Silicon carbide is widely recognized as an established technology that is transforming the power industry in many applications ranging from watts to megawatts across the industrial, energy, and automotive segments.
This is primarily due to its numerous advantages over previous silicon (Si) and insulated-gate bipolar transistor (IGBT) implementations, which include higher switching frequencies, lower operating temperatures, higher current and voltage capacities, and lower losses, all of which lead to increased power density, reliability, and efficiency.
SiC became a mature technology and a popular solution for systems requiring power delivery, particularly charging and discharging in energy storage applications such as electric vehicle charging and solar systems with batteries. These systems frequently include silicon carbide technology opportunities such as DC/DC boost converters, bidirectional inverters (with both AC and DC elements), and flexible battery charging circuitry.
In a nutshell, SiC increases system efficiency by up to 3%, increases power density by 50%, and reduces passive component volume and costs. SiC components are useful in most energy storage systems (ESS) that have multiple power stages.
Figure 1 depicts a typical ESS architecture that includes a power source (photovoltaic, or PV, in this case, but any alternative energy source can be used), a DC/DC converter, a battery charger, and an inverter for delivering energy to the home or back to the grid. SiC can improve efficiency, size, weight, and cost across all three power blocks in this configuration.
Fig. 1
Sic Advantages In Ess Power Blocks
When managing harvested energy and storing it for later use or powering a home/building, there are several power stages to consider.
The DC/DC conversion section, which frequently includes a boost converter for PV applications, benefits greatly from higher system efficiencies and power densities. When compared to traditional technologies like Si, typical benefits of SiC include a 70% reduction in system size, a 60%+ reduction in energy losses, and a 30% reduction in system cost.
Figure 2 depicts a 60-kW SiC-based interleaved boost converter (from the Wolfspeed reference design CRD-60DD12N) with multiple silicon carbide mosfets and diodes. With 850 VDC on the output, the four interleaved channels help scale output power up to 60 kW while maintaining efficiency of 99.5 percent.
Two C3M0075120K MOSFETs (in a TO-247-4L package with a Kelvin source pin), two C4D10120D diodes (per channel), and a CGD15SGOOD2 isolated discrete gate driver are used in this design.
Fig. 2
A BOM cost analysis/comparison was performed at different switching frequencies in the reference design shown above. Higher frequencies (100 kHz versus 60 kHz) may result in significant cost savings due to smaller, lighter-weight components/magnetics, whereas cooling may increase costs due to higher operating temperatures.
Higher frequencies, on the other hand, generally imply higher system efficiency, higher power density, and lower cost. This is how SiC can provide better performance at a lower cost.
Figure 3 depicts the benefits of SiC for both the inverter and the DC/DC charging circuitry. This design can operate in single-phase or three-phase mode, with charging and discharging efficiencies of more than 98.5 percent.
With only six SiC MOSFETs, the converter section consists of a simple two-level AC/DC converter that is compatible with single- and three-phase connections. Although this configuration is more expensive than most IGBT converters, it outperforms in terms of efficiency and loss. Though the T-type AC/DC converter has a similar switching frequency and efficiency, it frequently requires more complex control and has a higher part count with a lower power density.
Due to the electrical and thermal stress, the C3M0032120K 1200V 32-mΩ SiC MOSFET is well-suited due to its figure of merit, easy control and Vgs drive characteristics, and Kelvin source package, which reduces switching losses and crosstalk.
This topology supports advanced digital control schemes that perform various functions, such as a single-phase interleaved PFC scheme or a three-phase DQ transformation space-vector PWM scheme that balances switching losses in all devices, resulting in a very flexible reference platform. Using PWM control per switch can aid in sensing and balancing power dissipation while optimizing thermal performance, efficiency, and reliability.
When tested and measured for efficiency across a variety of loads and voltage ranges for single-phase charging, SiC achieves up to 98.5% efficiency, while IGBTs may top out at 96% resulting in 38% lower losses for SiC.
Figure 4 depicts two plots of the AFE for charging and discharging at various power levels.
The same peak efficiencies were obtained for three-phase charging, as well as thermal performance that was well within the system and device limitations. Although T-type topologies can achieve comparable performance, they are generally more complex and expensive.
To summarize the 22-kW inverter/AFE configuration, the C3M0032120K SiC MOSFETs and flexible control scheme enable high efficiency (>98.5%), high power density (4.6 W/L), lower loss (60%), and bidirectional chargers that can support the DC-link from both three-phase and single-phase AC inputs while producing a wide battery voltage range of 200–800 VDC.
Advantages of Sic Circuits in DC/DC Battery Chargers
Many topologies support isolated DC/DC converters. However, half-bridge and full-bridge LLC converters are the most widely used solutions.
A Wolfspeed reference design (CRD-22DD12N) demonstrates a 22-kW solution that can be configured for either a cascade or a single two-level converter. The cascade converter can use either 650-V Si MOSFETs or SiC components, but it will have a higher part count, higher conduction losses, greater control complexity, and a higher system cost. The single two-level converter operates at a much higher frequency of 200 kHz and employs SiC components for the higher voltage (1,200 V).
The main advantage of SiC-based technology here is higher efficiency/lower losses, along with some additional features such as zero-voltage turn-on, low-current turn-off, and lower EMI risk. The topology has fewer parts than the cascade converter, which reduces system costs and simplifies control. The differences between these two layouts are shown in Figure 5.
When considering power components for this 22kW design, it was discovered that the C3M0032120K 1,200-V 32-m MOSFET provides the best electrical stress and thermal characteristics for the converter. Furthermore, its Vgs can support 15 V, making it easy to drive.
A variable DC-link voltage control (based on sensed battery voltage) maximizes system efficiency and ensures that the CLLC operates at or near resonant frequency. When the battery voltage falls below a certain threshold, the control enters phase-shift mode, which reduces gain without running inefficiently outside of the resonant frequency range. This means that similar high efficiency can be obtained at lower output voltages using the same hardware.
If a lower battery voltage is required, the CLLC primary can be run as a half-bridge, which reduces gain while maintaining the efficiency zone. Because of the lower operating costs and less stringent thermal design, this lower efficiency may still be acceptable.
Figure 6 depicts the waveforms for a full-bridge configuration’s charging and discharging modes. The charging mode screenshot shows a zero voltage turn-on with a low-current turn-off, resulting in high-efficiency operation.
The efficiency results of the converter are comparable to the inverter reference design, with 98.5% peak efficiency across the majority of the load. Until the design enters half-bridge mode, which limits efficiency and power delivery capabilities during charging, the variable DC-link voltages and resulting efficiency remain above 97%.
In general, the SiC MOSFETs, combined with the flexible control scheme, enable high-efficiency (>98.5 percent efficiency for charging/discharging) and high-power–density (8 kW/L) bidirectional chargers that accept single-phase and three-phase AC inputs.
Due to the simplicity of the gate driver, thermal management components, reduced part counts, and smaller magnetics, higher efficiency and power density are achieved for significantly less cost when compared to Si.