A

S

A

OUT

VCC

N

=

N , where N

is the auxiliary winding turns

V

,

IN S

P(PK,MAX)

M

P T

I

= 2

L

RE F

M

2

I N S

OUT

RE C(P K ,M IN)

V

L

,

1

1

4P

+

nV

V

£

æ

ö

ç

÷

è

ø

f

,

REC(P K, MI N)

M A X

MA X

OUT

V

D

n =

1

D

V

-

OUT LED

IN

FLY

V

I

P

=

η

LM3447

SNOSC65 A – APRIL 2012 – REVISED MAY 2012

www.ti.com

DESIGN PROCEDURE

(1) (2)

STEP

VARIABLE

DESCRIPTION

1

PIN

where

VOUT = VLED = Typical LED string voltage,

ILED is the average LED current,

ηTOT = ηEMI × ηFLY,

ηTOT is the LED driver efficiency, ηEMI is the EMI input filter efficiency and ηFLY is the Flyback PFC efficiency.

2

DMAX

0.4 < DMAX < 0.5

where

DMAX is the maximum allowable Flyback PFC duty cycle

3

n:1

VSW = nVOUT + VREC(PK,MAX) + VOS and VSW < Maximum FET (Q1) breakdown voltage.

where

n is the transformer turns-ratio,

VREC(PK,MIN) is the minimum peak rectified input voltage,

VREC(PK,MAX) is the maximum peak rectified input voltage,

VIN(RMS,MIN) is the minimum input RMS voltage,

VIN(RMS,MAX) is the maximum input RMS voltage,

VSW is the switch drain to source voltage,

VOS is the overshoot voltage because of leakage inductance.

4

LM

where

VREF is the internal reference voltage; VREF = 1V,

fS is the fixed switching frequency; fS = 70 kHz.

5

IP(PK,MAX)

where

TS is the switching period, TS = 1/fS

8

NP, NA, NS

Transformer Design

•

Core geometry (EE, PQ, RM)

•

Bobbin (UL Class B or Class F)

LM is the magnetizing inductance referred to primary side

n:1 = NP:NS, primary to secondary winding turns-ratio

BMAX < 0.3T, where BMAX is the maximum operating flux density corresponding to IP(PK,MAX)

(1)

See the Electrical Characteristics Table for all constants and measured values, unless otherwise noted.

(2)

See Figure 18 for all component locations in the Design Procedure Table.

22

Submit Documentation Feedback

Copyright © 2012, Texas Instruments Incorporated

Product Folder Links: LM3447