STMicroelectronics' highly integrated PFC boost converter solves startup circuit design challenges

September 27, 2022, China – STMicroelectronics’ L4985A/B and L4986A/B power factor correction (PFC) boost converters integrate an 800V startup circuit and ST’s proprietary practical auxiliary functions to help simplify application design and increase design flexibility.

The chip's built-in high-voltage startup function can save auxiliary circuits, support "no auxiliary circuit" design, reduce bill of materials costs, and ensure startup reliability. In the additional auxiliary circuit, the internal logic circuit can perform a safe discharge operation on the input filter X capacitor when the AC line is disconnected, solving the power loss problem of traditional discharge resistors and simplifying the compliance design of safety regulations such as IEC 61010-1 or IEC 62368-1.

The two converters can be used as PFC pre-regulators to help various applications meet harmonic distortion requirements, such as IEC 61000-3-2 and equivalent standards. Target applications include power supplies for desktop computers, servers, game consoles, TVs and other products; switch-mode power supplies (SMPS) for industrial and medical equipment; and electric scooter chargers. The converters also integrate a proprietary multiplication emulator and dedicated circuits to minimize total harmonic distortion (THD) under all operating conditions, ensuring stable power quality for LED lamp power supplies.

The new converters benefit from ST's proprietary off-time modulator, which ensures that the converter maintains quasi-fixed frequency operation under full load conditions and enters burst mode at light load and zero load, further simplifying the design. The switching frequency of the L4985A and L4986A is 65kHz, while the L4985B and L4986B are 130kHz. The L4986A/B products have added a power normal input and output pin that can generate a logic signal to assist system management when the PFC output reaches the user-preset threshold.

For energy-saving design, in accordance with the latest European Eco-design directive, all products have an external standby pin to initiate a low-power idle mode.

The new products also have comprehensive built-in protection and monitoring functions, including overvoltage protection, feedback loop failure protection, setting error protection and boost inductor saturation protection. The soft-start circuit can limit the inrush current and help maintain the output voltage stability, while the brown-out/brown-in undervoltage detection ensures that the converter can start reliably under all AC line conditions and prevents excessive RMS current. The brownout protection function allows medical equipment to meet the requirements of IEC 60601-1-2. The IEC 60601-1-2 standard requires that the power supply must maintain a stable output voltage when the grid line voltage drops within 500ms.

Engineers can accelerate project design cycles with the EVL400W-80PL evaluation board, a total AC/DC power solution with CLEAResult® 80PLUS Platinum Level certification. The EVL400W-80PL board integrates the L4985 PFC controller with STMicroelectronics’ L6699D resonant controller and SRK2001 synchronous rectification controller, achieving 90.2% efficiency at full load and 92.49% at 50% load.

The L4985 and L4986 series are now in mass production. The L4985A/B is available in SO-8 package; the L4986A/B has an additional power-good pin and is available in SSOP-10 package.