AI Power Supplies Stuck at 96%: How Can SiC Break the Deadlock?
Capturing the final 1% of efficiency in PFC+LLC architectures
The AI compute boom is pushing server PSUs toward the Titanium efficiency threshold, and reverse-recovery losses in conventional silicon-based solutions at the PFC and LLC stages have become a bottleneck. SiC Schottky diodes, with zero reverse recovery, reduce switching losses in interleaved PFC and LLC secondary rectification, helping systems cross the 96% efficiency line. Selection hinges not on “whether to use SiC,” but on matching voltage class and current rating to the topology position.
Power consumption per AI server rack is climbing from tens of kilowatts to above 150 kW. One direct consequence of this surge is that the efficiency margin in power modules is being compressed sharply. The 80 PLUS Titanium standard at 230 V input requires 90% efficiency at 10% load, 94% at 20%, 96% at 50%, and 94% at 100%. Conventional silicon-based PSUs mostly plateau at 94%–95%, leaving only one or two percentage points to the 96% at 50% load—but crossing that gap is far harder than moving from Gold to Platinum.
The bottleneck is not the main switch; it is the diode. The boost diode in the PFC stage and the secondary rectification diode in the LLC stage are the two nodes where reverse-recovery loss concentrates. SiC Schottky diodes enter this scenario with Qrr approaching zero. Their role is not “blanket replacement of silicon devices,” but removing the loss component that silicon cannot eliminate within the critical efficiency window.
PFC Stage: How Interleaved Topologies Amplify Reverse-Recovery Loss
The front-end PFC of AI server PSUs is migrating toward interleaved totem-pole or bridgeless topologies. The more complex the topology and the higher the switching frequency, the greater the weight of the silicon fast-recovery diode’s reverse-recovery loss in total loss. At every main-switch turn-on, the diode’s reverse-recovery current spike superimposes on the switch current waveform, directly raising turn-on loss (Eon) and exciting voltage ringing across parasitic inductance. Published measurements indicate that replacing the PFC boost diode with a SiC Schottky improves PFC-stage total efficiency by 1–1.5 percentage points.
That magnitude of improvement lands precisely in the most critical assessment window of the Titanium standard at 50% load. Hypersemi offers 1200 V-class SiC SBD options for this position. CR30120L2X100 (1200 V / 30 A / VF 1.4 V / TO-247-2L) covers the PFC boost stage on an 800 V DC bus, and its TO-247-2L package aligns with the through-hole footprint of mainstream silicon FRDs, so replacement evaluation does not require reworking the heatsink mechanical structure. For lower-bus-voltage architectures, CR2065SX100 (650 V / 20 A / VF 1.25 V / TO-263) in a surface-mount format suits high-density PSU layouts.
Topology Position | Voltage Class | Core Loss Mechanism | Corresponding Model / Option |
PFC boost stage | 1200 V | Reverse-recovery current raises main-switch Eon and excites ringing | CR30120L2X100 (30 A / VF 1.4 V / TO-247-2L) |
PFC boost stage (low bus) | 650 V | Same, with surface-mount layout constraints | CR2065SX100 (20 A / VF 1.25 V / TO-263) |
LLC secondary rectification | 650 V / 1200 V | Qrr eliminates commutation spikes, enabling higher switching frequency | Filter by actual output voltage and current (see SiC diode selection page) |
LLC Secondary Rectification: Zero Qrr Buys More Than Efficiency
The secondary rectification diodes of an LLC resonant converter operate in continuous commutation. The reverse-recovery charge of a silicon FRD produces current spikes and voltage ringing at every commutation. SiC Schottky’s Qrr is negligible, so there is no stored charge to sweep out during commutation, and voltage spikes and the resulting EMI stress drop in tandem. For designs targeting overall efficiency above 97%, loss reduction in LLC secondary rectification is the second contribution point beyond the PFC stage.
Industry cases have already validated this path. A 3 kW-class server power reference design using SiC devices achieved 97.7% peak overall efficiency, well above the 96% Titanium threshold. Aukey’s 3200 W Titanium CRPS PSU uses a SiC Schottky diode in its front-end PFC, and Supermicro’s 2000 W Titanium PSU likewise uses a SiC SBD in the PFC boost circuit, confirming that this selection logic has reached volume production.
SiC SBD selection for LLC secondary rectification must match the actual output voltage and current class. The 650 V class covers rectification paths for 48 V output stages in flyback or LLC, while the 1200 V class serves higher output voltages or auxiliary supplies with power-factor correction. Hypersemi’s SiC SBD portfolio offers multiple package combinations from 650 V / 8 A to 1200 V / 30 A. The full voltage, current, and package filter is available on the SiC diode selection page for locating options by actual topology.
Conclusion: SiC Diodes Fill a Precise Gap Within the Efficiency Window
The efficiency race in AI server PSUs is not a route-choice question of “whether to adopt SiC,” but a judgment at the two loss nodes—PFC and LLC—of whether SiC diodes can push efficiency past the Titanium line at a reasonable cost. The PFC-stage benefit comes from eliminating reverse-recovery loss, on the order of 1–1.5 percentage points; the LLC secondary benefit comes from suppressing commutation spikes while freeing headroom for higher switching frequency. Together, the two nodes form the engineering path from a 94%–95% silicon platform to a 96%–97% Titanium platform.
One caveat in selection: SiC SBD forward voltage is typically higher than that of a silicon FRD at the same current class, and under light load this difference partially offsets the switching-loss benefit. The load-profile center of gravity therefore determines whether the net benefit of SiC replacement is positive. For Titanium certification scenarios where 50% load is the efficiency assessment point, the weight of switching-loss reduction at PFC and LLC stages at that load level is sufficient to cover the VF increment.