Part # AN1792 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN1792 APPLICATION NOTE DESIGN OF FIXED-OFF-TIME-CONTROLLED PFC PRE-REGULATORS WITH THE L6562 by C. Adragna Beside Transition Mode (TM) and fixed-Frequency Continuous Conduction Mode (FF-CCM) operationof PFC pre-regulators a third approach is proposed that couples the simplicity and affordability of TMoperation with the high current capability of FF-CCM operation: it is a peak current-mode control withfixed-OFF-time. After showing advantages and limits of this technique, in both its basic and advancedimplementation, design equations will be given and a practical design will be illustrated and evaluated. Introduction Two methods of controlling Power Factor Corrector (PFC) pre-regulators based on boost topology are cur-rently in use: the Fixed-Frequency (FF) PWM and the Transition Mode (TM) PWM (fixed ON-time, variablefrequency). The first method employs average current-mode control, a relatively complex technique requiringsophisticated controller IC s (e.g. STMicroelectronics L4981A/B) and a considerable component count. Thesecond one uses the simpler peak current-mode control, which is implemented with cheaper controller IC s(e.g. STMicroelectronics L6561, L6562), much fewer external parts and is therefore much less expensive. With the first method the boost inductor works in Continuous Conduction Mode (CCM), while TM makesthe inductor work on the boundary between continuous and discontinuous mode, by definition. For a givenpower throughput, TM operation then involves higher peak currents as compared to FF-CCM (see figure1). This, also consistently with the above mentioned cost considerations, suggests the use of TM in a lowerpower 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, sayaround 150-300W. The assessment of which approach gives the better cost/performance trade-off needs tobe done on a case-by-case basis, considering the cost and the stress of not only power semiconductors andmagnetics but also of the EMI filter: at the same power level, the switching frequency component to be fil-tered 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: a) FF-CCM PFC, b)TM PFC. Inductor current peak envelope 0000ILpk 000 0000000 0000000 0000 0000 000 0000000 0000 0000000 000000 Low frequency Switch current Inductor current 0000 0000 000000 000 0000000 0000 0000000 000000 Diode current 00 inductor current 0000 0000 00000 0000 000000 000 0000000 0000 0000000 0000 000000 0000 Low frequency 00 0000 0000 0000 00000 0000 000000 000 0000000 0000 0000000 0000 000000 0000 000inductor current 00 0000 0000 0000 000 0000 00000 0000 000000 000 0000000 0000 0000000 0000 000000 000 0000 000

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