MOSFET family, is proportional to the effective die area. As the voltage rating increases from 600V to 1200V, the conduction loss of a MOSFET rises significantly; consequently, even if a 1200V MOSFET has a comparable $R_{DS(ON)}$ rating, it may either be unavailable or prohibitively expensive.
For 600V-rated boost switches, super-junction MOSFETs are an excellent choice, offering optimal conduction loss for high-frequency switching applications. Options include MOSFETs in TO-220 packages with an $R_{DS(ON)}$ below 100 mΩ, or those in TO-247 packages with an $R_{DS(ON)}$ below 50 mΩ. For solar inverters requiring 1200V power switches, IGBTs are the appropriate solution. Advanced IGBT technologies-such as NPT Trench and NPT Field Stop-are optimized for lower conduction losses, but this comes at the cost of higher switching losses, making them less suitable for high-frequency boost applications. Based on legacy NPT (Non-Punch-Through) planar technology, the FGL40N120AND device has been developed to improve the efficiency of boost circuits operating at high switching frequencies. It features an $E_{OFF}$ (turn-off energy loss) of 43 µJ/A-a performance level that is otherwise very difficult to achieve, especially when compared to the 80 µJ/A $E_{OFF}$ typical of devices utilizing more advanced technologies. While the FGL40N120AND has a relatively high saturation voltage ($V_{CE(SAT)}$)-3.0 V (or 2.1 V at 125°C)-this is more than offset by its exceptionally low switching losses at high boost frequencies. The device also integrates an anti-parallel diode. Under normal boost operation, this diode remains in a non-conducting state; however, during startup or transient events, the boost circuit may be driven into an operating mode where the anti-parallel diode begins to conduct. Since an IGBT lacks an inherent body diode, this co-packaged diode is essential for ensuring reliable operation. For the boost diode itself, a fast-recovery type-such as a Stealth™ diode or a silicon carbide (SiC) diode-is required. SiC diodes, in particular, offer very low forward voltage and minimal losses. When selecting a boost diode, one must account for the impact of reverse recovery current (or, in the case of SiC diodes, junction capacitance) on the boost switch, as these factors contribute to additional losses. In this context, the newly introduced Stealth II diode (FFP08S60S) delivers superior performance: under conditions of $V_{DD} = 390\text{ V}$, $I_D = 8\text{ A}$, $di/dt = 200\text{ A/µs}$, and a case temperature of 100°C, the calculated switching loss is 205 mJ, compared to 225 mJ when using the ISL9R860P2 Stealth diode. Consequently, this solution also enhances the efficiency of the inverter at high switching frequencies. Following the MOSFET full-bridge and filtering stage, the output bridge generates a 50Hz sinusoidal voltage and current signal. A common implementation uses a standard full-bridge topology (Figure 2). In this configuration, turning on the top-left and bottom-right switches applies a positive voltage across the output terminals, while turning on the top-right and bottom-left switches applies a negative voltage. In this application, only one switch is active (conducting) at any given time. One switch operates at a high PWM frequency, while the other operates at a low 50Hz frequency. Since bootstrap circuits rely on the switching action of the low-side device, the low-side switch is operated at the high PWM frequency, whereas the high-side switch operates at the 50Hz frequency. Given that this application utilizes 600V power switches, 600V super-junction MOSFETs are highly suitable. Because these switches must withstand the full reverse-recovery current of the complementary devices during turn-on, fast-recovery super-junction devices-such as the 600V FCH47N60F-are an ideal choice. With an $R_{DS(ON)}$ of 73 mΩ, this device offers lower conduction losses compared to other similar fast-recovery components. Although the fast-recovery feature is not required when these devices switch at 50Hz, they offer superior $dv/dt$ and $di/dt$ characteristics, which enhance system reliability compared to standard super-junction MOSFETs.
Another option worth considering is the FGH30N60LSD, a 30A/600V IGBT with a saturation voltage ($V_{CE(SAT)}$) of only 1.1V. Due to its very high turn-off loss ($E_{OFF}$) of 10 mJ, it is suitable only for low-frequency switching. A 50 mΩ MOSFET, at operating temperature, exhibits an $R_{DS(ON)}$ of 100 mΩ; consequently, at a current of 11A, its $V_{DS}$ is equivalent to the $V_{CE(SAT)}$ of this IGBT. Furthermore, because this IGBT is based on older punch-through technology, its $V_{CE(SAT)}$ remains relatively stable with respect to temperature changes. Consequently, this type of IGBT can reduce total losses in the output bridge, thereby increasing the overall efficiency of the inverter. The FGH30N60LSD IGBT is also highly effective for applications that require switching between different power conversion technologies-or specific topologies-within each half-cycle; in such cases, the IGBT functions as a topology switch. For faster switching transitions, conventional or fast-recovery superjunction devices are employed. For 1200V-rated specialized topologies and full-bridge configurations, the aforementioned FGL40N120AND is an ideal switch for modern, high-frequency solar inverters. When a specific technology requires a diode, Stealth II™ diodes, Hyperfast™ II diodes, or silicon carbide (SiC) diodes serve as excellent solutions.