AN-9741
APPLICATION NOTE
Principle of Primary-Side Regulation
Figure 2 shows typical waveforms of a flyback converter.
Generally, Discontinuous Conduction Mode (DCM)
operation is preferred for primary-side regulation since it
allows better output regulation. The key of primary-side
regulation is how to obtain output voltage and current
information without directly sensing them. Once these
values are obtained, the control can be accomplished by the
conventional feedback compensation method.
Stage I
I
PK
Stage II
Stage III
Stage I
decreases linearly from the peak value to zero. At the end of
t
DIS
, all the energy stored in the inductor has been delivered
to the output.
Stage III
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
output capacitor of MOSFET.
Design Procedure
In this section, a design procedure is presented using the
schematic in Figure 3 as a reference.
V
O
V
ON
Constant Voltage
Operation
A
Constant Current
Operation
B
f
S
= 33kHz
UVLO
C Protection
I
O
I
DS
N
N
f
S
= 50kHz
P
S
I
PK
×
0.5*V
ON
I
O
= I
F_AVG
I
F
O
V
F
×
N
A
N
S
I
ON
Figure 3. CV and CC Operation Area
V
O
×
V
A
N
A
N
S
[STEP-1] Estimate the Efficiencies
Figure 3 shows the Constant Voltage (CV) and Constant
Current (CC) operation area. To optimize the power stage
design, the efficiencies and input powers should be
specified for operating point A (nominal output voltage
and current), B (50% of nominal output voltage), and C
(minimum output voltage).
1.
Estimated overall efficiency (η) for operating points
A, B, and C: The overall power conversion efficiency
should be estimated to calculate the input power. If
no reference data is available, set η = 0.7 ~ 0.75 for
low-voltage output applications and η = 0.8 ~ 0.85
for high-voltage output applications.
Estimated primary-side efficiency (η
P
) and
secondary-side efficiency (η
S
) for operating points A,
B, and C. Figure 4 shows the definition of primary-
side and secondary-side efficiencies, where the
primary-side efficiency is for the power transfer from
AC line input to the transformer primary side, while
the secondary-side efficiency is for the power transfer
from the transformer primary side to the power
supply output.
t
ON
t
DIS
t
S
Figure 2. Key Waveforms of PSR Flyback Converter
The operation principles of DCM flyback converter are:
Stage I
During the MOSFET on time (t
ON
), input voltage (V
DL
) 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 is turned off, the energy stored in the
inductor forces the rectifier diode (D
F
) to turn on. During
the diode conduction time (t
DIS
), the output voltage (V
O
),
together with diode forward-voltage drop (V
F
), are applied
across the secondary-side inductor and the diode current (I
F
)
© 2011 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 6/27/11
2
2.
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