This document describes the high-power factor flyback converter for LED driver with the FL7732 PSR controller. It provides several features to enhance the performance of low-power flyback converters. By using single-stage topology with primary-side regulation, a LED lighting board can be implemented with few external components and minimized cost, without requiring an input bulk capacitor and feedback circuitry. Constant on-time control ensures precise constant-current control across changes in input voltage and output voltage. The operating frequency is proportionally changed by the output voltage to guarantee DCM operation with higher efficiency and simple design. FL7732 provides protection functions such as open-LED, short-LED, and over-temperature protection. The current-limit level is automatically reduced to minimize the output current and protect external components in short-LED condition. This application note presents practical design consideration for an LED driver employing Fairchild Semiconductor PWM PSR controller FL7732. It includes designing the transformer, selecting the components, and implementing constant
The document introduces the FL7732, a highly integrated PWM controller designed to enhance low-power flyback converters. It utilizes a proprietary primary-side regulation topology that simplifies circuit design by eliminating the need for an input bulk capacitor and feedback circuitry while maintaining high power factor and low total harmonic distortion (THD). Key features include precise constant-current control and various protection functions such as open-LED, short-LED, and over-temperature protection.
The application note details practical design considerations including transformer design, component selection, and the implementation of constant current regulation. It explains the basic operation in Discontinuous Conduction Mode (DCM), linear frequency control to maintain DCM across a wide output voltage range, and Boundary Conduction Mode (BCM) control. Additionally, it describes specific protection mechanisms like Under Voltage Lockout (UVLO), Over-Voltage Protection (OVP), and Short-LED protection with automatic current limit reduction.
| Turn-on threshold voltage | 16V |
|---|---|
| Over-temperature protection limit | 150°C |
| OVP hysteresis | 10°C |
The built-in temperature-sensing circuit shuts down the PWM output.
It changes the OCP level to 0.2V when Vs voltage is lower than 0.4V to limit primary-side current.
It lengthens the switching period when output voltage decreases to retain DCM operation in a wide output voltage range.