AN-6067 datasheet(6/21 Pages) FAIRCHILD

No de pièce	AN-6067

Description	Design and Application of Primary-Side Regulation (PSR) PWM Controller

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Fabricant	FAIRCHILD [Fairchild Semiconductor]

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AN-6067

APPLICATION NOTE

www.fairchildsemi.com

Rev. 1.0.1 • 1/26/10

6

Constant Current Output Regulation

As shown in Figure 12, the output current IO can be

expressed by Equation 1 when the flyback converter is

operated in DCM. As a result, the output current IO can be

calculated by the signal i

pk, tdis. The PSR controller then

determines the on-time of the MOSFET to modulate input

power and provide constant output current.

P

i

S

i

Ts

in

p

V

L

2

o

-

p

nV

L

⋅

dis

t

on

t

pk

i

S

V

Gate

sampling voltage

O

I

Figure 12. Principal Operation Waveform of the

Flyback Converter (DCM)

The current-sense resistor can adjust the value of the

constant current. Through better design of the transformer

operations under discontinuous current mode, the PSR

controller’s proprietary control structure is able to achieve

accurate and constant current characteristics. Detailed

design guideline for the transformer is introduced in the

following section.

[]

⎥

⎦

⎤

⎢

⎣

⎡

⋅

=

⋅

=

⋅

=

dis

CS

p

dis

pk

p

pk

s

dis

t

R

V

n

Ts

t

i

n

Ts

i

t

Ts

Io

2

1

2

1

2

1

,

(1)

where:

is,pk is the peak inductor current of the secondary side,

ipk is the peak inductor of primary side.

tdis is discharge-time of transformer inductor current.

np is the turn ratio between primary and secondary winding.

RCS is the current-sense resistor.

VCS is the voltage on current-sense resistor.

Green-Mode Operation

The proprietary green-mode function of the PSR controller

provides off-time modulation to linearly decrease the PWM

frequency at light-load conditions, as low as 500Hz. With

the green-mode function, the power supply can easily meet

the most stringent of power conservation requirements.

Figure 13 shows the characteristics of the PWM frequency

vs. the output voltage of the error amplifier (V

COMV). The

PSR controller uses the positive, proportional, output load

parameter (V

COMV) as an indication of the output load for

modulating the PWM frequency. In heavy load conditions,

the PWM frequency is fixed at 42KHz. Once V

COMV is lower

than V

N, the PWM frequency starts to linearly decrease from

42KHz to 500Hz. Figure 14 is a measured waveform at

burst-mode operation.

COMV

V

Frequency

1KHz

G

V

N

V

40KHz

42KHz

500Hz

Figure 13. PWM Frequency vs. VCOMV

Figure 14. Measured Waveform at Burst-Mode Operation

Vo(AC)

100mV/Div

Gate

10V/Div

VCOMV

500mV/Div

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