AN2459 datasheet(5/35 Pages) STMICROELECTRONICS

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AN2459 - Application note

Power Factor Correction (PFC)

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Power Factor Correction (PFC)

Theoretically, any switching topology can be used to achieve a high power factor but, in

practice, the boost topology has become the most popular because of the advantages it

offers. These include:

●

Circuit requires the least external parts, thus it is the cheapest available.

●

Boost inductor, located between the bridge and the switch, lowers the input di/dt, thus

minimizing noise generated at the input and consequently reducing the EMI filter input

requirements.

●

Switch is source-grounded and therefore easy to drive.

Three methods of controlling the PFC preregulator are currently widely used. They are:

●

The Fixed Frequency Average Current Mode PWM.

●

The Transition Mode (TM) PWM (fixed on-time, variable frequency).

●

The peak current mode with fixed off-time.

Control of the first method is complicated and requires a sophisticated IC controller (e.g.

either ST's L4981A or ST’s L4981B which offers frequency modulation) and a considerable

component count.

Control of the second method is simpler (e.g. ST's L6561/2/3 family) and requires fewer

external parts. It is therefore much less expensive.

With the Fixed Frequency Average Current Mode method, the boost inductor operates in

continuous conduction mode, while the TM method causes the inductor to work on the

boundary between continuous and discontinuous modes. Thus, for a given throughput

power, TM operation involves higher peak currents, suggesting it is more efficient at lower

power ranges (typically below 200W). In contrast, the Fixed Frequency Average Current

Mode is recommended for higher power levels.

A third method of control, that of applying constant. Toff control, results in continuous

conduction mode. The same simple TM-controllers may be used, as may a small RC

network to set the off-time. This method is described in AN1792 (7) It is optimal for an input

power of between 200 and 400W.

2.1

Transition Mode operation

As mentioned above, the typical PFC topology used in electronic ballasts is a step-up

(boost) regulator (Figure 1) working in transition conduction mode. Figure 2 outlines the

Transition Mode principles. When the MOSFET is turned on, the inductor is charged from

the input voltage source. When the MOSFET is turned off, the boost inductor discharges its

energy into the load until its current falls to zero. When the latter occurs, the boost inductor

has no energy and a zero current (ZCD) signal is detected, due to a demagnetization

change on the auxiliary winding. This drives the MOSFET on again, whereby another

conversion cycle starts. As the drain voltage drops before turn-on, the turn-on switching

losses are minimized. Figure 2 indicates the geometric relationship of average and peak

currents. Due to the triangular shape of the inductor current, the peak current is twice the

average current.

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