What makes SiC the perfect solution for string inverters

In photovoltaic and energy storage systems, the high system voltage of 1500V requires a very low failure rate caused by cosmic radiation, and requires power devices to have higher system efficiency. Due to these conflicting requirements, the ANPC multilevel topology is currently the preferred solution because of its highest efficiency over the entire power factor operating range (Figure 1). Such an inverter is perfect for solar and battery storage applications.

Figure 1: Comparison of the two most common topologies for 1500V PV inverters

A cost-effective way to adapt the ANPC topology is to optimally combine Infineon’s 1200V CoolSiC™ MOSFET with TRENCHSTOP™ IGBT7 technology. Figure 2 shows one leg of the reference solution, where T1, T4, T5 and T6 are composed of silicon-based IGBTs and corresponding silicon freewheeling diodes (FWD). Transistors M2 and M3 consist of CoolSiC™ MOSFETs with body diodes. By using the modulation schemes [1] and [2] in Figure 2, the IGBT switches at the mains frequency of 50/60Hz. Therefore, IGBTs are optimized to have lower conduction losses. This way, switching losses only occur in fast and efficient SiC MOSFETs. Therefore, the number of SiC devices is reduced to a minimum, achieving the best cost-performance ratio.

Figure 2: ANPC topology and its modulation scheme in Easy 3B power module

Compared to IGBT inverter solutions, SiC MOSFET modules of comparable size can also handle more power. For example, an Infineon 950V EasyPACK™ 3B IGBT module operating at 16kHz can be replaced by two smaller EasyPACK™ 2B size 1200V CoolSiC™ modules operating at 32kHz. With power handling increased by 32% to 139 kVA, this solution reduces power conversion losses by almost 5%. This further increases the inverter's efficiency by 0.3% - a true "revolution you can trust!"

Figure 3: Comparison of a 950V EasyPACK™ 3B IGBT solution switching at 16kHz and an EasyPACK™ 2B CoolSiC™ MOSFET solution switching at 32kHz

To demonstrate the significant advantages of using SiC MOSFETs in PV strings and energy storage inverters, Infineon has developed a modular reference design for 1500 VDC systems with power ratings up to 300kW. The design uses a novel bidirectional 3-level ANPC topology with nearly 99% efficiency in both directions and a switching frequency of up to 96kHz (interleaved parallel structure). For a complete solution including radiator and all controls, the power density is greater than 5 kW/kg, resulting in an output of 300 kW at an ideal maximum cabinet weight of 80 kg.

Energy use and cost savings can be easily calculated from the overall efficiency improvements by using SiC; for example, 1200V CoolSiC™ MOSFETs can cut losses in an ESS installation in half compared to Super Junction Si MOSFET solutions and provide the usual 2% extra energy and run time. For similar performance, SiC MOSFETs generally have higher unit costs than IGBTs. But at a system level, hardware costs are significantly reduced because higher switching frequencies allow the use of smaller, cheaper magnetics and heat sinks. For example, in a 1500V PV string inverter, the cost per kilowatt can be saved by at least 5-10% (Figure 4).

Figure 4: String inverters using SiC MOSFETs offer significantly lower system costs compared to IGBTs

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[Infineon Industrial Semiconductors]