FA5640 Datasheet, PDF (18/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
8. Basic operation(The values in the following description are typical values unless otherwise specified.)
The basic operation of the power supply using IC is not switching
operation with fixed frequency using an oscillator but switching with
self-excited oscillation. This is shown in Fig.1 Schematic circuit diagram
and Fig.2 Waveform in the basic operation.
OUT
(Q1 gate)
t1 to t2
Q1 turns ON and then Q1 drain current Id (current of primary windings
Q1
of T1) begins to rise from zero. Q1 current is converted into the voltage
Vds
by Rs and is input into IS pin.
t2
When the current of Q1 get to the reference voltage of the current
Q1
Id
comparator that is fixed by the voltage of FB pin, a reset signal is input
into RS flip-flop and Q1 turns OFF.
t2 to t3
D1
When Q1 turns OFF, then the windings voltage of the transformer
IF
turns over and the current IF is provided from the transformer into the
secondary side through D1.
t3 to t4
Vsub
When the current from the transformer into the secondary side stops
and the current of D1 gets to zero, the voltage of Q1 turns down
rapidly due to the resonance of the transformer inductance and the
ZCD Pin
capacitor Cd. At the same time the transformer auxiliary windings
60mV
voltage Vsub also drops rapidly.
ZCD pin receives this auxiliary windings voltage but then it has a little
delay time because of CR circuit composed with RZCD and CZCD on
the way.
1 shot
out put
(Valley
signal、set)
t4
If ZCD pin voltage turns down lower than the threshold voltage 50mV
of Bottom detection, a set signal is input into R-S flip-flop and Q1 turns
Current
comparator
out put
(reset)
ON again. If the delay time of CR circuit placed between the auxiliary
windings and ZCD pin is adjusted properly, Q1 voltage can be turned
t1
t2
t3 t4
on at the bottom. This operation makes the switching loss of TURN
Fig.2 Waveform in basic operation
ON to the minimum.
(Return to t1)
IF
D1
Subsequently repeat from t1 to t4 and continue switching.
1 shot
(290ns)
Driver
Bottom Skip
control
Bottom
detection
QS
Set
Current
Comparator
R
Rset
Level
shift
OUT
5
ZCD
1
Q1
R1
Cd ^Vds
Rs
C1
IS
3
2
FB
Vsub
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
Fig.1 Schematic circuit diagram in basic operation
18
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