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AN-6961 Datasheet(PDF) 2 Page - Fairchild Semiconductor

Part # AN-6961
Description  Critical Conduction Mode PFC Controller
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Manufacturer  FAIRCHILD [Fairchild Semiconductor]
Direct Link  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

AN-6961 Datasheet(HTML) 2 Page - Fairchild Semiconductor

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AN-6961
APPLICATION NOTE
© 2009 Fairchild Semiconductor Corporation
www.fairchildsemi.com
Rev. 1.0.2 • 4/8/09
2
Basic Operation of the Boost Converter
The typical boost converter and its operational waveforms
are shown in Figure 2, 3, and 4, respectively.
()
g
vt
+
()
L
vt
+−
D
Co
Ro
o
V
+
()
L
it
Q
b
L
Figure 2. Boost Converter
()
g
vt
+
()
L
vt
+−
()
L
it
Q
b
L
()
g
vt
+
()
L
vt
+−
Co
Ro
o
v
+
()
L
it
b
L
(a) Switch Q is ON
(b) Switch Q is OFF
Figure 3. Switching Sequences of the Boost Converter
()
og
b
vv t
L
()
L
vt
on
t
off
t
Q
()
L
it
()
g
b
vt
L
()
g
vt
g ()
o
vv t
T
,
(t)
Lavg
i
Figure 4. One-cycle Waveform of the Boost Converter
Operation Principle
Switch Q is ON:
When Q turns on, the rectifier diode D is
reverse-biased and output capacitor CO supplies load
current. The rectified AC line input voltage Vg(t) is applied to
the inductor Lb so that inductor current IL ramps up linearly
and can be expressed as:
b
g
on
L
L
(t)
V
t
I
=
)
(
(1)
Switch Q is OFF:
When Q turns off, the voltage VO-Vg(t) is
applied to inductor Lb and the polarity on the inductor Lb is
reversed. The diode D is forward-biased in this stage. The
energy stored in the inductor Lb is delivered to supply load
current and output capacitor CO. The inductor current iL can
be expressed as:
b
g
o
off
L
L
(t)
V
-
V
t
I
=
)
(
(2)
Controlled On-Time:
The on-time of the power MOSFET Q
is determined by the output of the error amplifier that
monitors the preregulator output voltage. With a low-
bandwidth error amplifier, the feedback signal is almost
constant during a half AC cycle, resulting a fixed on-time of
the power MOSFET at a specific AC voltage and some
certain output power level. Therefore, the peak inductor
current ILpk automatically follows the input voltage Vg(t),
achieving a natural power factor correction mechanism.
Figure 5 shows the typical inductor current waveform during
a half AC cycle.
on
Off
,
()
Lavg
it
()
g
vt
max
T
Gate
,
Lpk
i
min
T
-
fixed On Time
Figure 5. Controlled On-Time Inductor Current Waveform
Referring to Figure 4, considering one switching period the
average inductor current IL,ave(t) can be calculated by the
average area of triangle waveform of inductor current:
s
2
b
s
on
g
T
L
2
T
t
)
t
(
V
)
(
+
=
(t)
V
-
V
(t)
V
t
I
g
o
2
g
avg
L,
(3)


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