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Design Considerations. AN4105 Datasheet

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Design Considerations. AN4105 Datasheet






AN4105 Considerations. Datasheet pdf. Equivalent




AN4105 Considerations. Datasheet pdf. Equivalent





Part

AN4105

Description

Design Considerations



Feature


www.DataSheet4U.com www.fairchildsemi.c om Application Note AN4105 Design Con siderations for Switched Mode Power Sup plies Using A Fairchild Power Switch (F PS) in a Flyback Converter Introduction Flyback switched mode power supplies ( SMPS) are among the most frequently use d power circuits in household and consu mer electronics. The basic function of an SMPS is to supp.
Manufacture

Fairchild Semiconductor

Datasheet
Download AN4105 Datasheet


Fairchild Semiconductor AN4105

AN4105; ly regulated power to the load on the se condary, or output side. An SMPS typica lly incorporates a power transformer, s econdary-side rectifier diodes, a switc hing semiconductor device with control IC, and peripheral circuitry. If the le vel of integration of the switching and control circuitry is not high enough, then additional, separate circuits will be required to ac.


Fairchild Semiconductor AN4105

commodate all functions. Such additional components raise the overall SMPS cost and not uncommonly reduce reliability. Fairchild Power Switches are highly-in tegrated ICs for power supply applicati ons. They combine a high-voltage power MOSFET (SenseFET) and pulse width modul ation (PWM) based control IC in one pac kage. Moreover, they provide enhanced I C functionality, t.


Fairchild Semiconductor AN4105

hereby minimizing the number of addition al components needed in an SMPS. Fairch ild Power Switch (SPS) ICs are widely u sed in the power circuits of a variety of equipment, such as color TVs, printe rs, PCs, monitors, battery chargers and ac adapters. They typically incorporat e a variety of enhanced protection func tions and they permit a much reduced po wer consumption in.

Part

AN4105

Description

Design Considerations



Feature


www.DataSheet4U.com www.fairchildsemi.c om Application Note AN4105 Design Con siderations for Switched Mode Power Sup plies Using A Fairchild Power Switch (F PS) in a Flyback Converter Introduction Flyback switched mode power supplies ( SMPS) are among the most frequently use d power circuits in household and consu mer electronics. The basic function of an SMPS is to supp.
Manufacture

Fairchild Semiconductor

Datasheet
Download AN4105 Datasheet




 AN4105
www.DataSheet4U.com
www.fairchildsemi.com
Application Note AN4105
Design Considerations for Switched Mode Power
Supplies Using A Fairchild Power Switch (FPS) in a
Flyback Converter
Introduction
Flyback switched mode power supplies (SMPS) are among
the most frequently used power circuits in household and
consumer electronics. The basic function of an SMPS is to
supply regulated power to the load on the secondary, or
output side. An SMPS typically incorporates a power
transformer, secondary-side rectifier diodes, a switching
semiconductor device with control IC, and peripheral
circuitry. If the level of integration of the switching and
control circuitry is not high enough, then additional, separate
circuits will be required to accommodate all functions. Such
additional components raise the overall SMPS cost and not
uncommonly reduce reliability.
Fairchild Power Switches are highly-integrated ICs for
power supply applications. They combine a high-voltage
power MOSFET (SenseFET) and pulse width modulation
(PWM) based control IC in one package. Moreover, they
provide enhanced IC functionality, thereby minimizing the
number of additional components needed in an SMPS.
Fairchild Power Switch (SPS) ICs are widely used in the
power circuits of a variety of equipment, such as color TVs,
printers, PCs, monitors, battery chargers and ac adapters.
They typically incorporate a variety of enhanced protection
functions and they permit a much reduced power
consumption in standby modes.
This application note considers the three major functional
blocks of an SMPS: Fairchild Power Switch (SPS) , flyback
converter, and transformer. It discusses a variety of issues
important to their design and use in the overall SMPS.
#3 Vcc
#4
Feedback
#5
Soft Start
& Sync.
6 .3V
2uA 1mA
Sync.
OSC.
3 2V
Vck
UVLO
Voltage
Ref.
2 . 5R
R
S
Q
R
Voffset
LEB
5V
Rsense
Sense
Reset
7 .5V
Thermal
Protection
OVP
Reset
S
Q
R
Control IC
#1
Drain
#2
Source
GND
Sense
FET
Figure1. Internal block diagram of a Fairchild Power Switch(FPS).
©2002 Fairchild Semiconductor Corporation
Rev. 1.0.2




