VIPER100B datasheet(12/20 Pages) STMICROELECTRONICS

12 / 20 page

OPERATION DESCRIPTION :

CURRENT MODE TOPOLOGY:

The current mode control method, like the one

integrated in the VIPer100B/BSPuses 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 voltage VS proportional to this

current. When VS reaches VCOMP (the amplified

output voltage 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

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

reaches

the

maximum

limitation

current

internally set and finally stops because the power

supply on VDD is no longer correct. For specific

applications the maximum peak current internally

set can be overridden by externally limiting the

voltage

excursion

the

COMP

pin.

integrated

blanking

filter

inhibits

the

PWM

comparator output for a short time after the

integrated Power MOSFET is switched on. This

function

prevents

anomalous

premature

termination of the switching pulse in the case of

current

spikes

caused

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

burst mode operation happens for a power PSTBY

given by :

STBY =

P ISTBY

Where:

LP is the primary inductance of the transformer.

FSW is the normal switching frequency.

ISTBY

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 :

STBY =

b + td) VIN

tb +td is the sum of the blanking time and of the

propagation time of the internal current sense

and comparator, and represents roughly the

minimum on time of the device. Note that PSTBY

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

state (VCOMP <VCOMPth). This situation leads to

the shutdown mode where the power switch is

maintained in the off state, resulting in missing

cycles and zero duty cycle. As soon as VDD gets

back to the regulation level and the VCOMPth

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

VIPer100B/BSP to meet the new German ”Blue

Angel” Norm with less than 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 power get back

to higher levels than PSTBY.

HIGH

VOLTAGE

START-UP

CURRENT

SOURCE

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

VIPER100B/BSP

12/20

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