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AN2782 Datasheet(PDF) 1 Page - STMicroelectronics |
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AN2782 Datasheet(HTML) 1 Page - STMicroelectronics |
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1 / 39 page  March 2010 Doc ID 14763 Rev 2 1/39 AN2782 Application note Solution for designing a 400 W fixed-off-time controlled PFC preregulator 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). 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. Figure 1. Line, inductor, switch and diode currents in FF-CCM PFC Figure 2. Line, inductor, switch and diode currents in TM PFC IL IAC ON MOSFET OFF "CCM" type IL IAC ON MOSFET OFF "TM" type www.st.com |
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