Dual-PWM Combination Controller for Flyback and Forward Converter

The highly integrated FAN6791 dual PWM combination controller provides several features to enhance the performance of converters. To minimize standby power consumption, a proprietary green-mode function provides off-time modulation to linearly decrease the switching frequency at light-load conditions. To avoid acoustic-noise problems, the minimum PWM frequency is set above 20kHz. This green-mode function enables the power supply to meet international power conservation requirements. With the internal high-voltage startup circuitry, the power loss due to bleeding resistors is also eliminated. To further reduce power consumption, FAN6791 is manufactured using the CMOS process, which allows an operating current of only 6mA. FAN6791 integrates a frequency-jittering function internally to reduce EMI emissions of a power supply with minimum line filters. The built-in synchronized slope compensation achieves stable peak-current-mode control. The proprietary internal line compensation ensures constant output power limit. FAN6791 provides many protection functions, including brownout protection, cycle-by-cycle current limiting, and an internal open-loop protection circuit to ensure safety should an open-loop or output short-circuit failure occur. PWM output is disabled until VDD drops below the UVLO lower limit when the controller restarts. As long as VDD exceeds ~24.5V, the internal OVP circuit is triggered.