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AN2761 Datasheet(PDF) 4 Page - STMicroelectronics |
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AN2761 Datasheet(HTML) 4 Page - STMicroelectronics |
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4 / 36 page  Introduction to power factor correction AN2761 4/36 Doc ID 14690 Rev 2 1 Introduction to power factor correction The front-end stage of conventional offline converters, typically consisting of a full-wave rectifier bridge with a capacitor filter, has an unregulated DC bus from the AC mains. The filter capacitor must be large enough to have a relatively low ripple superimposed on the DC level. This means that the instantaneous line voltage is below the voltage on the capacitor most of the time, thus the rectifiers conduct only for a small portion of each line half-cycle. The current drawn from the mains is then a series of narrow pulses whose amplitude is 5-10 times higher than the resulting DC value. Many drawbacks result such as a much higher peak and RMS current down from the line, distortion of the AC line voltage, overcurrents in the neutral line of the three-phase systems and, consequently, a poor utilization of the power system's energy capability. This can be measured in terms of either total harmonic distortion (THD), as norms provide for, or power factor (PF), intended as the ratio between the real power (the one transferred to the output) and the apparent power (RMS line voltage times RMS line current) drawn from the mains, which is more immediate. A traditional input stage with capacitive filter has a low PF (0.5-0.7) and a high THD (>100%). By using switching techniques, a power factor corrector (PFC) preregulator, located between the rectifier bridge and the filter capacitor, allows drawing a quasi-sinusoidal current from the mains, in phase with the line voltage. The PF becomes very close to 1 (more than 0.99 is possible) and the previously mentioned drawbacks are eliminated. Theoretically, any switching topology can be used to achieve a high PF but, in practice, the boost topology has become the most popular thanks to the advantages it offers: ● primarily because the circuit requires the fewest external parts (low-cost solution) ● the boost inductor located between the bridge and the switch causes the input di/dt to be low, thus minimizing the noise generated at the input and, therefore, the requirements on the input EMI filter ● the switch is source-grounded, therefore easy to drive However, boost topology requires the DC output voltage to be higher than the maximum expected line peak voltage (400 VDC is a typical value for 230 V or wide-range mains applications). In addition, there is no isolation between the input and output, thus any line voltage surge is passed on to the output. Two methods of controlling a PFC preregulator are currently widely used: the fixed frequency average current mode PWM (FF PWM) and the transition mode (TM) PWM (fixed ON-time, variable frequency). The first method needs a complex control that requires a sophisticated controller IC (ST's L4981A, with the variant of the frequency modulation offered by the L4981B) and a considerable component count. The second one requires a simpler control (implemented by ST's L6562A), much fewer external parts and is therefore much less expensive. With the first method the boost inductor works in continuous conduction mode, while TM makes the inductor work on the boundary between continuous and discontinuous mode, by definition. For a given throughput power, TM operation involves higher peak currents. This, also consistently with cost considerations, suggests its use in a lower power range (typically below 200 W), while the former is recommended for higher power levels. For completion, FF PWM is not the only alternative when CCM operation is desired. FF PWM modulates both switch ON and OFF times (their sum is constant by definition), and a given converter operates in either CCM or DCM depending on the input voltage and the load conditions. Exactly the same result can be achieved if the ON-time only is modulated and the OFF-time is kept constant, in which case, however, the switching frequency is no longer fixed. This is referred to as "fixed-OFF-time" (FOT) control. Peak-current-mode control can still be used. In this application note transition mode is studied in depth. |
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