Fairchild AN-8025 Specifications Manual

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AN-8025 Design Guideline of Single-Stage Flyback AC-DC Converter Using FAN7530 for LED Lighting

output should be controlled by CC mode in the normal state

Summary

while CV mode only works as over voltage protection.

This application note describes the single-stage power factor correction (PFC) and presents the design guidelines of a 75W universal-input, single-stage PFC for LED lighting applications. Flyback converter topology controlled by the critical conduction mode control IC, FAN7530 is applied and several functions; such as CV/CC mode feedback circuits, cycle-by-cycle current limit, soft-starting function, and so on, are considered for LED lighting applications.

Introduction

Figure 2. Flyback AC-DC Converter

Despite large output voltage ripple, single-stage AC-DC conversion is a more attractive solution than two-stage conversion from the standpoint of the cost and power density. Especially in applications like battery chargers, Plasma Display Panel (PDP)-sustaining power supplies, and LED lighting; low frequency, 100Hz or 120Hz, large output voltage ripple is inconsequential. Single-stage AC-DC converter directly converts AC input voltage to the DC output voltage without a pre-regulator, as shown in Figure 1. This application note presents a 75W single-stage AC-DC converter for LED lighting. As a power-conversion topology, flyback converter is normally chosen because it doesn’t need an inductive output filter; the main transformer works as an inductive filter itself.

Figure 3. Block Diagram of FAN7530

Figure 3 shows the block diagram of FAN7530. Its major features are: Fixed On Time CRM PFC Controller Zero Current Detector (ZCS) & Valley Switching MOSFET Over-Current Protection Figure 1. Single-Stage AC-DC Converter Low Startup (40μA) and Operating Current (1.5m A) Totem Pole Output with High State Clamp

Figure 2 shows the circuit diagram of a flyback AC-DC

+500/-800m A Peak Gate Drive Current

converter. FAN7530 is used as a controller and both CV (constant voltage) and CC (constant current) mode feedback FAN7530 is a voltage-mode CRM PFC controller; the turn- circuits are applied to prevent overload and over-voltage on time of switch is fixed while the turn-off time is varied conditions. In LED lighting, the output is always full-load during the steady state. Therefore, the switching frequency condition and the forward voltage drop of LED decreases if varies in accordance with the input voltage variation shown the junction temperature of LED increases. Therefore the in Figure 4.

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