FSEZ1307 Fairchild Semiconductor, FSEZ1307 Datasheet - Page 10

• Manufacturer Part Number: FSEZ1307
• Description: This third-generation Primary Side Regulation (PSR) PWM controller combination power MOSFET, FSEZ1307, provides several features to enhance the performance of low-power flyback converters
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FSEZ1307 • Rev. 1.0.2
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 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 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 the 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 because output current is same as the 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 the error voltage (V
determines the duty cycle of the MOSFET in CC mode.
With
technique, constant current (CC) output can be
precisely controlled.
Among the two error voltages, V
smaller one determines the duty cycle. Therefore, during
constant voltage regulation mode, 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
Fairchild’s
in
pk
Figure
). During this time, the energy is drawn
pk
×N
DIS
ds
) increases linearly from zero to the
) and switching period (t
p
/N
innovative
25.
s
DIS
) to zero. At the end of inductor
COMV
), all the energy stored in the
w
) begins to oscillate by the
Generally,
D
) decreases linearly from
ON
), which determines the
), input voltage (V
COMV
COMV
TRUECURRENT™
and V
determines the
discontinuous
COMI
m
COMI
), which
). Then
o
s
+V
). This
m
m
) and
DL
F
×N
, the
F
), is
) ×
) is
s
2
/
10
duty cycle while V
constant current regulation mode, V
duty cycle while V
Figure 24. Simplified PSR Flyback Converter Circuit
V
COMV
Primary-Side Regulation
V
Control
PWM
AC
EA_V
Figure 25. Key Waveforms of DCM Flyback
V
EA_I
COMI
Controller
V
F
Ref
Ref
⋅
N
N
Estimator
Estimator
Detector
A
S
t
V
I
DIS
O
O
COMV
COMI
V
is saturated to HIGH.
V
I
+
O
-
DL
CS
pk
V
I
V
is saturated to HIGH. During
⋅
pk
S
DD
N
N
⋅
R
A
S
R
R
Converter
N
N
CS
S1
S2
P
S
L
V
N
+
-
m
w
N
I
A
COMI
ds
p
:N
s
determines the
D
I
+ V
. D avg
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I
D
F
-
=
I
o
V
+
-
I
O
O
O
D
L
A