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VIPER20B Datasheet(PDF) 11 Page - STMicroelectronics

Part No. VIPER20B
Description  SMPS PRIMARY I.C.
Download  17 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER20B Datasheet(HTML) 11 Page - STMicroelectronics

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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 :
P
STBY =
1
2
L
P ISTBY
2
F
SW
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 :
I
STBY =
(t
b + td) VIN
L
P
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
VIPer20B / VIPer20BSP
11/17


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