STMicroelectronics Demo board is a 12 V to 400 W converter

     STMicroelectronics EVL400W-80PL Demo board is a 12 V to 400W converter tailored to the typical specifications of AC/DC adapters, with a wide input power range, extremely low power consumption at light loads, and good average efficiency. It consists of a mother board for installing high-voltage power devices, a main control board for installing IC controllers L4985A and L6699, and a secondary control board for synchronous rectification.

     The application architecture is based on two phases: a front-end PFC preregulator based on a CCM (continuous on-mode) boost PFC controller using the L4985A, and a downstream LLC resonant half-bridge converter designed around the L6699. On the secondary side, synchronous rectification is achieved by SRK2001, ensuring very high rectification efficiency while reducing the required radiator size.

     The PFC portion uses the L4985A, a peak current mode PFC controller for boost converters with a proprietary multiplier "simulator" that guarantees extremely low total harmonic distortion (THD) performance under all operating conditions, in addition to the innovative THD optimizer.

     Thanks to a proprietary turn-off time modulator, the device operates at a quasi-fixed frequency in all operating conditions and includes a high voltage start module and circuit for discharging the EMI filter's X capacitors. This integration provides a solution with a small number of components for the boost PFC pre-voltage regulator. The LLC part uses the L6699, which is a two-ended controller specific to the series resonant half-bridge topology. Output voltage regulation is achieved by modulating the operating frequency.

     The main focus of the demo board is light load efficiency, achieved through the burst mode function of the PFC and LLC controllers and the adaptive dead time of the L6699, modulated by internal logic according to the half-bridge node transition time, which maximizes the transformer's excitation inductance and reduces the primary current during light load operation.