AN-9719 View Datasheet(PDF) - Fairchild Semiconductor

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AN-9719

Applying Fairchild Power Switch (FPS™) FSL1x7 to Low- Power Supplies

Fairchild Semiconductor 

AN-9719

APPLICATION NOTE

(Design Example) As can be seen in Table 2, it is

recommended to use rectifier diode with 100V voltage

rating to maximize efficiency. Assuming that the

nominal voltages of MOSFET and diode are less than

80% of their voltage rating, the reflected output voltage

is given as:

·

373 · 12 0.85

12 0.8 · 100 80

Î

373 · 12 0.85

70.5

68

0.8 · 700 560

Î

560 373 187

Figure 9. Output Voltage Reflected to the Primary

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

MOSFET can be reduced by reducing VRO. However, this

increases the voltage stresses on the rectifier diodes in the

secondary side, as shown in Equation 6. Therefore, VRO

should be determined by a balance between the voltage

stresses of MOSFET and diode. Especially for low output

voltage applications, 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 2 shows the forward-voltage drop for

Schottky diodes with different voltage rating.

The actual drain voltage and diode voltage rise above the

nominal voltage is due to the leakage inductance of the

transformer as shown in Figure 9. It is typical to set VRO

such that VDSNOM and VDONOM are 70~80% of voltage

ratings of MOSFET and diode, respectively.

Table 2. Diode Forward-Voltage Drop for Different

Voltage Ratings (3A Schottky Diode)

Part Name

VRRM

VF

SB320

SB330

SB340

SB350

SB360

SB380

SB3100

20V

30V

40V

50V

60V

80V

100V

0.5V

0.74V

0.85V

By determining as 74V,

74

0.48

74 79

373 74 447

·

373 · 12 0.85

12 76.8

74

[STEP-4] Determine the Transformer Primary-Side

Inductance (LM)

The transformer primary-side inductance is determined for

the minimum input voltage and nominal-load condition.

With the DMAX from Step-3, the primary-side inductance

(LM) of the transformer is obtained as:

·

(7)

2· · ·

where fSW is the switching frequency and KRF is the ripple

factor at minimum input voltage and nominal load

condition, defined as shown in Figure 10.

For DCM operation, KRF = 1, and, for CCM operation, KRF

< 1. The ripple factor is closely related to 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 transformer size. When designing the flyback

converter to operate in CCM, it is reasonable to set KRF =

0.25-0.5 for the universal input range and KRF = 0.4-0.8 for

the European input range.

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 11/2/10

5

www.fairchildsemi.com

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