LM3450 datasheet(12/28 Pages) NSC

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FIGURE 2. PFC System Architecture

PFC BACKGROUND

Power factor (PF) is a number between 0 and 1 that indicates

how well energy is transmitted from input to output of a sys-

tem. It can be described by average power (P

AVG), RMS

voltage (V

RMS), and RMS current (IRMS):

Or by distortion factor (K

DIST)

and displacement factor

DISP):

With a purely resistive system, PF = 1. The addition of reactive

elements necessary in any converter, such as EMI filters and

energy storage, will induce some amount of displacement

(phase shift between the input voltage and input current). The

addition of switching devices will also create distortion (ener-

gy present in the harmonics relative to the switching frequen-

cies). These non-idealities decrease the PF towards zero.

Active power factor correction attempts to make the input

impedance look as resistive as possible to the power source.

Since the output of the converter is usually a regulated voltage

or current, there is a need for large energy storage elements

to remove the twice line frequency (100Hz or 120Hz) ripple.

A power factor control architecture, as shown in Figure 2, has

very little capacitance at the input. Instead, the twice line fre-

quency content is removed with large energy storage capac-

itance at the output.

With minimal input capacitance, the converter is able to pro-

vide two important functions at the same time:

•

Shape the input current

•

Regulate the output voltage

The PFC control approach requires two separate control

loops to achieve both functions: a fast loop which shapes the

input current, and a slow loop that regulates the output volt-

age.

The fast control loop shapes the input current to have the

same sinusoidal shape as the AC input voltage. Assuming

both are perfect sinusoids with zero distortion or phase shift,

the power factor will be perfect (unity). Unfortunately, distor-

tion is always present in switching converters. An input filter,

which is required to comply with EMI standards, helps to at-

tenuate the switching content, thereby reducing distortion.

However, the added filter capacitance will increase the phase

shift at the same time. Though perfect PF is not achievable

within real applications, extremely high PF (>.99) is possible

using most active PFCs.

The output voltage has to be regulated slowly to ensure the

converter ignores the twice line frequency ripple present on

the output. Therefore, the voltage loop containing the error

amplifier should have a bandwidth at least an order of mag-

nitude slower (<20Hz is common). Sometimes the bandwidth

is increased to improve transient response, which is the case

with off-line dimmable LED drivers. Though PF decreases

with the increase in bandwidth, high PF (>.95) is still possible.

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