Design Of Fixed-off-time-controlled Pfc Pre-regulators With The L6562

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, variable frequency). The first method employs average current-mode control, a relatively complex technique requiring sophisticated controller IC's (e.g. STMicroelectronics' L4981A B) and a considerable component count. The second 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 makes the inductor work on the boundary between continuous and discontinuous mode, by definition. For a given power throughput, TM operation then involves higher peak currents as compared to FF-CCM (see figure 1). This, also consistently 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, say around 150-300W. 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 magnetics 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.


Design Of Fixed-off-time-controlled Pfc Pre-regulators With The L6562 application circuits