FAN103 Fairchild Semiconductor, FAN103 Datasheet - Page 10

• Manufacturer Part Number: FAN103
• Description: This third-generation Primary Side Regulation (PSR) and highly integrated PWM controller provides several features to enhance the performance of low-power flyback converters
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN103 • Rev. 1.0.5
Functional Description
Figure 21 shows the basic circuit diagram of a primary-
side regulated flyback converter with typical waveforms
shown
conduction mode (DCM) operation is preferred for
primary-side regulation since it allows better output
regulation. The operation principles of DCM flyback
converter are as follows:
During the MOSFET on time (t
applied across the primary-side inductor (L
MOSFET current (I
peak value (I
from the input and stored in the inductor.
When the MOSFET is turned off, the energy stored in
the inductor forces the rectifier diode (D) to be turned
on. While the diode is conducting, the output voltage
(V
applied across the secondary-side inductor (L
N
the peak value (I
current discharge time (t
inductor has been delivered to the output.
When the diode current reaches zero, the transformer
auxiliary winding voltage (V
resonance between the primary-side inductor (L
the effective capacitor loaded across MOSFET.
During the inductor current discharge time, the sum of
output voltage and diode forward-voltage drop is
reflected to the auxiliary winding side as (V
N
as current decreases, the auxiliary winding voltage
reflects the output voltage best at the end of diode
conduction time, where the diode current diminishes to
zero. Thus, by sampling the winding voltage at the end
of the diode conduction time, the output voltage
information can be obtained. The internal error amplifier
for output voltage regulation (EA_V) compares the
sampled voltage with internal precise reference to
generate error voltage (V
duty cycle of the MOSFET in CV mode.
Meanwhile, the output current can be estimated using
the peak drain current and inductor current discharge
time since output current is same as average of the
diode current in steady state.
The output current estimator recognizes the peak value
of the drain current with a peak-detection circuit and
calculates the output current using inductor discharge
time (t
information is compared with internal precise reference
to generate error voltage (V
duty cycle of the MOSFET in CC Mode. With Fairchild’s
innovative
current (CC) output can be precisely controlled.
Among the two error voltages, V
small one determines the duty cycle. Therefore, during
constant voltage regulation mode, V
duty cycle while V
constant current regulation mode, V
duty cycle while V
p
a
o
2
/N
), together with diode forward voltage drop (V
) and the diode current (I
s
. Since the diode forward-voltage drop decreases
DIS
in
) and switching period (t
TRUECURRENT
pk
Figure
). During this time, the energy is drawn
pk
COMV
N
ds
COMI
) increases linearly from zero to the
p
/N
is saturated to HIGH.
22.
s
DIS
) to zero. At the end of inductor
is saturated to HIGH. During
COMV
), all the energy stored in the
w
COMI
) begins to oscillate by the
Generally,
D
®
) decreases linearly from
ON
), which determines the
), which determines the
), input voltage (V
technique,
COMV
COMV
COMI
s
). This output
and V
determines the
determines the
discontinuous
m
COMI
). Then,
constant
o
+V
m
F
m
), are
) and
DL
N
, the
F
) 
) is
s
2
/
10
Figure 21. Simplified PSR Flyback Converter Circuit
V
COMV
Primary-Side Regulation
Control
PWM
V
EA_V
Gate
Figure 22. Key Waveforms of DCM Flyback
AC
V
EA_I
COMI
V
Controller
F
Ref

N
Ref
N
A
S
Estimator
Estimator
Detector
t
Vo
Io
DIS
V
I
O
V
Converter
pk
I
+
-
DL

CS
pk
V
N
N
Vs
DD

A
S
N
N
R
R
P
S
S1
S2
R
CS
L
V
N
+
-
m
w
N
I
A
ds
p
:N
s
I
D
. D avg
+ V
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I
D
F
-

I
o
V
+
-
I
O
o
O
D
L
A