AN3009 datasheet(4/36 Pages) STMICROELECTRONICS

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Introduction to the power factor correction (PFC)

AN3009

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Doc ID 16032 Rev 4

Introduction to the power factor correction (PFC)

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’s half-cycle.

The current drawn from the mains then becomes a series of narrow pulses whose amplitude

is five to ten 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, 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 correction (PFC) pre-regulator, 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 power factor 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 power factor but, in

practice, the boost topology has become the most popular thanks to the advantages it

offers.

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Primarily because the circuit requires the fewest external parts (low-cost solution).

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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, a 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 pre-regulator

are currently widely used: the fixed-frequency average current mode pulse-width modulation

(FF PWM) and the transition mode pulse-width modulation (TM PWM), the latter having a

fixed ON time and 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

method requires a simpler control (implemented by ST's L6564), fewer external parts and is

therefore much more economical. With the first method, the boost inductor works in

continuous conduction mode, while the transition mode makes the inductor work on the

boundary between continuous and discontinuous mode by definition. For a given throughput

power, transition mode operation involves higher peak currents. This, also consistently with

cost considerations, suggests its use in a lower power range (typically up to 250 W), while

the former is recommended for higher power levels. To conclude, FF PWM is not the only

alternative when continuous current mode (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 (discontinuous current mode), depending on the input

voltage and the load conditions.

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