Data Sheet

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9
FSL106HR — Green Mode Power Switch
Functional Description
Startup
At startup, an internal high-voltage current source
supplies the internal bias and charges the external
capacitor (C
A
) connected w ith the V
CC
pin, as illustrated
in Figure 14. When V
CC
reaches the start voltage of
12V, the FPS™ begins sw itching and the internal high-
voltage current source is disabled. The FPS continues
normal sw itching operation and the pow er is provided
from the auxiliary transformer w inding unless V
CC
goes
below the stop voltage of 8V.
Figure 14. Startup Circuit
Oscillator Block
The oscillator frequency is set internally and the FPS
has a random frequency fluctuation function. Fluctuation
of the sw itching frequency of a sw itched pow er supply
can reduce EMI by spreading the energy over a w ider
frequency range than the bandw idth measured by the
EMI test equipment. The amount of EMI reduction is
directly related to the range of the frequency variation.
The range of frequency variation is fixed internally;
how ever, its selection is randomly chosen by the
combination of external feedback voltage and internal
free-running oscillator. This randomly chosen sw itching
frequency effectively spreads the EMI noise nearby
sw itching frequency and allow s the use of a cost-
effective inductor instead of an AC input line filter to
satisfy the w orld-w ide EMI requirements.
t
SW
Dt
I
DS
t
t
f
SW
f
SW
+1/2Df
SW
MAX
f
SW
-1/2Df
SW
MAX
no repetition
several
mseconds
several
milliseconds
t
SW
=1/f
SW
Figure 15. Frequency Fluctuation Waveform
Feedback Control
FSL106HR employs current-mode control, as show n in
Figure 16. An opto-coupler (such as the FOD817A) and
shunt regulator (such as the KA431) are typically used
to implement the feedback netw ork. Comparing the
feedback voltage w ith the voltage across the R
SENSE
resistor makes it possible to control the sw itching duty
cycle. When the shunt regulator reference pin voltage
exceeds the internal reference voltage of 2.5V, the opto-
coupler LED current increases, the feedback voltage
V
FB
is pulled dow n, and the duty cycle is reduced. This
typically occurs w hen the input voltage is increased or
the output load is decreased.
Figure 16. Pulse-Width-Modulation Circuit
Leading-Edge Blanking (LEB)
At the instant the internal SenseFET is turned on, the
primary-side capacitance and secondary-side rectifier
diode reverse recovery typically cause a high current
spike through the SenseFET. Excessive voltage across
the R
SENSE
resistor leads to incorrect feedback operation
in the current-mode PWM control. To counter this effect,
the FPS employs a leading-edge blanking (LEB) circuit
(see the Figure 16). This circuit inhibits the PWM
comparator for a short time (t
LEB
) after the SenseFET is
turned on.
Protection Circuits
The FPS has several protective functions, such as
overload protection (OLP), over-voltage protection
(OV P), output-short protection (OSP), under-voltage
lockout (UVLO), abnormal over-current protection
(AOCP), and thermal shutdow n (TSD). Because these
various protection circuits are fully integrated in the IC
w ithout external components, the reliability is improved
w ithout increasing cost. Once a fault condition occurs,
sw itching is terminated and the SenseFET remains off.
This causes V
CC
to fall. When V
CC
reaches the UVLO
stop voltage, V
STOP
(8V), the protection is reset and the
internal high-voltage current source charges the V
CC
capacitor via the V
STR
pin. When V
CC
reaches the UVLO
start voltage, V
START
(12V), the FPS resumes normal
operation. In this manner, the auto-restart can
alternately enable and disable the sw itching of the
pow er SenseFET until the fault condition is eliminated.