OPERATION DESCRIPTION :

This device is intended to be used in off line

AC/DC

adapter

where

the

desired

output

characteristic

must

present

a

rectangular

characteristic. For output voltage values lower

than a fixed value, the average output current

must be constant, whatever are the input or

output voltages. If the output current consumed

by the load is lower than the previous constant

current value, the output voltage value must be

limited. In addition, the device provides protection

against output short circuits and overtemperature

events.

The two modes of operation are described in the

following paragraphs. Figure 10 presents a typical

application of which the output characteristic can

be seen on figure 11.

CONSTANT OUTPUT CURRENT

The power topology to be used with this device is

a simple discontinuous flyback, as shown on

figure 10. The average output current of such a

topology cannot be easily kept constant, as it

depends on the output voltage. Actually, if the

peak primary current is fixed, the converter

behaves as a constant power generator.

Therefore, a modulation of the peak primary

current versus output voltage must be done in

order

to

get

the

constant

output

current

characteristic. A conventional way consists to use

an optocoupler between primary and secondary,

with

additional circuitry

on

secondary side

(Reference, error amplifier and current sense

resistor).

This device avoids the use of all the secondary

circuitry by controlling from primary side the

secondary average output current. Figure 12

presents the internal constitution of the current

control function. It is built around a constant

current source Iref, and a mosfet switch driven

with the complemented signal EOD, in series with

a resistance R. The middle point of these

elements is available on the CREF pin.

The

EOD

signal

is

generated

by

the

demagnetisation function, which is monitoring the

voltage of the main transformer auxiliary winding.

Figure 12: Constant Current Operation

R

R2

RS

C

R1

D2

D1

C1

IOUT

GND

C2

Iref

VIPer31

Oscillator

Demag.

R

Q

PWM

Latch

-

+

Ic

Ip

Is

n

T

EOD

Uc

VCC

DSENSE

DRAIN

SOURCE

CS ENSE

CREF

GND

+Vin

Tsw

Ip

Is

EOD

Ic

tonsec

t

t

t

t

VCREF

RS

VCREF

RS

n.

1

0

Iref

Iref -

Uc

R

SC12140

VIPer31SP

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