Power supply designers are looking for semiconductor solutions that offer ease of design, a host of protection features and enable power supplies to meet increasingly stringent energy-efficiency specifications. This article will explore a highly integrated semiconductor solution for power supplies that consists of boost PFC and dual-forward PWM with very few external components to achieve versatile protections /compensation and meets IEC-1000-3-2 specifications.

The FAN480x is composed of average current mode control of Power Factor Correction (PFC) and Pulse-Width Modulation (PWM). Switching charge multiplier technology for the PFC stage could provide higher Power Factor (PF) and lower Total Harmonic Distortion (THD) for the power supply, which the PWM stage can use for current mode control or voltage mode control. The PFC and PWM gate output uses different modulation control to reduce output voltage ripple on the PFC output capacitor with Leading-Edge Modulation control on PFC stage and Trailing-Edge Modulation on PWM stage. FAN480x also provides programmable Two-Level PFC output voltage for improved efficiency at light load and low-line.

Basic Circuit Diagram of Dual-Forward Converter Using PFC/PWM Combo Solution Click on image to enlarge.

FAN480x has several protect functions, e.g. PFC and PWM Soft-Start, Over Voltage Protection / Under Voltage Protection, Cycle-by-Cycle Current Limit, and PFC Brownout Protection, etc. Figure 1 shows a 300W FAN480x ATX application circuit with 10W standby power supply and the PFC section provides 380VDC to a dual-transistor voltage-mode forward converter. The circuit operates from 75VAC to 264VAC with both power sections switching at 65 kHz.

While FAN480x PFC is working in continuous conduction mode, it could help to reduce current variation from the boost inductor and for large wattage applications. The gain modulator could provide higher Power Factor and lower Total Harmonic Distortion for power supply and is the key for PFC stage as a circuit block response to the current loop for line voltage, frequency, RMS line voltage and PFC output voltage as shown in equation (1). The function of gain modulator is to generate a control signal for PFC stage duty cycle and maintain output voltage. The divider Vrms² could provide a constant power at high line and low line condition. The figures below shows FAN480x gain modulator working principle and application circuitry.

Gain Modulator Working Principle and FAN480x Gain Modulator Application Circuitry Click on image to enlarge.