AN-6961

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 4/8/09

6

Determine Current-Sense Resistor

The MOSFET on-time and the input current increase with

the decreasing AC input voltage or increasing load. The

FAN6961 can establish the maximum on-time limit (25µs is

recommended) of power MOSFET. Once the voltage on

typically, the FAN6961 stops the PWM output immediately.

Thus, the maximum output power can be designed by the

current-sense resistor and maximum on-time limit. In

general operation, the maximum on-time occurs at minimum

AC input voltage and maximum loading conditions.

When the output power increases from full load to

maximum load, the on-time is restricted to the maximum on-

time limit first, then the current limit. In the design example,

the voltage on the current-sense pin is set to 0.57V at full

load and minimum input voltage conditions. At this condition,

the maximum power is about 156% of full load at minimum

input voltage condition. The current-sense resistor can be

calculated from Equation 7. The calculated curve of the

MOSFET turn-on time at different loading conditions are

shown in Figure 16. The calculated waveforms of the PFC

inductor current at two kinds of current limit are shown in

Figure 17.

1.00E-05

1.20E-05

1.40E-05

1.60E-05

1.80E-05

2.00E-05

2.20E-05

2.40E-05

2.60E-05

0

35

70

105

140

175

210

245

280

315

Time(S/20000)

Full load Pin=105.9W

110% Full load Pin=116.46W

120% Full load Pin=127W

150% Full load Pin=158.8W

Max. Pin=164.7W

(0.82 /4.55A)

Current Sense Limit

V

.

-

(25

)

Max On Time Limit

uS

Figure 16.

Calculated Curve of the MOSFET

Turn-on Time at Different Loading Conditions

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0

35

70

105

140

175

210

245

280

315

Time(S/20000)

Max. Pin=164.7W,Current Limit=4.55A

Max. Pin=140.4W,Current Limit=3.727A

150% Full load Pin=158.8W, Current Limit=4.55A

(3.727 )

0.7

.

133%

Current Limit

A

Full load

Vcs

V

Max Load

Full Load

→=

→

(4.55 )

0.57

.

156%

Current Limit

A

Full load

Vcs

V

Max Load

Full load

→=

→

Figure 17.

Calculated Waveforms of the PFC

Inductor Current at Two Current Limits

95%

I

0.57

R

PK

L,

S

•

=

(7)

From Faraday’s law, the number of turns for PFC inductor

can be obtained by:

6

e

max

pk

,

L

b

b

10

A

B

I

L

N

•

•

•

=

(8)

where:

Determine the Auxiliary Winding

The FAN6961 can perform zero-current detection by

sensing the information on an auxiliary winding of the PFC

inductor. As discussed previously, when the ZCD voltage is

lower than the threshold voltage (1.75V typical), the PWM

output is high again and initiates a new switching cycle.

However, there is a prerequisite: the zero-current detector

voltage must exceed the rising-edge threshold voltage (2.1V

typical) before it falls below 1.75V. The minimum rising-

edge voltage of zero-current detector input occurs at the

peak of the highest AC line voltage, which is equal to

n

/

V

2

-

V

max

rms,

O

•

and must be larger than the ZCD input

rising-edge threshold voltage (2.1V typical). The ZCD

to have a safe margin; therefore, the number of turns for

auxiliary winding is obtained as:

b

max

.

rms

o

zcd

aux

N

V

2

V

2

.

1

V

•

•

−

•

=

N

(9)

input.

Figure 18.

Simplified Power Stage

The fixed on-time for the specific output power, inductor,

and input voltage can be calculated by:

η

•

•

•

=

2

rms

b

o

fix

.

on

V

L

P

2

t

(10)

where:

η is conversion efficiency,