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Functional Description
Figure 20.
Basic Circuit of a PSR Flyback Converter for LED Illumination
Figure 20 shows the basic circuit diagram of a primary-
side regulated flyback converter with typical
waveforms shown in Figure 21. Generally,
Discontinuous 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:
Stage I
During the MOSFET on time (t ON ), input voltage (V DC ) is
applied across the primary-side inductor (L m ). Then
MOSFET current (I DS ) increases linearly from zero to the
peak value (I PK ). During this time, the energy is drawn
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 F ) to be turned
on. While the diode is conducting, the output voltage
I
PK
×
N
N
P
S
(V O ), together with diode forward-voltage drop (V F ), is
applied across the secondary-side inductor and the
diode current (I F ) decreases linearly from the peak value
(I PK × N P /N S ) to zero. At the end of inductor current
discharge time (t DIS ), all the energy stored in the
inductor has been delivered to the output.
Stage III
V F ×
N A
N S
V O ×
N A
N S
When the diode current reaches zero, the transformer
auxiliary winding voltage (V A ) begins to oscillate by the
resonance between the primary-side inductor (L m ) and
the effective capacitor loaded across MOSFET (Q1).
Constant Voltage Regulation
During the inductor current discharge time (t DIS ), the
sum of output voltage (V O ) and diode forward-voltage
drop (V F ) is reflected to the auxiliary winding side as
Figure 21.
Waveforms of DCM Flyback Converter
(V O +V F ) × N A /N S . Since the diode forward-voltage drop
(V F ) decreases as current decreases, the auxiliary
winding voltage (V A ) reflects the output voltage (V O ) at
the end of diode conduction time (t DIS ), where the diode
current (I F ) diminishes to zero. By sampling the winding
? 2012 Fairchild Semiconductor Corporation
FL103 ? Rev. 1.0.2
10
www.fairchildsemi.com
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