FSL136HR View Datasheet(PDF) - Fairchild Semiconductor
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FSL136HR Datasheet PDF : 13 Pages
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Functional Description
Startup
At startup, an internal high-voltage current source
supplies the internal bias and charges the external
capacitor (CA) connected with the VCC pin, as illustrated
in Figure 14. When VCC reaches the start voltage of 12V,
the FPS™ begins switching and the internal high-
voltage current source is disabled. The FPS continues
normal switching operation and the power is provided
from the auxiliary transformer winding unless VCC goes
below the stop voltage of 8V.
Feedback Control
The FSL136HR employs current-mode control, as
shown in Figure 16. An opto-coupler (such as the
FOD817A) and shunt regulator (such as the KA431) are
typically used to implement the feedback network.
Comparing the feedback voltage with the voltage across
the RSENSE resistor makes it possible to control the
switching 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 VFB is pulled down, and the duty
cycle is reduced. This typically occurs when the input
voltage is increased or the output load is decreased.
Figure 14. Startup Circuit
Oscillator Block
The oscillator frequency is set internally and the FPS
has a random frequency fluctuation function. Fluctuation
of the switching frequency of a switched power supply
can reduce EMI by spreading the energy over a wider
frequency range than the bandwidth 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;
however, its selection is randomly chosen by the
combination of external feedback voltage and internal
free-running oscillator. This randomly chosen switching
frequency effectively spreads the EMI noise nearby
switching frequency and allows the use of a cost-
effective inductor instead of an AC input line filter to
satisfy the world-wide EMI requirements.
Figure 15. Frequency Fluctuation Waveform
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 RSENSE 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 (tLEB) after the SenseFET is
turned on.
Protection Circuits
The FPS has several protective functions, such as
overload protection (OLP), over-voltage protection
(OVP), output-short protection (OSP), under-voltage
lockout (UVLO), abnormal over-current protection
(AOCP), and thermal shutdown (TSD). Because these
various protection circuits are fully integrated in the IC
without external components, the reliability is improved
without increasing cost. Once a fault condition occurs,
switching is terminated and the SenseFET remains off.
This causes VCC to fall. When VCC reaches the UVLO
stop voltage VSTOP (8V), the protection is reset and the
internal high-voltage current source charges the VCC
capacitor via the VSTR pin. When VCC reaches the UVLO
start voltage VSTART (12V), the FPS resumes normal
operation. In this manner, the auto-restart can
alternately enable and disable the switching of the
power SenseFET until the fault condition is eliminated.
© 2011 Fairchild Semiconductor Corporation
FSL136HR • Rev. 1.0.0
9
www.fairchildsemi.com
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