FA5640 Datasheet, PDF (29/35 Pages) Fuji Electric – Fuji Switching Power Supply Control IC

FA5640/41/42
Advice for designing
(1) Insertion of a filter
Since this IC has a leading edge blanking (minimum ON
width: 290 ns), malfunction due to surge current generated
at the MOSFET is switched on does not occur. However, if
the surge current generated at the leading edge of OUT is
large, or external noise is added, malfunction may occur. In
such cases, add a CR filter to the IS pin as shown in Fig.
22.
The filter constant depends on the magnitude of the noise,
but as the time constant of Ris  Cis, about 500 ns or less is
recommended. Note, however, the overload detection level
and the load level of starting burst operation may vary, thus
audible noise may be generated or standby power may vary.
Pay special attention to the above phenomena.
(2) Burst operation point adjustment 1 under light load
If burst operation is started under heavy load, the audible
noise may be generated at transformer. To decrease the
burst point slightly, add resistor R4 between the IS pin and
the OUT pin (Fig. 23). If R4 is connected, the positive bias
voltage is applied to the IS pin voltage when the MOSFET
is turned-on, and consequently, the FB pin voltage also
remains high level. Since burst operation occurs if the FB
pin voltage decrease to 0.45 V (at low line voltage) or lower,
burst operation does not tend to occur if the FB pin voltage
remains high. Even if a resistor is added between the IS pin
and the OUT pin, the effect of resistor R4 may not be
obtained if Ris is small. In this case, decrease Cis and
increase Ris, while fixing the time constant of the filter (Ris
= 470  is recommended when R4 is added).
Note, however, that the standby power may increase, or
overload detection level may vary. With this IC, though the
dependency of overload detection level on line voltage has
been compensated, this ratio may deviate. Confirm there is
no problem in application enough.
by external signals
Diodes D1 and D2 in Fig. 21 can share parts with the diode
connected between the VCC pin and the auxiliary winding.
While the MOSFET is turned-off, the voltage of auxiliary
windings is depended on output voltage on the secondary
side and ratio of the number of turns of secondary windings
and number of turns of auxiliary windings. As the pull-up
level of the IS pin voltage, determine the value of R3 so that
the IS pin voltage reaches IS pin standby detection voltage
0.55 V, or higher within 2 s after the turn OFF. In this case,
if Ris is small, the IS pin voltage may not increase.
Therefore, adjust the constant of the filter, following the
description in (2) “Burst operation point adjustment 1 under
light load.”
Diode D2 is added to prevent heating of the MOSFET in the
event diode D1 is short-circuited, causing negative voltage
to be applied to the IS pin and allowing the ON width to
increase abnormally.
Current
Comparator
Ris
3
IS
Cis
Rs
Fig.22 IS pin filter
OUT
5
R4 _Ris
3
IS
4
GND
Cis
Rs
Fig.23 Burst operation point adjustment 1
(3) Burst operation point adjustment 2 under light load
To allow bust operation to occur under slightly heavier load,
thus to improvement the efficiency under light load, on
condition that there is no transformer audible noise problem,
add resistor R5 between the IS pin and the OUT pin (Fig.
24). Note, however, that the overload detection level varies
in this case also. With this IC, though the dependency of
overload detection level on line voltage has been
compensated, this ratio may deviate. Confirm there is no
problem in application enough.
(4) Switching of current limiting maximum threshold voltage
C1
IS
3
Ris
Rs
R5
VCC
6
4
GND
Fig.24 Burst operation point adjustment 2
Fuji Electric Co., Ltd.
AN-064E Rev.1.1
April 2011
29
http://www.fujielectric.co.jp/products/semiconductor/