FA5590 데이터 시트보기 (PDF) - Fuji Electric

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FA5590

Power Supply Control IC(Power Factor Correction)

Fuji Electric 

FA5590N,FA5591N

10. Description of each circuit block

(1) Error amplifier

The error amplifier is to make the output voltage constant with

feedback control. For this IC, a transconductance type is used for the

error amplifier.

The non-inverting input terminal is connected to internal reference

voltage of 2.5V (typ.).

The inverting input terminal is fed with output voltage of the

power-factor correction converter, and normally use divided voltage

with resistors. To the inverting input, internal constant current source

of 1.8μA is connected for FB open detection function.

The output of the error amplifier (COMP) is connected to the PWM

comparator and controls the on time of the OUT output.

The output voltage of PFC contains much of ripple of frequency 2

times AC power line (50 or 60Hz). When this ripple component

becomes largely appears in the output of the error amplifier, the

power-factor correction converter does not stably operate. In order to

obtain the stable operation, connect capacitors and a resistor at Pin

No. 2 (COMP) as shown in Fig.5.

Vout

1.8uA

R1

FB

1

R2

C3

RAMP

OSC

ERRAMP

VREF(2.5V)

OUT

F/F

PWM

Comp

COMP

2

R3

C4

C5

Maximum

最o大nオtiンm幅e

Fig.5 Error amplifier circuit

(2) Soft start circuit

FA5590/91 is equipped with soft start function to suppress rushing

startup and overshoot of output voltage when starting.

The soft start circuit works after UVLO and standby is released

and before the soft start cancellation voltage is exceeded. In the

meantime, the soft start function restricts the startup speed of the

output voltage by limiting the maximum on time to about 80% when

the FB terminal voltage is lower than the reference voltage. (Fig. 6)

The on time limited by the soft start is cancelled when the FB terminal

voltage becomes higher than the soft start cancellation voltage.

(3) Overvoltage protection circuit (OVP)

This circuit is to limit the voltage when the output voltage of the

power-factor correction converter exceeds the set value.

When this IC starts up or load changes sharply, the output voltage

of the converter may exceed the set value. In such a case, this

protect circuit works to control the output voltage.

FA5590/91 has dynamic OVP function to narrow the on tme when

the FB terminal voltage becomes above 2.5V, and static OVP function

to stop the output when it becomes higher than 1.09 times the

reference voltage.

Normally the voltage of the FB terminal is 2.5V, approximately same

as the reference voltage of the error amplifier. When the output

voltage rises due to starting up or sharp load changes and the voltage

of the FB terminal becomes higher than 2.5V, the on time narrows by

the dynamic OVP function. When the voltage further rises and

exceeds the comparator reference voltage, output voltage of the

comparator(OVP) inverts to stop the OUT pulse.(Fig. 7)

When the output voltage turns below 1.05 times the reference

voltage, the OUT pulse resumes.

100%

80%

VF voltage

FB電圧

Vfb*0.95 Vfb

(2.5V)

Fig.6 maximum on time at soft start

Oオnンti幅me

100%

70%

Vfb

(2.5V)

VF voltage

Vfb*1.05 Vfb*1.09

FB電圧

Fig.7 on time at overvoltage

Fuji Electric Co., Ltd.

AN-016E Rev.1.2

April-2011

13

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

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