OPERATION DESCRIPTION :

CURRENT MODE TOPOLOGY:

The current mode control method, like the one

integrated in the VIPer20B uses two control loops

- an inner current control loop and an outer loop

for voltage control. When the Power MOSFET

output transistor is on, the inductor current

(primary side of the transformer) is monitored with

a SenseFET technique and converted into a

error) the power switch is switched off. Thus, the

outer voltage control loop defines the level at

which the inner loop regulates peak current

through the power switch and the primary winding

of the transformer.

Excellent open loop D.C. and dynamic line

regulation is ensured due to the inherent input

voltage feedforward characteristic of the current

mode control. This results in an improved line

regulation,

instantaneous

correction

to

line

changes and better stability for the voltage

regulation loop.

Current mode topology also ensures good

limitation in the case of short circuit. During a first

phase

the

output

current increases

slowly

following the dynamic of the regulation loop. Then

it

reaches

the

maximum

limitation

current

internally set and finally stops because the power

applications the maximum peak current internally

set can be overridden by externally limiting the

voltage

excursion

on

the

COMP

pin.

An

integrated

blanking

filter

inhibits

the

PWM

comparator output for a short time after the

integrated Power MOSFET is switched on. This

function

prevents

anomalous

or

premature

termination of the switching pulse in the case of

current

spikes

caused

by

primary

side

capacitance or secondary side rectifier reverse

recovery time.

STAND-BY MODE

Stand-by operation in nearly open load condition

automatically leads to a burst mode operation

allowing voltage regulation on the secondary

side. The transition from normal operation to

given by :

P

1

2

L

2

F

SW

Where:

is

the

minimum

controllable current,

corresponding to the minimum on time that the

device is able to provide in normal operation. This

current can be computed as :

I

(t

L

P

propagation time of the internal current sense

and comparator, and represents roughly the

may be affected by the efficiency of the converter

at low load, and must include the power drawn on

the primary auxiliary voltage.

As soon as the power goes below this limit, the

auxiliary secondary voltage starts to increase

above the 13V regulation level forcing the output

voltage of the transconductance amplifier to low

the shutdown mode where the power switch is

maintained in the off state, resulting in missing

threshold is reached, the device operates again.

The above cycle repeats indefinitely, providing a

burst mode of which the effective duty cycle is

much lower than the minimum one when in

normal

operation.

The

equivalent

switching

frequency is also lower than the normal one,

leading to a reduced consumption on the input

mains lines. This mode of operation allows the

VIPer20B to meet the new German ”Blue Angel”

Norm with 1W total power consumption for the

system when working in stand-by. The output

voltage remains regulated around the normal

level, with a low frequency ripple corresponding

to the burst mode. The amplitude of this ripple is

low, because of the output capacitors and of the

low output current drawn in such conditions.The

normal operation resumes automatically when the

HIGH

VOLTAGE

START-UP

CURRENT

SOURCE

An

integrated

high

voltage

current

source

provides a bias current from the DRAIN pin

during the start-up phase. This current is partially

absorbed by internal control circuits which are

placed into a standby mode with reduced

VIPer20B / VIPer20BSP

11/17