AN-6961

APPLICATION NOTE

© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.2 • 4/8/09

3

Block Operation Description

Multi-Vector Error Amplifier

The FAN6961 has a trans-conductance type amplifier that

provides better dynamic performance. Referring to Figure 6,

waveform to generate a fixed on-time. To achieve a low

input current THD, the variation of the on-time within one

input AC cycle should be very small. Therefore, the

bandwidth of the feedback loop should be set below 20Hz

to maintain a constant on-time for a line half-cycle.

COMP and GND is suggested.

t

_

EA out

V

PWM

Gate

ds

V

()

L

it

t

t

t

()

g

b

Vt

L

-( )

og

b

VV t

L

on

t

Sawtooth Generator

()

g

Vt

off

t

Figure 6. Operation Waveforms of Fixed On Time

Technique

For fast transient response and precise clamping of the

output voltage overshoot and undershoot, the FAN6961 has

a built-in multi-vector error amplifier. Figure 7 shows the

block diagram of the multi-vector error amplifier. When the

variation of the feedback voltage exceeds +6% and -8% of

the reference voltage, the multi-vector error amplifier

adjusts its output impedance to increase the loop response.

1

2

FAN6961

2.65V

2.3V

INV

COMP

Error

Amplifier

Figure 7. Block Diagram of the Multi-Vector Error

Amplifier

Total Harmonic Distortion (THD) Optimization

As discusses previously, the FAN6961 uses the controlled

on-time technique to achieve power factor correction

mechanism. However, to get better THD at light load

condition, especially at high input voltage, a THD

optimization circuit is inserted into the FAN6961. With this

internal THD optimization circuit, the on-time of the power

MOSFET is modulated to further improve the THD

performance. The calculated on-time variation within one

line voltage period with the fixed on-time technique, and

after the THD optimization is added, are shown in Figure 8.

The calculated input current waveform is shown in Figure 9.

5.00E-07

2.50E-06

4.50E-06

6.50E-06

8.50E-06

1.05E-05

1.25E-05

1.45E-05

1.65E-05

1.85E-05

0

20

40

60

80

100

120

140

160

Time(S/10000)

THD Optimization(Vac=90V,Vo=250V)

Fixed On Time(Vac=90V,Vo=250V)

THD Optimization(Vac=264V,Vo=400V)

Fixed On Time(Vac=264V,Vo=400V)

Figure 8. MOS Turn-on Time Calculational Curve

(Before and After THD Optimization Circuit Added)

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0

20

40

60

80

100

120

140

160

Time(S/10000)

THD Optimization(Vac=90V,Vo=250V)

Fixed On Time(Vac=90V,Vo=250V)

THD Optimization(Vac=264V,Vo=400V)

Fixed On Time(Vac=264V,Vo=400V)

Figure 9. Calculated Waveforms of the Input Current

(Before and After THD Optimization Circuit Added)