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STMicroelectronics EVL400W-EUPL7 Converter Demonstration Board is a 12V – 400W converter, tailored for the typical specification of an AC/DC adapter with wide input mains range. It also includes a very low power consumption at light load and good average efficiency. The architecture is based on a two-stage approach. The first stage is a front-end PFC pre-regulator based on a CCM (continuous conduction mode) boost PFC controller using the L4984, and a downstream LLC resonant half-bridge converter, designed around the L6699. At the secondary side, synchronous rectification is implemented utilizing the SRK2001 that assures a very high rectification efficiency with consequent small heat sink required.
The PFC section uses a proprietary LM-FOT (line-modulated fixed-off-time) that allows control methodology at fixed frequency operation for boost PFC converters, as long as it’s operating in CCM. The LLC section uses the L6699, a double-ended controller specific to the series-resonant half-bridge topology. The output voltage regulation is obtained by modulating the operating frequency.
The main focus of this demonstration board is the light-load efficiency, achieved through the burst mode function of both PFC and LLC controllers with the self-adaptive dead time of the L6699. The L6699 is modulated by the internal logic according to the half-bridge node transition times. This allows the maximization of the transformer magnetizing inductance, reducing the primary current at light load operation. An active high voltage start-up circuitry, based on a depletion MOSFET, provides a very fast start-up time and minimizes the residual consumption during normal operation to a negligible level.
