FIP4F8S datasheet(29/33 Pages) FUJI

29 / 33 page

AN-087E Rev.1.0

April-2011

FA5526/5527/5528/5536/5537/5538

Fuji Electric Co., Ltd.

http://www.fujielectric.co.jp/products/semiconductor/

(7) Improvement of input power at light load

This IC is provided with a function to lower switching

frequency at light load in order to reduce power dissipation.

However,

depending

the

circuit

used,

switching

frequency cannot be sufficiently reduced, leading to

insufficient reduction of power dissipation at light load.

In such a case, connect R6 between the auxiliary winding

and the IS pin as shown in Fig.36. When R4 is 1k ohm, R6

is several hundreds of k ohm to 1Meg ohm. The smaller the

R6 is, the lower the switching frequency at light load.

However, negative voltage is applied to the IS pin due to

R6 for some time while MOSFET is ON. Be aware that the

negative voltage shall not be lower than absolute maximum

rationg,-0.3V.

In addition, when switching frequency is set too low at

light load, transformer or other apparatus may produce

noise.

(8) Prevention of malfunction caused by noise

This IC is an analog IC, and noise applied to anyone of

the pins may cause malfunction. If malfunction is detected,

use the IC through referring to the following description and

fully checking a power supply unit.

In addition, arrange the capacitors connected to pins as

close to the IC as possible and take great care of wiring, for

effective noise suppression.

(8-1) FB pin

The FB pin sets the threshold voltage of the current

comparator. Any noise applied to the FB pin may disturb

output pulses. Usually the capacitor C5 is connected as

shown in Fig.37 to suppress noise.

(8-2) IS pin

As described in “9.-(4) Blanking,” this IC has a blanking

function, and malfunction caused by a surge current

produced at turn-on of the MOSFET is hard to occur.

A malfunction, however, may occur when a surge current

is excessively large or when any noise is externally applied

at other than turn-off.

In such a case, add a CR filter to the IS pin as shown in

Fig.38.

(8-3) VCC pin

Relatively large noise may occur at the VCC pin because

a large current flows from the VCC pin at the instant of

driving the MOSFET. If noise is excessively large, a

malfunction may occur of the IC. Pay full attention to

capacitance and characteristics of the capacitor between

the VCC pin and GND to reduce noise as much as possible.

Fig.36

Input power improvement circuit at light load

Fig.37

Prevention of malfunction caused by noise

(FB pin)

Fig.38

Prevention of malfunction caused by noise

(IS pin)

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