原标题：Fairchild FAN6791 RD-343 250W电源解决方案

　　Fairchild 公司的FAN6791 / FAN6793是高度集成双PWM组合控制器，具有高压起动，低工作电流。FAN6791/3集成的频率抖动功能降低了EMI，内置的斜率补偿可得到稳定的峰值电流模式控制。主要用在开关电源和反激电源转换器，包括PC-ATX电源。本文介绍了FAN6791 / FAN6793主要特性，功能方框图，典型应用电路图，参考电路图及其材料清单(BOM)，以及参考设计RD-343主要特性，电路图和所用变压器特性与电路。

　　FAN6791 / FAN6793 Highly Integrated, Dual-PWM Combination Controller

　　The highly integrated FAN6791/3 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, FAN6791/3 is manufactured using the CMOS process, which allows an operating current of only 6mA. FAN6791/3 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.

　　FAN6791/3 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 shortcircuit 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.

　　FAN6791 / FAN6793主要特性：

　　High-Voltage Startup

　　Low Operating Current

　　Interleaved Stand-by PWM / Forward PWM Switching

　　Green Mode Stand-by PWM / Forward PWM

　　Linearly Decreasing Stand-by PWM Frequency to 20kHz

　　Remote On / Off

　　AC Brownout Protection

　　Forward PWM with Soft-Start

　　Frequency Hopping to Reduce EMI Emissions

　　Cycle-by-Cycle Current Limiting for Stand-by PWM /Forward PWM

　　Leading-Edge Blanking for Stand-by PWM / Forward PWM

　　Synchronized Slope Compensation for Stand-by PWM / Forward PWM

　　GATE Output Maximum Voltage Clamp

　　VDD Over-Voltage Protection (OVP)

　　VDD Under-Voltage Lockout (UVLO)

　　Internal Open-Loop Protection for Stand-by PWM /Forward PWM

　　Constant Power Limit for Stand-by PWM / Forward PWM

　　FAN6791 / FAN6793应用：

　　General-purpose switch-mode power supplies and flyback power converters, including:

　　PC-ATX Power Supplies

　　图1。FAN6791 / FAN6793功能方框图

　　图2。FAN6791 / FAN6793典型应用电路图

　　材料清单(BOM)：

　　参考设计RD-343

　　参考设计RD-343主要特性：

　　FAN6791

　　High-Voltage Startup

　　Low Operating Current

　　Interleaved Standby PWM / Forward PWM Switching

　　Green Mode Standby PWM / Forward PWM

　　Linearly Decreasing Standby PWM Frequency to 20kHz

　　Remote On / Off

　　AC Brownout Protection

　　Forward PWM with Soft-Start

　　Frequency Hopping to Reduce EMI Emissions

　　Cycle-by-Cycle Current Limiting for Standby PWM / Forward PWM

　　Leading-Edge Blanking for Standby PWM / Forward PWM

　　Synchronized Slope Compensation for Standby PWM / Forward PWM

　　GATE Output Maximum Voltage Clamp

　　VDD Over-Voltage Protection (OVP)

　　VDD Under-Voltage Lockout (UVLO)

　　Internal Open-Loop Protection for Standby PWM / Forward PWM

　　Constant Power Limit for Standby PWM / Forward PWM

　　SG6521

　　Two 12V Sense-Input Pins: VS12 and VS12B

　　Over-Voltage Protection (OVP), Over-Current Protection (OCP), and Under-Voltage Protection (UVP) for 3.3V, 5V, and Two 12V

　　Open-Drain Output for PGO and FPO Pins

　　300ms Power-Good Delay

　　300ms Turn-On Delay for 3.3V, 5V, and Two 12V

　　2.8ms PSON Control to FPO Turn-Off Delay and 48ms PSON Control Delay

　　No Lockup During the Fast AC Power On / Off

　　Wide Supply Voltage Range: 4.2V to 15V

　　Additional Protection Input (Pext)

　　变压器电路框图

　　变压器线绕指标：

　　反激转换器变压器(TX2)

　　变压器电路框图

　　变压器线绕指标：