L4984D datasheet(18/35 Pages) STMICROELECTRONICS

18 / 35 page

Application information

L4984D

18/35

DocID024474 Rev 1

Along a line half-cycle, TOFF goes all the way from a minimum near the zero-crossing to a

maximum on the sinusoid peak. It is important to check that the off-time occurring on the

peak of the voltage sinusoid at minimum input voltage is greater then the minimum

programmable value:

Equation 9

This constraint limits the maximum programmable frequency at:

Equation 10

As the line RMS voltage is increased and/or the output load is decreased, the boost inductor

current tends to become discontinuous starting from the region around the zero-crossings.

As a result, the switching frequency is no longer constant and tends to increase. However,

the frequency rise is significantly lower as compared to that in a transition-mode (TM)

operated boost PFC stage, as illustrated in Figure 25. The switching frequency can exceed

fsw.max in the region where the inductor current is discontinuous.

Figure 27. Typical frequency change along a line half-cycle in a boost PFC operated

in LM-FOT (left) and TM (right)

In this example the voltage ripple appearing across the output capacitor Cout has been

neglected. This ripple at twice the line frequency fL has peak amplitude

ΔVout proportional

to the output current Iout:

Equation 11

As a consequence, fsw is not exactly constant but is modulated at 2fL, which spreads the

spectrum of the electrical noise injected back into the power line and facilitates the

compliance with conducted EMI emission regulations. The relative frequency change due to

the output voltage ripple is:

1.45

Vpk

min

TIMER

min

OFF

Vout

Vpk

690

min

max

[kHz]

AM13243v1

0.52

1.05

1.57

2.09

2.62

3.14

0.52

1.05

1.57

2.09

2.62

3.14

0.4

0.6

0.8

1.2

1.4

1.6

Vin = 88 Vac

Vin = 264 Vac

Vin = 230 Vac

Line voltage phase angle (rad)

Transition -mode operated PFC

Vin = 88 Vac

Vin = 264 Vac

Vin = 230 Vac

Line voltage phase angle (rad)

LM -FOT operated PFC

Vin = 115 Vac

Cout

Iout

Vout

L4984D датащи(PDF) 18 Page - STMicroelectronics

L4984D датащи(HTML) 18 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

датащи скачать

ссылки URL