FAN302UL Fairchild Semiconductor, FAN302UL Datasheet - Page 11

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FAN302UL

Manufacturer Part Number
FAN302UL
Description
This highly integrated PWM controller provides several features to enhance the performance of flyback converters
Manufacturer
Fairchild Semiconductor
Datasheet

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© 2011 Fairchild Semiconductor Corporation
FAN302UL • Rev. 1.0.2
With a given current sensing resistor, the output current
can be programmed as:
The peak value of primary-side current is obtained by an
internal peak-detection circuit while diode current
discharge time is obtained by detecting the diode
current zero-crossing instant. Since the diode current
cannot be sensed directly with primary-side control, the
Zero Crossing Detection
indirectly by monitoring the auxiliary winding voltage.
When the diode current reaches zero, the transformer
winding voltage begins to drop by the resonance
between
transformer magnetizing inductance. To detect the
starting instant of the resonance, the V
85% of the diode current discharge time of the previous
switching cycle and compared with the instantaneous V
voltage. When instantaneous voltage of the VS pin
drops below the sampled voltage by more than 200mV,
ZCD of diode current is obtained as shown in Figure 27.
Frequency Reduction in CC Mode
The transformer should be designed to guarantee DCM
operation over the whole operation range since the
output current is properly estimated only in DCM
operation. As can be seen in Figure 28, the discharge
time (t
voltage decreases in CC Mode. The converter tends to
go into CCM as output voltage drops in CC Mode when
operating at the fixed switching frequency. To prevent
this CCM operation and maintain good output current
estimation in DCM, FAN302UL decreases switching
frequency as output voltage drops, as shown in Figure
28 and Figure 29. FAN302UL indirectly monitors the
output voltage by the sample-and-hold voltage (V
V
time of the previous switching cycle, as shown in Figure
27. Figure 30 shows how the frequency reduces as the
sample-and-hold voltage of the VS pin decreases.
S
, which is taken at 85% of diode current discharge
where K is the design parameter of IC, which is 12
for FAN302UL.
I
O
DIS
Figure 27. Detailed Waveform for ZCD
) of the diode current increases as the output
K R
the
1.25
SENSE
MOSFET
N
N
P
S
output
(ZCD) is accomplished
capacitance
S
is sampled at
SH
and
(2)
) of
S
11
Figure 30. Frequency Reduction Curve in
Figure 29. Frequency Reduction with V
Figure 28. t
CC Regulation
DIS
Variation in CC Mode
V
f
S
G
CC
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SH

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