Part # Application Notes Lighting Control L6562 AN1895 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: EVAL6562-375, 375W FOT-CONTROLLED PFC PRE-REGULATOR WITH THE L6562


Part Details:

AN1895 Application note EVAL6562-375W Evaluation Board L6562-based 375W FOT-controlled PFC Pre-regulator Introduction This application note describes a 375W evaluation board based on the L6562 Transition-mode Power Factor Correction (PFC) controller (order code: EVAL6562-375W). The board implements a 375W, wide-range mains input, PFC pre-regulator that is suitable for a 300/350W ATX12V power supply unit (PSU). To enable the use of a low-cost device like the L6562 at a power level that is usually prohibitive for this device, the chip operates with a Fixed-Off-Time (FOT) control system. This allows Continuous Conduction Mode operation, normally achievable with more expensive control chips and more complex control architectures. EVAL6562-375W evaluation board August 2006 Rev 3 1/16 www.st.com Contents AN1895 Contents 1 Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Power stage design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 Setting up FOT control with the L6562 . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Getting started with the evaluation board . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Testbench results and significant waveforms . . . . . . . . . . . . . . . . . . . . . . . 9 5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix A Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2/16 AN1895 Board description 1 Board description The EVAL6562-375W evaluation board includes a Power Factor Correction (PFC) pre-regulator for a 300W ATX 12V power supply unit (PSU). It is able to deliver 375W continuous power on a regulated 400V rail from a wide range of mains voltage. This rail will be the input for the cascaded isolated DC-DC converter (typically a forward converter) that will provide the output rails of the silver box. Although the ATX specification envisages air cooling, typically realized with a fan capable of an airflow in the range of 25-35 CFM, this is not allowed for in the design of this evaluation board. Enough heat sinking will be provided to allow full-load operation in still air. With an appropriate airflow and without any change in the circuit, the evaluation board can easily deliver up to 400-420W. The L6562 controller chip is designed for Transition-Mode (TM) operation where the boost inductor works next to the boundary between Continuous (CCM) and Discontinuous Conduction Mode (DCM). However, with a slightly different usage, the chip can operate so that the boost inductor works in CCM, hence surpassing the limitations of TM operation in terms of power handling capability. The gate-drive capability of the L6562 (±0.8A min.) is also adequate to drive the MOSFETs used at higher power levels. This approach, which couples the simplicity and cost-effectiveness of TM operation with the high-current capability of CCM operation, is the Fixed-Off-Time (FOT) control. The control modulates the ON-time of the power switch, while its OFF-time is kept constant. More precisely, it will be used the Line-Modulated FOT (LM-FOT) where the OFF-time of the power switch is not rigorously constant but is modulated by the instantaneous mains voltage. Please refer to [2] for a detailed description of this technique. Table 1 summarizes the electrical specification of the application and Table 3 lists transformer specifications. The electrical schematic is shown in Figure 1 and the PCB layout in Figure 2. Appendix A lists the bill of materials. Table 1. Electrical specifications Parameter Value Line voltage range 90 to 265 VAC Minimum line frequency (fL) 47 Hz


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