FAN102 Fairchild Semiconductor, FAN102 Datasheet - Page 10

• Manufacturer Part Number: FAN102
• Description: The primary-side PWM controller significantly simplifies power supply design that requires CV and CC regulation capabilities
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN102 Rev. 1.0.3
Functional Description
Figure 24 shows the basic circuit diagram of 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 turn on.
While the diode is conducting, the output voltage (V
together with diode forward voltage drop (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. 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
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 picks up the peak value of
the drain current with a peak detection circuit and
calculates the output current using the inductor
discharge time (t
output information is compared with internal precise
reference to generate error voltage (V
determines the duty cycle of the MOSFET in CC mode.
Among the two error voltages, V
smaller actually determines the duty cycle. During
constant voltage regulation mode, V
duty cycle while V
constant current regulation mode, V
duty cycle while V
p
A
2
/N
) and the diode current (I
S
. Since the diode forward voltage drop decreases
in
COMV
pk
Figure
), which determines the duty cycle of the
). During this time, the energy is drawn
pk
COMV
×N
DIS
ds
COMI
) increases linearly from zero to the
) and switching period (t
p
/N
is saturated to HIGH.
25.
s
DIS
) to zero. At the end of inductor
is saturated to high. During
), all the energy stored in the
W
) begins to oscillate by the
Generally,
D
) decreases linearly from
ON
), input voltage (V
COMV
COMV
COMI
and V
determines the
determines the
discontinuous
COMI
m
), which
COMI
). Then
S
F
O
m
). The
m
), are
+V
) and
×N
, the
DL
) is
F
o
s
)×
2
),
/
10
Figure 24. Simplified PSR Flyback Converter Circuit
V
I
COMV
Primary-Side Regulation
I
d s
D
Control
PWM
(Diode Current)
V
(MOSFET Drain-to-Source Current)
V
EA_V
Gate
Figure 25. Key Waveforms of DCM Flyback
W
AC
V
EA_I
COMI
(Auxiliary Winding Voltage)
Controller
V
F
Ref
•
Ref
t
N
N
ON
Estimator
Estimator
A
S
Detector
t
V o
I o
DIS
V
t
V
I
+
O
-
S
DL
pk
CS
I
V
•
t
V
pk
DD
N
N
S
DI S
•
S
R
R
A
Converter
N
N
S1
S2
R
P
S
CS
L
V
N
+
-
m
w
N
I
A
ds
p
:N
s
D
+ V
I
D avg
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I
.
D
F
-
=
I
V
+
-
I
o
O
o
O
D
L
A