AN-8024

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 9/18/09

3

NOM

MAX

DSIN

RO

VV

V

=+

(5)

()

MAX

NOM

IN

O

F

DOO

RO

VV

V

VV

V

⋅+

=+

(6)

Figure 3. Output Voltage Reflected to the Primary

As can be seen in Equation (5), the voltage stress across

increases the voltage stresses on the rectifier diodes in the

should be determined by a trade-off between the voltage

stresses of MOSFET and diode. Especially for low output

voltage application, the rectifier diode forward-voltage drop

is a dominant factor determining the power supply

efficiency. Therefore, the reflected output voltage should be

determined such that rectifier diode forward voltage can be

minimized. Table 1 shows the forward-voltage drops for

Schottky diodes with different voltage ratings.

Because the actual drain voltage and diode voltage rise

above the nominal voltage due to the leakage inductance of

MOSFET and diode, respectively.

Table 1. Diode Forward-Voltage Drop for Different

Voltage Ratings (3A Schottky Diode)

Part Name

SB320

20V

0.5V

SB330

30V

SB340

40V

SB350

50V

0.74V

SB360

60V

SB380

80V

0.85V

SB3100

100V

(Design Example)

As can be seen in Table 1, it is necessary

to use a rectifier diode with 40V voltage rating to maximize

efficiency. Assuming that the nominal voltages of MOSFET

and diode are less than 68% of their voltage rating, the

reflected output voltage is given as:

()

373 (5

0.5)

5

0.68 40

27.2

MAX

NOM

IN

O

F

DO

O

RO

RO

VV

V

VV

V

V

⋅+

=+

⋅+

=+ <

⋅

=

373 (5

0.5)

92.4

22.2

RO

VV

⋅ +

⇒>

=

0.68 700

476

NOM

MAX

DS

IN

RO

VV

V

=+

<

⋅

=

476

373 103

RO

VV

⇒<−

<

100

0.47

100 113

RO

MAX

MIN

RO

IN

V

D

VV

==

=

+

+

373 100

473

NOM

MAX

DS

IN

RO

VV

V

V

=+

=

+

=

()

373 (5

0.5)

5

25.5

100

MAX

NOM

IN

O

F

DO

O

RO

VV

V

VV

V

V

⋅+

=+

⋅+

=+ =

[STEP-4] Determine the Transformer Primary-Side

The transformer primary-side inductance is determined for

the minimum input voltage and nominal load condition.

of the transformer is obtained as:

2

()

2

MIN

IN

MAX

M

IN

SW

RF

VD

L

Pf K

⋅

=

(7)

factor at minimum input voltage and nominal load condition,

defined as shown in Figure 4. The ripple factor is closely

related with the transformer size and the RMS value of the

MOSFET current. Even though the conduction loss in the

MOSFET can be reduced by reducing the ripple factor, too

small a ripple factor forces an increase in