AN-9741

APPLICATION NOTE

© 2011 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.0 • 6/27/11

4

2

1

T

T

D

ch

=

Figure 5. DC Link Voltage Waveforms

The maximum DC link voltage is given as:

max

max

2

LINE

DL

V

V

×

=

(14)

The minimum input DC link voltage at 50% nominal output

voltage is given as:

L

DL

ch

B

IN

LINE

B

DL

f

C

D

P

V

V

×

−

−

×

=

)

1

(

)

(

2

@

2

min

min

@

(15)

The minimum input DC link voltage at minimum output

voltage are given as:

L

DL

ch

C

IN

LINE

C

DL

f

C

D

P

V

V

×

−

−

×

=

)

1

(

)

(

2

@

2

min

min

@

(16)

[STEP-3] Determine Transformer Turns Ratio

Figure 6 shows the MOSFET drain-to-source voltage

waveforms. When the MOSFET is turned off, the sum of

the primary is imposed across the MOSFET as:

RO

DL

nom

DS

V

V

V

+

=

max

(17)

(

)

F

O

P

S

RO

V

V

N

N

V

+

×

=

(18)

are number of turns for the primary side and secondary

side, respectively.

When the MOSFET is turned on; the output voltage,

together with input voltage reflected to the secondary, are

imposed across the diode as:

max

DL

P

S

O

F

V

N

N

V

V

×

+

=

(19)

As observed in Equations (5) and (6), increasing the

of MOSFET, while it leads to reduced voltage stress of

rectifier diode. Therefore, the transformer turns ratio

MOSFET and diode voltage stresses. When determining the

voltage should be also considered. The maximum voltage

stress of MOSFET is given as:

OS

RO

DL

DS

V

V

V

V

+

+

=

max

max

(20)

typically 1~1.5 times the reflected output voltage. It is also

typical to have a margin of 15~20% of breakdown voltage

for maximum MOSFET voltage stress.

S

P

F

O

N

N

V

V

×

+

)

(

Figure 6. Voltage Stress of MOSFET

The transformer turns ratio between the auxiliary winding

and minimum output voltage in constant current. When the

with the output voltage, as seen Figure 7. The overshoot of

auxiliary winding voltage caused by the leakage inductance

voltage is given as:

(

)

FA

F

O

S

A

DD

V

V

V

N

N

V

−

+

×

=

1

min

(21)

Equation (21) due to the overshoot by the leakage

inductance, which is proportional to the voltage overshoot

of MOSFET drain-to-source voltage shown in Figure 7.

voltages for nominal output voltage and minimum output

voltage are given as:

FA

OS

P

S

F

O

S

A

DD

V

V

N

N

V

V

N

N

V

−













×

+

+

×

≅

max

(22)

FA

OS

P

S

F

O

S

A

DD

V

V

N

N

V

V

N

N

V

−













×

+

+

×

≅

min

2

min

(23)

winding diode.