FL6961 datasheet(3/11 Pages) FAIRCHILD

Part #	FL6961

Description	Design Guideline for Single-Stage Flyback AC-DC Converter Using FL6961 for LED Lighting

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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AN-9737

APPLICATION NOTE

www.fairchildsemi.com

Rev. 1.0.0 • 4/13/11

3

As a result, designers should consider two conditions before

component selection: voltage and current capacity on

primary-side MOSFET(Q1) and secondary-side diode (D3)

to make a stable system with margin.

Figure 4 shows a guide to deciding two components on the

boundary

condition

of

flyback

converter

topology.

Figure 4. Boundary Conditions of Flyback Converter

Topology (Refer to AN-8025)

Design Example

A. Transformer Design

A design guideline of 16.8W single-stage flyback AC-DC

converter using FL6961 is presented. The applied system

parameters are shown in Table 1.

Table 1.

System Parameters

Parameter

Value

Main Input Voltage Range, VAC(main)

90V~265V

Output Voltage, VOUT

24V

Output Current, IOUT

0.7A

Minimum Switching Frequency at VAC(min)_pk

50kHz

Diode Voltage Drop, Vd

1V

MOSFET On Resistance, RMOS

1

Window Utilization

0.4

Target System Efficiency

0.82

Maximum Duty at Vac(min)_pk

0.35

Operating Maximum Flux Density

0.35

Regulation,

α

0.5%

Note:

1.

Regulation is strongly related with the copper loss and

0.5% regulation means 0.084W loss on transformer.

There are many ways to decide core and coil size and turns,

such as using AL value and following common practices. In

this note, use the Kg value related with the core geometry to

find optimum core and coil information.

Step 1. Calculate the total period, T:

20

1 =

=

f

T

[µs]

Step 2. Calculate the maximum on-time at MOSFET in

primary side.

7

)

35

.

0

)(

10

20

(

6

max

=

×

=

−

TD

t

on

[µs]

Step 3. Calculate the output power:

5

.

17

)

1

24

(

7

.

0

)

(

=

+

=

+

=

d

o

V

I

P

[W]

Step 4. Calculate the maximum input current, Imax:

168

.

0

)

82

.

0

)(

90

2

(

5

.

17

min

(max)

=

×

=

η

V

P

I

o

in

[A]

Step 5. Calculate the MOSFET voltage drop, Vvd:

168

.

0

(max)

=

MOS

in

vd

R

I

V

[V]

Step 6. Calculate the primary voltage on transformer, Vp:

127

168

.

0

127

min

≈

−

=

−

=

vd

P

V

[V]

Vp=126.83 use 127

Step 7. Calculate the primary peak current, Ippk:

96

.

0

)

10

7

)(

127

(

82

.

0

)

5

.

17

)(

10

20

(

2

6

(max)

=

×

=

−

on

p

ppk

t

V

TP

I

η

[A]

Step 8. Calculate the primary rms current, Iprms:

32

.

0

)

10

20

(

3

)

10

7

(

96

.

0

3

6

=

×

=

−

T

t

I

on

ppk

prms

[A]

Step 9. Calculate the required minimum inductance, L:

926

.

0

96

.

0

)

10

7

(

127

6

(max)

=

×

=

−

ppk

on

p

I

t

V

L

[mH]

L=0.926[mH] use 1[mH]

Step 10. Calculate the energy-handing capability in watt-

seconds, w-s:

0004608

.

0

2

)

96

.

0

)(

10

1

(

2

3

2

=

×

=

−

ppk

LI

ENG

[w-s]

Step11. Calculate the electrical conditions, Ke:

00003108

.

0

10

)

35

.

0

)(

5

.

17

(

145

.

0

10

145

.

0

4

2

4

2

=

×

=

×

=

−

m

e

PB

K

Step 12. Calculate the core geometry, Kg:

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