 AN4105
AN4105
APPLICATION NOTE
1. Block Diagram and Basic Operation
of a Fairchild Power Switch(FPS)
IC current is less than 300µA. IC operation starts when CCC
(Figure 2) charges to 15V. Because only a small current (<1
1.1 Block Diagram
mA) is allowed to flow in through the resistor during normal
Figure 1 presents a block diagram of a Fairchild Power
Switch (FPS). It can be divided into several large, functional
sections: under voltage lockout circuitry (UVLO); reference
voltage; oscillator (OSC); pulse width modulation (PWM)
operation this technique reduces the current dissipation in
the SMPS start up resistor.
DC
LINK
Rg
Ccc
block; protection circuits; and gate driving circuits.
Vcc
1.2 Under Voltage Lockout (UVLO)
A Fairchild Power Switch (FPS) under voltage lockout
(UVLO) circuitry (Figure 2) guarantees stable operation of
the IC’s control circuit by stopping and starting it as a
function of the value of VCC (Figure 3). The tur off and turn
on voltage thresholds are fixed internally at 10V and 15V,
respectively. Therefore the UVLO circuitry turns off the
control circuit when VCC is lower than 10V and starts it
when VCC is higher than 15V. Once the control circuit starts
operating, VCC must drop below the 10V level for the
UVLO to stop the circuit again. Before switching starts, the
3
Power On
Reset
+
Latch
Comp ara to r
Fairchild Power
SSwiPtchS(FPS)
-
6V
In te rn al Bia s
1 5V /1 0 V
Vz
UV L
5V
V re f
O Good Logic
Figure 2. Detail of the undervoltage lockout (UVLO)
circuitry in a Fairchild Power Switch. The gate
operating circuit holds in a low state during UVLO,
thereby maintaining the SenseFET at turnoff.
Icc
[mA]
20
Power On
Reset Range
0.3
6V 10V 15V
Vc
Vz [[VcV]]
Figure 3. Fairchild Power Switch(FPS) control circuit status vs. Vcc.
1.3 Feedback Control Circuit
The Fairchild Power Switch(FPS) control IC uses a current
mode PWM and operates such that MOSFET current is
proportional to the feedback voltage Vfb. This limits the
MOSFET current at every cycle. It also offers other
advantages, such as a well regulated SMPS output voltage
with input voltage changes. This method of control also
works successfully in SMPSs used for monitors, which may
have a broad range of synchronizing frequencies to deal
with. As shown in Figure 4, the Fairchild Power Switch
(FPS) oscillator turns on the MOSFET. The feedback
comparator operates to turn it off again, when the MOSFET
current reaches a set value proportional to Vfb. The
MOSFET turn off operation is as follows:(1) the internal
(R+2.5R) voltage divider sets the voltage fed back to one
input of the feedback comparator at Vfb /3.5; (2) a current
proportional to the drain current flows to the MOSFET sense
terminal making Vsense proportional to the drain current;
and, (3) when Vsense becomes greater than Vfb, the output of
the feedback comparator goes high, turning off the
MOSFET. Figure 4 also shows that the circuit is designed to
use an opto isolator in the feedback loop. This is appropriate
for an off line design where input to output
2 ©2002 Fairchild Semiconductor Corporation




 AN4105
APPLICATION NOTE
AN4105
isolation is required. Cfb improves the noise characteristics.
If the control IC incorporates an error amp however, as in
Fairchild’s KA3842B/3B/4B/5B current mode PWM
controller ICs for SMPSs then a resistor and capacitor are
required to provide the feedback to the error amp. This
would provided the same functions as those provided by the
circuit of Figure 4, e.g., fine control of the output voltage
through Vfb. Similarly, other appropriate devices here are
Fairchild’s LM431/TL431/KA431 series of three terminal
shunt regulators. These have a very sharp turn on
characteristic much like a Zener diode and are widely used in
SMPS secondary side error amplifiers.
Vfb
#4
Cfb
Vss
#5
Precision
Current Source
2uA 1mA
5V 2.5R
Vfb*
R
Vck
OSC.
On
Off
S
Q
R
Css Vsense
Reset
Rsense
Fairchild Power SwSitcPhS(FPS)
Idrain
#1
Ioffset
Sense
#2
Isense
Figure 4. Fairchild Power Switch(FPS) feedback circuit appropriate for off line SMPS use (current mode PWM).
1.4 Example Fairchild Power Switch (FPS)
Control Circuit
Figure 5 shows two approaches to control feedback with a
Fairchild Power Switch (FPS). The design in Figure 5a uses
the LM431 regulator, and that of Figure 5b uses a Zener diode.
Even though zenor diode approach is cost effective, the output
regulation is relatively poor.
In Figure 5a, C1 together with R1 produce high frequency
pole formed by the internal 3.5kresistor and Cfb in the
compensation networrk. R4 limits the maximum current of the
photodiode to 2.3mA [(12V - 2.5V - 2V) /3.3K, where 2.5V
is the LM431's saturation voltage and 2 V is the photodiode's
voltage drop]. Cfb should be determined by considering the
shutdown delay time (see Section 2.1). In Figure 5b, R3 sets a
fixed current to the Zener diode to stabilize its voltage.
©2002 Fairchild Semiconductor Corporation
3






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