.

OUT

FLY IN

P

= η

P

,

2

2

2

2

IN(PK)

S

I N(RMS)

IN(RMS)

M

IN(AVG)

e

2

M

e

S

M

2

S

V

D T

V

V

2L

1

P

=

=

=

; R =

4

L

R

D T

2L

D T

æ

ö

ç

÷

ç

÷

è

ø

L

L

T

T

2

2

2

2

IN (PK)

S

2

IN (AVG)

in

in

L

L

M

L

0

0

V

D T

2

2

1

2

P

=

v (t)

i (t)dt =

sin (

t)dt;

T

T

2

L

T

´

ò

ò

p

,

S

2

in

in

P

S

T

M

v (t)

1

i (t) = Average (I )

=

D T

2 L

in

IN(PK)

L

2

v (t) = V

sin(

t),

T

p

in

REC

P(PK)

S

S

M

M

v (t)

v

(t)

I

=

DT =

DT , for

L

L

VREC

IP

IIN(AC)

TL

2TL

t

t

t

IP

IS

VSW

DTS

D2TS TS

2TS

3TS

IS(PK)

IP(PK)

t

t

t

(a) Over line period, T

L

(b) Over switching period, T

S

LM3447

www.ti.com

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

POWER FACTOR CORRECTION

Figure 22. DCM Flyback PFC Waveforms

Power factor correction is performed by operating the Flyback converter in discontinuous conduction mode

(DCM). In this mode, the peak primary current, IP(PK) is given by

(1)

(2)

where vIN(t) is the input voltage, vREC = ||vin|| is the rectified input voltage, LM is the transformer magnetizing

inductance referred to the primary winding, D is the duty cycle, TS is the switching period and TL is the line

period. For a fixed switching frequency controller, if duty cycle D, is held constant over a line cycle, then the peak

primary current, IP(PK), varies in proportion to input voltage, vIN(t), as shown in Figure 22 (a). The resulting input

current, IIN, is obtained by averaging the area under primary current, IP, shown in Figure 22 (b),

(3)

is sinusoidal and in-phase with input voltage, vIN(t). As a result, the DCM Flyback converter behaves much like a

resistor and exhibits a power factor close to unity.

The input power, PIN(AVG) drawn by the Flyback PFC is derived by averaging the product of input voltage, vin(t)

and input current, iin(t), over half line cycle TL/2,

(4)

(5)

The low frequency behavior of the DCM Flyback is defined by an effective resistance, Re. The expression for

average input power is given by Equation 5 and is based on Re and the input RMS voltage VIN(RMS). For a single

stage Flyback PFC driver, the output power, POUT, delivered to the LED load is a function of the converter

efficiency,

ηFLY, and is given by

(6)

The average LED current through the string with forward voltage drop VLED = VOUT is

Copyright © 2012, Texas Instruments Incorporated

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