SiC Schottky Diodes in PFC Applications
Power factor correction (PFC) is a mandatory function in switch mode power supplies (SMPS) above a certain power level, required by international energy efficiency standards such as 80 PLUS and Energy Star. The boost PFC topology is the most common implementation, and the boost diode plays a critical role in determining PFC efficiency and EMI performance.
Advantages of SiC Schottky Diodes
Zero Reverse Recovery
Unlike Si PiN diodes, SiC Schottky diodes are majority carrier devices with essentially zero reverse recovery charge (Qrr). This eliminates the reverse recovery loss that plagues Si diode-based PFC designs, particularly at high switching frequencies and high input voltages.
Reduced EMI
The absence of reverse recovery current eliminates a major source of EMI in PFC circuits. This allows for simpler EMI filter designs and reduces the overall component count and cost of the PFC stage.
Temperature-Independent Performance
SiC Schottky diodes maintain their excellent switching performance across the entire operating temperature range. Unlike Si diodes, whose Qrr increases significantly with temperature, SiC diodes provide consistent performance at any junction temperature.
AstSiC SiC Diode Product Line
AstSiC offers SiC Schottky diodes in 650V and 1200V ratings. The ASC10D650DF2C (650V, 10A, TO-220F) suits low-power PFC applications up to 500W. For medium-power applications (1-3kW), the ASC30D650DT2C (650V, 30A, TO-220) provides ample current margin. High-power PFC stages (3-10kW) benefit from the ASC100D1200DT2R (1200V, 100A, TO-220), which supports three-phase PFC architectures.
PFC Design Optimization with SiC Diodes
Continuous Conduction Mode (CCM) PFC
In CCM PFC, the SiC Schottky diode eliminates reverse recovery losses, allowing the use of higher switching frequencies without the efficiency penalty associated with Si diodes. This enables smaller boost inductor values and reduced output capacitor ripple current.
Totem-Pole PFC
The totem-pole PFC topology, which was impractical with Si diodes due to reverse recovery issues, becomes viable with SiC Schottky diodes. This bridgeless PFC topology eliminates diode bridge conduction losses and can achieve efficiency above 99%.
Application Examples
In a 2kW server power supply with 80 PLUS Titanium efficiency requirements, replacing a Si ultrafast diode with an AstSiC ASC20D650DT2C SiC Schottky diode in the PFC stage typically yields a 0.5-1% efficiency improvement at full load and up to 2% improvement at light load. This efficiency gain is critical for meeting the stringent requirements of modern efficiency standards.
