FA5695 Datasheet, PDF (11/21 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5695
8. Outline of circuit operation
This IC is a power-factor correction converter
utilizing a boosting chopper, operating in critical
mode. Hereinafter is outline of the operation
consisting of switching operation and power-factor
correction operation using the circuit diagram
shown in Fig. 1.
(1) Switching operation
This IC performs the switching operation in the
critical mode applying self-oscillation without using
an oscillator. Fig. 2 shows the outline of waveforms
of the switching operation in steady state. The
operation is as follows.
t1. Q1 turns on, the current through inductor (L1)
rises from zero. At the timing of Q1 on, Vramp;
output of ramp generator states to rise.
t2. Vramp and Vcomp; output of the error amplifier
are compared by the PWM comparator, and
when Vramp>Vcomp, Q1 turns off and the
output of the ramp generator drops.
When Q1 turns off, the voltage across L1 inverts
and the current through L1 decreases while the
current is supplied to the output side through D1.
t3.The current through L1 is detected by Is terminal,
and when the current becomes zero, the output
of the current detection comparator becomes
High to turn on Q1 after delay given by the delay
circuit, thus moving to the next switching cycle
(t1).
Iin
IL1
AC
C1
By repeating the operations of t1 ~ t3, the switching
in critical mode is continued.
With the power-factor correction circuit in the
critical mode, the switching frequency is always
changing due to instantaneous values of the AC
input voltage. The switching frequency also
changes when the input voltage or load changes.
OUT
(Q1 gate)
Q1
Vds
IL1
Vcomp
Vramp
PWM.comp.
output
(reset)
ZCD.comp.
output
(set)
Fig. 2 Switching Operation, Waveforms
t1
t2
t3 t1
L1
D1
Vo
Q1
Vds
Rs
IS
5
CUR.comp
ZCD.comp
-0.6V
Restart
-10mV
RT
3
Delay
RAMP
OSC
PWM.comp
COMP
2
Dynamic
OVP
Static
OVP
OUT
7
R
Q
UVLO
S
SP
TIMER
Restart
FB
1
ERRAMP 2.5V
SPC7011F
4
OVP
Fig.1 Block diagram of operating circuit
Fuji Electric Co., Ltd.
AN-073E Rev.1.1
April-2011
http://www.fujielectric.co.jp/products/semiconductor/
11