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Project Background:
A client manufacturing a 300W industrial switching power supply encountered issues during EMC certification testing, where conducted emissions exceeded limits in the 150 kHz–500 kHz frequency range. Investigation revealed that the root cause was high-frequency noise—generated by the reverse recovery current of the freewheeling diode in the Boost PFC circuit—coupling into the input stage.
Product Models Used:
• FRED (Fast Recovery Epitaxial Diode) with ultra-fast, soft-recovery characteristics—used for freewheeling in PFC circuits; products with soft-recovery characteristics and trr < 35ns are recommended.
• Fast recovery rectifier—used for rectification at the power supply output stage.
Problem Addressed:
• The di/dt resulting from diode reverse recovery is a primary source of EMI in PFC circuits.
• In a 200 kHz LLC power supply, switching losses associated with a conventional UF4007 diode (trr < 75 ns) accounted for 18% of total energy loss; replacing it with a device featuring trr < 35 ns reduced this loss to 8%.
Project Results:
• Conducted emissions improved by 10 dB, meeting EN 55032 Class B limits.
• Selected an ultra-fast recovery diode with soft-recovery characteristics and optimized the layout (connecting the diode cathode directly to the output capacitor), successfully passing certification.
Note: This is a standard industry technical case study; your company can implement a replacement or upgrade based on actual customer applications of your own FRED products (such as the DSEK10S06 and DSE10S06 series).
