FAN100 Fairchild Semiconductor, FAN100 Datasheet - Page 10

• Manufacturer Part Number: FAN100
• Description: The primary-side PWM controller FAN100 significantly simplifies power supply design that requires CV and CC regulation capabilities
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN100 Rev. 1.0.2
Functional Description
0 shows the basic circuit diagram of a primary-side
regulated flyback converter and its typical waveforms
are shown in 0. 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:
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.
When the MOSFET is turned off, the energy stored in
the inductor forces the rectifier diode (D) to be turned
on. While the diode is conducting, the output voltage
(V
applied across the secondary-side inductor (L
N
the peak value (I
inductor current discharge time (t
stored in the inductor has been delivered to the output.
When the diode current reaches zero, the transformer
auxiliary winding voltage (V
resonance between the primary-side inductor (L
the effective capacitor loaded across the MOSFET.
During the inductor current discharge time, the sum of
output voltage and diode forward-voltage drop is
reflected to the auxiliary winding side as (V
Since the diode forward-voltage drop decreases as
current decreases, the auxiliary winding voltage reflects
the output voltage best at the end of diode conduction
time where the diode current diminishes to zero. Thus,
by sampling the winding voltage at the end of the diode
conduction time, the output voltage information can be
obtained. The internal error amplifier for output voltage
regulation (EA_V) compares the sampled voltage with
internal precise reference to generate error voltage
(V
MOSFET in CV mode.
Meanwhile, the output current can be estimated using
the peak drain current and inductor current discharge
time since output current is the same as average of the
diode current in steady state.
The output current estimator detects the peak value of
the drain current with a peak detection circuit and
calculates the output current using the inductor
discharge time (t
output information is compared with the internal precise
reference to generate error voltage (V
determines the duty cycle of the MOSFET in CC mode.
With
TRUECURRENT™, constant current (CC) output can
be precisely controlled.
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10
Of the two error voltages, V
determines the duty cycle. During constant voltage
regulation mode, V
V
regulation mode, V
V
Figure 23. Simplified PSR Flyback Converter Circuit
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Figure 24. Key Waveforms of DCM Flyback
is saturated to HIGH.
is saturated to HIGH. During constant current
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