FSEZ1016A Fairchild Semiconductor, FSEZ1016A Datasheet - Page 10

• Manufacturer Part Number: FSEZ1016A
• Description: This highly integrated PWM controller provides several features to enhance the performance of low-power flyback converters
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FSEZ1016A • Rev. 1.0.2
Functional Description
Figure 24 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 because 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
inductor current discharge time (t
stored in the 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 the same as the average of
the diode current in steady state.
The output current estimator detects the peak value of
the drain current by a peak detection circuit and
calculates the output current by the inductor discharge
time (t
information is compared with the internal precise
reference to generate error voltage (V
determines the duty cycle of the MOSFET in CC mode.
With
TRUECURRENT™, constant current (CC) output can
be precisely controlled.
Of the two error voltages, V
determines the duty cycle. During constant voltage
regulation mode, V
V
COMI
p
A
2
/N
) and the diode current (I
S
. Since the diode forward-voltage drop decreases
is saturated to HIGH. During constant current
DIS
in
) and switching period (t
COMV
Fairchild’s
pk
Figure
), which determines the duty cycle of the
). During this time, the energy is drawn
ds
pk
COMV
) increases linearly from zero to the
× N
25.
determines the duty cycle while
p
/N
W
s
COMV
) to zero. At the end of
innovative
) begins to oscillate by the
Generally,
D
) decreases linearly from
ON
), input voltage (V
and V
DIS
), all the energy
S
COMI
). This output
discontinuous
COMI
, the smaller
technique
m
), which
). Then
F
O
m
), are
m
+V
) and
×N
DL
) is
F
O
s
)×
2
),
/
10
regulation mode, V
V
Figure 24. Simplified PSR Flyback Converter Circuit
V
I
COMV
COMV
I
Primary-Side Regulation
d s
D
V
Control
PWM
AC
(Diode Current)
V
(MOSFET Drain-to-Source Current)
EA_V
Figure 25. Key Waveforms of DCM Flyback
W
V
EA_I
COMI
is saturated to HIGH.
(Auxiliary Winding Voltage)
Controller
V
F
Ref
•
Ref
t
N
N
ON
Estimator
Estimator
A
S
Detector
t
V
I
DIS
O
O
COMI
V
t
V
I
O
+
-
S
DL
determines the duty cycle while
pk
CS
I
V
•
V
t
pk
S
DD
N
N
DI S
•
R
S
R
R
A
Converter
N
N
CS
S1
S2
P
S
L
V
N
+
-
m
w
N
I
A
ds
p
:N
s
D
+ V
I
D avg
www.fairchildsemi.com
I
.
D
F
-
=
I
V
+
-
I
o
O
O
O
D
L
A