Jeffrey Casady & Paul Kierstead, Cree
Silicon carbide (SiC) power semiconductor technology has now reached a point in its evolution where SiC power devices can serve as the catalyst for a new generation of ultra-efficient power electronics systems, especially in the emerging market for small- to medium-sized solar power inverters. Several component manufacturers already offer a comprehensive portfolio of discrete SiC power MOSFETs (metal-oxide semiconductor field-effect transistors) and Schottky diodes, and the subsequent introduction of all-SiC integrated power modules – a form factor familiar to power electronics design engineers – has demonstrated the potential to revolutionize solar inverters by enabling the design of smaller, lighter, and higher-efficiency units for three-phase power conversion than can be achieved with conventional silicon technology. This development, in turn, can lower the overall system costs for solar energy and thus encourage more widespread commercial and residential installations.
SiC’s electrical properties
As a compound semiconductor material, silicon carbide has a wider bandgap, higher thermal conductivity, and a much higher breakdown voltage than silicon (Si), which enables it to outperform silicon devices in high voltage switching applications. Since IGBTs (silicon insulated gate bipolar transistors) experience significantly higher switching losses as operating frequency increases, design engineers are forced to over-specify silicon devices (at higher amperage ratings) and/or to lower the operating frequency of their system to mitigate the Si IGBT’s switching losses, consequently reducing overall system efficiency.
SiC MOSFETs have inherently better switching efficiency at high frequencies and high voltages, in addition to significantly better thermal performance. As such, SiC power devices can achieve power densities and switching frequencies that are just not possible with conventional silicon components.
Replacing silicon devices with SiC power module boosts inverter output from 10kW to 40kW. The paradigm change potential of SiC power technology is demonstrated by the following example, in which a 10kW solar inverter from one of the leading manufacturers was retrofitted with a Cree SiC MOSFET module (see Figure 1) replacing the incumbent silicon power module to determine how these advanced power switching devices could significantly increase delivered power while maintaining equivalent system physical form-factor.
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Figure 1 – Cree’s 1200V-50A 6-Pack Module
SiC simplifies inverter topology
The existing solar inverter unit employed a silicon-based power module of the same housing shown in Figure 1 that combined 600–650V Si super-junction MOSFETs for high frequency switching and 600–650V Si IGBTs for low frequency switching, reflecting the frequency limitations of Si IGBTs. The silicon module utilizes a relatively complex topology and control scheme to produce 10kW of power with a modest efficiency performance of 98.1% for the overall system.
Replacing the Si devices with a SiC MOSFET module rated for 1.2kV/50A in an integrated three-phase, hard-switched configuration significantly simplified the topology, as SiC components are much smaller than comparable silicon devices. Consequently, they enabled much more power in a full three-phase configuration. The SiC module allows generation of 40kW at twice the frequency and while maintaining similar losses versus the original 10kW silicon-based device. The result is 4X the delivered power in the same physical form-factor as the incumbent silicon system. What follows are the key design optimizations enabled by SiC device advantages.
SiC’s lower switching losses
Since the SiC MOSFET module exhibits considerably lower switching losses than the Si devices (as shown in Figure 2), it was possible to increase the inverter’s switching frequency from ~20kHz up to 48kHz. Filter elements are critical components of any solar inverter that ties into the utility grid. The passive inductors and capacitors required to design the line filters are inversely proportional to the ripple frequencies of the power conversion electronics and are essential to shrinking the size, weight, and cost per delivered watt in the SiC based solution. The higher frequency operation enabled by the SiC switching devices reduced the relative volume and weight of the inductors required for the output power. As a result, the 40kW SiC inverter was able to use approximately the same volume inductor as was required by the 10kW Si inverter.
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Figure 2: Switching losses for 150A Si IGBT and 100A SiC MOSFET at 150°C
In addition, many solar power inverter applications specify aluminum electrolytic capacitor banks. Due to their lower cost, these devices are much more likely to be primary failure points than their polypropylene film counterparts, which are typically more expensive. However, the higher frequency operation enabled by the SiC switching components reduces the number of film capacitors required while maintaining the same DC link voltage ripple, effectively enhancing overall system reliability.
As shown in Figure 3, the 40kW SiC inverter board realized significant PCB space savings when compared to the 10kW Si inverter layout. The bank of 22 aluminum electrolytic capacitors required for the 10kW output of the Si inverter are pictured on the left and the eight polypropylene film capacitors required for the 40kW output power of the SiC inverter are pictured on the right. Converting to SiC switching devices at the higher frequency greatly reduced the volume of capacitors per kW of delivered power. Even factoring in the higher unit cost of the film capacitors (8 polypropylene caps for ~$118 at a 500-piece cost vs. 22 aluminum electrolytic caps at ~$123 at a 500-piece cost), the component cost averages out in the much more efficient SiC-enabled inverter.
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Figure 3: Comparison between Si 10kW inverter electrolytic capacitor bank (left) and the SiC 40kW inverter polypropylene capacitor bank (right)
Thermal management requirements
Even though the power output of the inverter was boosted from 10kW to 40kW by implementing the SiC power devices, the superior thermal characteristics of SiC allowed for the continued employment of the original heatsink, successfully avoiding adding additional volume or weight to the system despite a 4X increase in power output.
Moreover, in high-level simulations used to determine the link ripple voltage and output harmonic distortions of each baseline unit for comparison purposes, the SiC MOSFET module exhibited extremely low conduction losses, enabling the SiC system to double the delivered current while maintaining the same die size as the super-junction Si MOSFETs used in the 10kW inverter. Consequently, the SiC system was able to deliver four times the output power within the same footprint as the 10kW system.
SiC delivers 4X the output power
The example above clearly demonstrates that replacing inefficient Si switching devices with SiC MOSFETs and diodes has the potential to quadruple the power density of an existing 10kW PV inverter system while also simplifying the topology, effectively delivering robust and reliable three-phase 480VAC output power in the exact same enclosure.
Higher output power, higher operating efficiencies, improved thermal characteristics, and higher operating frequencies can all be realized by using an all-SiC module design for solar inverters. Further, these inherent advantages enable designs with fewer and more reliable capacitive and inductive components, ultimately contributing to a lighter weight, more compact, and lower cost system capable of delivering more output power per dollar.
The aforementioned simulation and evaluation validates the considerable benefits of replacing the Si switching devices in existing solar inverter systems with all-SiC modules. However, it is reasonable to assume that a new design for next-generation solar power inverters optimized for SiC devices from the earliest stages of the design cycle could help achieve even higher efficiencies, cost savings, and power densities through optimized circuit design and magnetic/filter element selection.