Page 66 - NEW Armstrong Book - 2

generate and supply continuous energy, some form of energy storage must be inserted into the system to “buffer” the supply and demand.
Here again, the power-conversion properties of SiC — higher switching frequencies, greater system efficiencies, smaller systems, and lower system costs — can be transposed to the bidirectional battery charging for the energy storage section. Figure 2 shows how a renewable solar energy system is con- nected to a battery energy storage system that then supplies a point of load — in this case, a fast DC off-board charging station — and supplies the load when in an excess energy condition. Effectively, this is a DC-coupled system.
SiC’s real design impact: solar
At present, SiC is proving to be more efficient than tradition- ally used silicon. Solar-string systems implement MPPT between a series of panels and the grid-tied inverter. The MPPT circuit is essentially a boost converter in which effi- ciency and power density are critical to the performance of
FIGURE 3: THE PHYSICAL FOOTPRINT OF AN IGBT 50-KW MPPT BOOSTER
(LEFT) IN COMPARISON WITH WOLFSPEED’S SiC 60-KW MPPT BOOSTER (RIGHT) (SOURCE: WOLFSPEED)
the system design. In past designs, the booster would be IGBT-based, with devices switching at 15–30 kHz and effi- ciency in the range of ~97%.
By implementing the same booster circuit with Wolfspeed’s C3M MOSFET and C4D diodes, system-level efficiency now achieves 99.5% peak, with a dramatic improvement in overall MPPT size and cost (Figure 3).
The design implementation with SiC technology is simple: Increase the switching frequency of the SiC MOSFETs and utilize the near-zero reverse-recovery characteristics of the
SiC boost diodes. This helps in achieving the lowest circuit loss while minimizing the size and, hence, cost of the boost inductors, capacitors, and cooling systems.
Battery charging
Inserting battery storage into any renew- able energy system means some form of DC/DC power conversion to charge the bat-
tery. Here again, SiC is proving to be more efficient than traditionally used silicon. Isolated bidirectional DC/DC conversion
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ASPENCORE GUIDE TO SILICON CARBIDE
FIGURE 2: ENERGY STORAGE AND DC FAST-CHARGING POWER CONVERSION (SOURCE: WOLFSPEED)