FA5540 Fuji Electric holdings CO.,Ltd, FA5540 Datasheet - Page 19

• Manufacturer Part Number: FA5540
• Description: Fuji Switching Power Supply Control Ic
• Manufacturer: Fuji Electric holdings CO.,Ltd

8. Basic operation
Rev.0 July-2007
After this, repeat from t1 to t4 and continue switching.
t1 to t2
t2
t2 to t3
t3 to t4
t4
The basic operation of the power supply using this IC is not
switching operation using fixed frequency with an oscillator but
switching using self-excited oscillation. This is shown in Fig.1
Basic circuit diagram and Fig.2 Waveform in operation.
Q1 turns ON, and Q1 drain current Id (primary current of T1)
begins to rise from zero. Q1 current is converted into voltage at
Rs and input into IS pin.
When the current of Q1 gets up to the reference voltage of the
current comparator that is fixed by the voltage of FB pin, a reset
signal is input into RS flip-flop and Q1 turns OFF.
When Q1 turns OFF, and the winding voltage of the transformer
turns over and current IF is provided from the transformer into
secondary side through D1.
When ZCD pin voltage drops lower than the threshold voltage
(62mV(typ.)) of Valley detection, a set signal is input into R-S
flip-flop and Q1 turns ON again. If the delay time of CR circuit
placed between the auxiliary winding and ZCD pin is adjusted
suitably, Q1 can be turned on at the bottom of the voltage.
Switching loss of TURN ON can be controlled to the minimum
by this operation. (Return to t1)
When the current from the transformer into secondary side
goes out and the current at D1 gets to zero, the voltage of Q1
drops rapidly due to the resonance of transformer inductance
and capacitor Cd. At the same time the transformer auxiliary
winding voltage Vsub also drops rapidly.
ZCD pin receives this auxiliary winding voltage but it has a little
amount of delay time due to CR circuit composed of R
CZCD on the way.
Q
F.F.
Driver
S
R
com parator
Current
1 shot
com parator
ZCD
detection
Valley
Level
shift
FA5540/5541/5542
5
1
3
2
OUT
ZCD
FB
IS
C
ZCD
R
ZCD
ZCD
19
and
Q1
Fig.1 Circuit outline for basic operation
Vsub
Cd
Rs
Vds
comparator
signal、set)
Current
out put
out put
(reset)
(Q1 gate)
1 shot
(Valley
ZCD Pin
T1
OUT
Vsub
Vds
Q1
Q1
D1
Id
I
F
Fuji Electric Device Technology
Fig.2 Waveform in basic operation
D1
t1
62mV
t2
t3
t4