FL103 FAIRCHILD [Fairchild Semiconductor], FL103 Datasheet - Page 10

• Manufacturer Part Number: FL103
• Description: Primary-Side-Regulation PWM Controller for LED Illumination
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2012 Fairchild Semiconductor Corporation
FL103 • Rev. 1.0.1
Functional Description
Figure 20 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.
Figure 21.
in
V ×
F
Figure
Waveforms of DCM Flyback Converter
N
N
S
A
Figure 20.
V ×
O
21.
N
N
A
S
I
Generally,
PK
×
Basic Circuit of a PSR Flyback Converter for LED Illumination
N
N
P
S
Discontinuous
10
The operation principles of DCM flyback converter are
as follows:
Stage I
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.
Stage II
When the MOSFET (Q1) is turned off, the energy stored
in the inductor forces the rectifier diode (D
on. While the diode is conducting, the output voltage
(V
applied across the secondary-side inductor and the
diode current (I
(I
discharge time (t
inductor has been delivered to the output.
Stage III
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 (Q1).
Constant Voltage Regulation
During the inductor current discharge time (t
sum of output voltage (V
drop (V
(V
(V
winding voltage (V
the end of diode conduction time (t
current (I
PK
O
O
F
) decreases as current decreases, the auxiliary
), together with diode forward-voltage drop (V
+V
× N
F
) × N
F
P
) is reflected to the auxiliary winding side as
F
/N
) diminishes to zero. By sampling the winding
S
A
) to zero. At the end of inductor current
PK
/N
F
). During this time, the energy is drawn
S
) decreases linearly from the peak value
. Since the diode forward-voltage drop
DIS
DS
A
) reflects the output voltage (V
) increases linearly from zero to the
), all the energy stored in the
O
) and diode forward-voltage
A
) begins to oscillate by the
ON
), input voltage (V
DIS
), where the diode
F
) to be turned
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m
DIS
). Then
m
), the
) and
DC
F
O
), is
) at
) is