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PFC boost converter simplifies startup circuit design

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By Nick Flaherty

STMicroelectronics has developed two power-factor correction (PFC) boost converters to simplify the design of startup circuitry in desktop PCs, servers, game consoles, and televisions, industrial and medical switched-mode power supplies (SMPS) and e-scooter chargers

The L4985A/B and L4986A/B include built-in high-voltage startup capability saves auxiliary circuitry, allowing a “no aux” design that reduces bill-of-materials costs and ensures reliable starting. These circuits also include internal logic to safely discharge input-filter X capacitors when the AC line is disconnected. This eliminates the power losses incurred in a traditional discharge resistor and simplifies meeting safety regulations such as IEC 61010-1 or IEC 62368-1.

The converters suit multiple applications as PFC pre-regulators in equipment that must meet requirements for harmonic distortion, such as IEC 61000-3-2 and equivalents.

Integrating a proprietary multiplier emulator and adding dedicated circuitry to minimize total harmonic distortion (THD) in all operating conditions also ensures consistent power quality for LED luminaires.

The converters benefit from a proprietary off-time modulator that further simplifies design by ensuring quasi fixed-frequency operation in all load conditions, entering burst mode at light load and zero load.

The L4985A and L4986A operate at 65kHz, while the L4985B and L4986B are designed for 130kHz operation. The L4986A/B variants have additional power-good input and output pins that generate a logic signal to aid system management when the PFC output reaches a user-determined threshold.

All the variants have an external pin to initiate idle mode with low power consumption.

Comprehensive protection and monitoring are also built-in, including overvoltage protection and protection against feedback loop failures, erroneous settings, and boost-inductor saturation. Soft-start limits inrush current and helps maintain output-voltage regulation, while brown-out/brown-in detection ensures reliable starting under all AC-line conditions and prevents excessive RMS current. The brownout protection enables medical equipment to meet IEC 60601-1-2, which requires output regulation to be maintained if the line voltage slumps for up to 500ms.

Engineers can accelerate their projects using the EVL400W-80PL evaluation board (above). This is a complete 80PLUS Platinum AC/DC power supply with 90.2% efficiency at full load and 92.49% at 50% load. It combines the L4985 PFC controller with ST’s L6699D resonant controller and SRK2001 synchronous rectifier controller.

All L4985 and L4986 variants are in production now. The L4985A/B are packaged as SO-8 devices, while the L4986A/B with additional power-good pins are housed in SSOP-10. Pricing starts from $1.21 for orders of 1000 pieces.

www.st.com

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