Benefits Of SiC For String Inverters

SiC MOSFETs in solar and energy storage applications have clear benefits compared to silicon technologies, addressing the pressing need for energy and cost savings, particularly when bidirectional power conversion is required.

Ease of installation is one of the key features of high-power solar string inverters. It is beneficial if only two workers are needed to carry and install the system. Hence, size and weight are of special importance. Latest generation silicon carbide semiconductors enable a significant increase in power conversion efficiency. This does not only save energy, but also results in smaller and lighter equipment with lower associated capital, installation, and maintenance costs.

Key application requirements and their challenges

The high operation voltage of 1500 V in utility scale installations requires a low cosmic-radiation-induced failure rate combined with a higher system efficiency for the power devices. Because of these contradictory requirements, ANPC multilevel topologies are the preferred solutions today due to their highest efficiency over the full range of power factor operation (figure 1). Such inverters are perfectly designed for both solar and battery storage applications.

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

A cost-effective way of adapting the ANPC topology is to optimally combine Infineon’s 1200 V CoolSiC MOSFET with TRENCHSTOP IGBT7 technology. Figure 2 shows a phase leg of the proposed implementation, whereby T1, T4, T5 & T6 are composed of silicon-based IGBTs with corresponding silicon freewheeling diodes (FWD). Transistors M2 and M3 consist of CoolSiC MOSFETs with internal body diodes. By using the modulation scheme [1] & [2] presented below in figure 2, the IGBTs are switched continuously ON or OFF with grid frequency of 50/60 Hz. Hence the IGBTs are optimized for lower conduction losses. In this way, switching losses only occur in the fast and highly efficient SiC MOSFETs. Thus the number of SiC devices is reduced to a minimum, achieving an optimal cost-performance ratio.

Fig. 2: ANPC topology in an Easy 3B power module and its modulation scheme.

Compared with an IGBT inverter solution, comparable sized SiC MOSFET modules can also handle more power. For example, an Infineon 950 V EasyPACK 3B IGBT module operating at 16 kHz can be replaced by two smaller EasyPACK 2B size 1200 V CoolSiC modules at 32 kHz. With a 32% increase in power handling to 139 kVA, this solution has almost 5% lower power conversion losses. This further improves the inverter efficiency by 0.3 % – which is a true “Revolution to rely on!”

Fig. 3: Comparison of 950 V IGBT solution switched at 16 kHz against SiC MOSFET solution switched at 32 kHz.

Reference design proves the benefits

In order to demonstrate the significant benefits of using SiC MOSFETs in PV string and energy storage inverters, Infineon has developed a modular reference design for 1500 VDC systems rated up to 300 kW. The design uses a novel bidirectional 3-level ANPC topology which achieves an efficiency close to 99% in both directions and a switching frequency up to 96 kHz (interleaving configuration). Power density is greater than 5 kW/kg for a complete solution including heatsinking and all control, allowing 300 kW throughput in the ideal 80kg maximum cabinet weight.

Energy usage and cost savings by using SiC can be easily calculated from overall efficiency improvements; for example, compared with a super-junction Si MOSFET solution, 1200 V CoolSiC MOSFETs can halve the losses in an ESS installation and provide typically 2% extra energy and run-time. SiC MOSFET unit cost is not yet generally less than an IGBT for similar performance. But on system level, hardware costs are reduced significantly, as high efficiency at higher switching frequencies allows to use smaller and cheaper magnetic components and heatsink. For example, in a 1500 V PV string inverter, at least 5-10% saving in cost per kW can be expected (figure 4).

Fig. 4: System costs in string inverters are significantly lower with SiC MOSFETs compared with IGBTs.

References:
[1] D. Floricau et al. “The efficiency of three level active NPC converter for different PWM strategies,” EPE, 2009
[2] B. Sahan et al. “Combining the benefit of SiC T-MOSFET and Si IGBT in a novel ANPC power module for highly com-pact 1500-V grid-tied inverters”, PCIM Europe, 2019