VIPER20B 查看數據表（PDF） - STMicroelectronics

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VIPER20B

SMPS PRIMARY I.C.

STMicroelectronics 

VIPer20B / VIPer20BSP

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

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 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

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 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

burst mode operation happens for a power PSTBY

given by :

PSTBY

=

1

2

LP

IS2 TBY

FSW

Where:

LP is the primary inductance of the transformer.

FSW is the normal switching frequency.

ISTBY 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 :

IST BY

=

(tb

+ td)

LP

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

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

power get back to higher levels than PSTBY.

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

11/17

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