FA5546 Datasheet, PDF (24/27 Pages) Fuji Electric – Switching Power Supply Control IC

FA5546/47
(6) Current sensing unit
As described in 8-(4) One-shot circuit, the minimum ON
width is set for this IC to minimize malfunction due to surge
current that occurs when the power MOSFET is turned on.
However, if the surge current that occurs at the time of
power ON is large, or noise is applied externally at the time
of power ON, malfunction might occur.
In such cases, add RC filters C5 and R7 as shown in
Fig.30.
To ensure efficient operation of the capacitor C5, place it as
close to the IC as possible, and lay wiring with extreme
care.
(7) Protection against noise
Since large current is fed to the VCC terminal when the
MOSFET is started, relatively large noise is likely to occur.
In addition, noise is also generated from the current
supplied from the auxiliary winding.
A large noise may cause IC malfunction. To minimize the
noise generated at the VCC terminal, connect a bypass
capacitor C6 (0.1 F or larger), as shown in Fig.31,
adjacent to the VCC terminal of the IC, and between VCC
and GND, in addition to the electrolytic capacitor C2.
5
OUT
34
IS
GND
C5
Rs
R7
Fig. 30 IS terminal filter
Fig. 31 Noise malfunction prevention (VCC terminal)
If the load is made larger, or due to surge voltage, the VCC
voltage increases, actuating the overvoltage protection
function. To prevent this phenomenon from occurring,
connect resistor R10 (several  to several 10) in series
with the diode as shown in Fig.31.
(8) Improvement of input power at light load
The IC integrates the function of reducing standby power
consumption by lowering oscillation frequency under light
load.
However, since load conditions vary depending on the
power supply setting, the settings within the IC may be
insufficient to reduce standby power consumption.
In such cases, connect resistor R8 as shown in Fig.32 to
reduce oscillation frequency.
If R7 is 1k, the resistance of R8 should be approximately
several k to 100k. The smaller the R8 value, the lower
the switching frequency under light load (positive IS
terminal correction).
Also check overload characteristics, which are affected by
the correction described above.
By connecting R8 and C7 in series and connecting them
between IS-OUT terminals as shown in Fig.33, the Rs
resistance can be set small, allowing the loss to be
minimized. Example: R8 = 4.7k, C7 = 100pF
Fuji Electric Co., Ltd.
AN-027E Rev.0.4
April -2011
Fig. 32 Correction circuit for improvement of input
power under light load
Fig. 33 Correction circuit for improvement of input
power under light load
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