Part # Application Notes Power Supply And Power Management L6562A AN2782 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: Solution for designing a 400 W fixed off-time controlled PFC pre-regulator with the L6562A


Part Details:

AN2782 Application note Solution for designing a 400 W fixed off-time controlled PFC pre-regulator with the L6562A Introduction In addition to the transition mode (TM) and fixed-frequency continuous conduction mode (FF-CCM) operation of PFC pre-regulators, a third approach is proposed that couples the simplicity and affordability of TM operation with the high-current capability of FF-CCM operation. This solution is a peak current-mode control with fixed off-time (FOT). Design equations are given and a practical design for a 400 W board is illustrated and evaluated. Two methods of controlling power factor corrector (PFC) pre-regulators based on boost topology are currently in use: the fixed-frequency (FF) PWM and the transition mode (TM) PWM (fixed on-time, variable frequency). The first method employs average current-mode control, a relatively complex technique requiring sophisticated controller ICs (e.g. the L4981A/B from STMicroelectronics) and a considerable component count. The second one uses the simpler peak current-mode control, which is implemented with cheaper controller ICs (e.g. the L6561, L6562, L6562A from STMicroelectronics), much fewer external parts and is therefore much less expensive. In the first method the boost inductor works in continuous conduction mode (CCM), while TM makes the inductor work on the boundary between continuous and discontinuous mode, by definition. For a given power throughput, TM operation involves higher peak currents as compared to FF-CCM (Figure 1 and 2). Figure 1. Line, inductor, switch and diode Figure 2. Line, inductor, switch and diode currents in FF-CCM PFC currents in TM PFC IL IL "CCM" type "TM" type IAC IAC ON ON MOSFET MOSFET OFF OFF This demonstration, consistent with the above mentioned cost considerations, suggests the use of TM in a lower power range, while FF-CCM is recommended for higher power levels. This criterion, though always true, is sometimes difficult to apply, especially for a midrange power level, around 150-300 W. The assessment of which approach gives the better cost/performance trade-off needs to be done on a case-by-case basis, considering the cost and the stress of not only power semiconductors and magnetic but also of the EMI filter. At the same power level, the switching frequency component to be filtered out in a TM system is twice the line current, whereas it is typically 1/3 or 1/4 in a CCM system. August 2008 Rev 1 1/39 www.st.com Contents AN2782 Contents 1 Introduction to FOT control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Operation of an FOT-controlled PFC pre-regulator . . . . . . . . . . . . . . . . 5 3 The circuit implementing the line-modulated fixed off-time with the new L6562A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 Designing a fixed off-time PFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1 Input specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2 Operating condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.3 Power section design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3.1 Bridge rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3.2 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3.3


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