PFC boost controllers eliminate startup-circuit design challenges, says ST
Two PFC (power factor correction) boost controllers, the L4985A / L4985B and L4986A / L4986B, integrate 800V start up circuitry to simplify design and enhance flexibility, said ST Microelectronics.
The built-in high-voltage start up capability saves auxiliary circuitry, allowing a ’no aux’ design that reduces bill of materials costs and is 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 controllers suit multiple applications as PFC pre-regulators in equipment that must meet requirements for harmonic distortion, such as IEC 61000-3-2 and equivalents. Example applications include power supplies for desktop PCs, servers, game consoles, and televisions, industrial and medical switched-mode power supplies (SMPS), and e-scooter chargers. Integrating a proprietary multiplier emulator together with dedicated circuitry to minimise total harmonic distortion (THD) in all operating conditions, means the converters also ensure consistent power quality for LED luminaires.
A proprietary off-time modulator 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 / L4986B 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 models have an external pin to initiate idle mode with low power consumption for power-saving in accordance with leading eco-design directives.
Protection features include over-voltage 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.
An evaluation board, the EVL400W-80PL is available. This includes a complete AC/DC power supply certified 80PLUS Platinum Level by CLEAResult. Reaching 90.2 per cent efficiency at full load and 92.49 per cent at 50 per cent load, the EVL400W-80PL combines the L4985 PFC controller with ST’s L6699D resonant controller and SRK2001 synchronous rectifier controller.
The L4985 and L4986 boost controllers are in production now. The L4985A / L4985B are packaged as SO-8 devices, the L4986A / L4986B have additional power-good pins and are housed in SSOP-10 packages.
