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AN-6094
2. Operation Principle
2-1 Constant-Voltage Regulation Operation
Figure 2 shows the internal PWM control circuit of
FAN302. The constant voltage (CV) regulation is
implemented in the same way as the conventional isolated
power supply, where the output voltage is sensed using
voltage divider and compared with the internal 2.5 V
reference of the shut regulator (KA431) to generate a
compensation
signal.
The
transferred to the primary side using an opto-coupler and
applied to the PWM comparator (PWM.V) through
attenuator Av to determine the duty cycle.
CC regulation is implemented internally without directly
sensing the output current. The output current estimator
reconstructs output current information (V
transformer primary-side current and diode current
discharge time. V
is then compared with a reference
CCR
voltage (2.5 V) by an internal error amplifier and generates
a V
signal to determine the duty cycle.
EA.I
V
and V
are compared with an internal sawtooth
EA.I
EA.V
waveform (V
) by PWM comparators PWM.I and
SAW
PWM.V, respectively, to determine the duty cycle. As seen
in Figure 2, the outputs of two comparators (PWM.I and
PWM.V) are combined with the OR gate and used as a
reset signal of flip-flop to determine the MOSFET turn-off
instant. The lower signal, V
duty cycle, as shown in Figure 3. During CV regulation,
V
determines the duty cycle while V
EA.V
HIGH. During CC regulation, V
cycle while V
is saturated to HIGH.
EA.V
Figure 2. Internal PWM Control Circuit
© 2012 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 9/27/12
compensation
signal
) using the
CCR
and V
, determines the
EA.V
EA.I
is saturated to
EA.I
determines the duty
EA.I
is
Figure 3. PWM Operation for CC and CV
Output Current Estimation
Figure 4 shows the key waveform of a flyback converter
operating in Discontinuous Conduction Mode (DCM),
where the secondary side diode current reaches zero before
the next switching cycle begins. Since the output current
estimator of FAN302 is designed for DCM operation, the
power stage should be designed such that DCM is
guaranteed for the entire operating range. The output
current is obtained by averaging the triangular output diode
current area over a switching cycle, as calculated by:
=
=
AVG
I
I
I
O
D
PK
where I
is the peak value of the primary-side current;
PK
NP and NS are the number of turns of the transformer,
primary side and secondary side, respectively; t
diode current discharge time; and t
Figure 4. Key Waveforms of DCM Flyback Converter
2
N
t
⋅
P
DIS
2
N
t
S
S
is switching period.
s
I
P K
N
⋅
P
I
PK
N
S
I
AVG
D
N
⋅
A
V
O
N
S
www.fairchildsemi.com
(1)
is the
DIS
=
I
O
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