Energy Storage PCS Efficiency Stuck at 98%: How Much Can SiC SBDs Change?

Energy Storage PCS Efficiency Stuck Below 98%: How Much Difference Can SiC SBDs Make?

     In the AC/DC and DC/DC power stages of energy storage PCS, the reverse recovery loss of traditional silicon fast-recovery diodes is one of the key factors limiting further efficiency improvements. SiC Schottky barrier diodes (SBDs) have zero reverse recovery charge (Qrr≈0), which can almost eliminate diode-related losses in high-frequency switching scenarios. Measured data shows that SiC solutions can reduce total losses of 125 kW-class PCS by 20%–30%, pushing maximum system efficiency from the 96%–97% range to above 99%.

The “last mile” of PCS efficiency: where do the losses go?

     Energy storage converters (PCS) perform bidirectional power conversion between the battery and the grid, and their efficiency denominator is the megawatt-level power that runs year-round. A difference of a few tenths of a percentage point in efficiency translates into an annual cumulative energy difference on the order of hundreds of thousands of kWh.

     In hard-switching topologies, the reverse recovery current and reverse recovery loss (Err) generated by silicon fast-recovery diodes (FRDs) at turn-off are a non-negligible component of switching loss. The higher the switching frequency, the larger the share of this loss. Traditional silicon IGBT solutions limit switching frequency below 20 kHz, largely to control the loss increase caused by diode reverse recovery.

     The Schottky barrier structure of SiC SBDs physically eliminates the minority-carrier storage effect, so the reverse recovery charge Qrr is theoretically zero. This means that at the same switching frequency, the turn-off loss of the diode branch is greatly reduced, clearing a key obstacle for PCS to evolve toward higher frequency and higher power density.

Specific locations of SiC SBDs in PCS

     In mainstream topologies for commercial and industrial energy storage PCS, SiC SBDs mainly appear in the following locations:

The midpoint clamping branch of the T-type three-level topology. The diode in this branch experiences reverse recovery stress in every switching cycle. The zero-Qrr characteristic of SiC SBDs directly eliminates recovery loss at this node and also reduces the turn-on loss of the associated switch (because the diode reverse recovery current at turn-on is eliminated).

     The freewheeling and rectification paths of the bidirectional DC-DC stage. The round-trip charge/discharge efficiency of an energy storage system depends on the loss symmetry when current flows in both directions. SiC SBDs produce no reverse recovery penalty in either the charging (Buck) or discharging (Boost) direction, making the efficiency curve flatter over a wide load range.

     HyperSemi’s SiC SBD product line covers 650V / 1200V / 1700V voltage classes, with forward currents from 6 A to above 40 A, and packages including TO-247-2L, TO-247-3L, TO-220AC, TO-252, TO-263, DFN8×8, etc., directly matching the diode selection requirements for the 650 V bus (two-level/three-level low-voltage side) and 1200 V bus (high-voltage DC side) in commercial and industrial PCS. For specific part numbers and parameters, see the SiC diode product page.

     The high-frequency capability enabled by SiC SBDs allows PCS switching frequency to increase from the 20 kHz level to above 50 kHz, with magnetic component size decreasing accordingly. Smaller inductors and cooling systems mean higher power density, which directly translates into increased installable capacity under the space-constrained conditions of commercial and industrial energy storage sites. At the same time, the positive temperature coefficient of forward voltage drop in SiC SBDs gives them a natural current-sharing tendency when multiple devices are paralleled, reducing the thermal runaway risk of parallel designs.