viper20sp STMicroelectronics, viper20sp Datasheet - Page 12

• Manufacturer Part Number: viper20sp
• Description: Smps Primary I.c.
• Manufacturer: STMicroelectronics

OPERATION DESCRIPTION:
CURRENT MODE TOPOLOGY
The current mode control method, like the one
integrated in the VIPer20/20A 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 V
current. When V
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 D.C. open loop 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,
changes and better stability for the voltage
regulation loop.
Current mode topology also ensures good
limitation in the case of short circuit. During the
first phase the output current increases slowly
following the dynamic of the regulation loop. Then
it reaches
internally set and finally stops because the power
supply on V
applications the maximum peak current internally
set can be overridden by limiting the voltage
excursion externally 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
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 P
by:
Where:
L
P
P
STBY
is the primary inductance of the transformer.
=
prevents
1
-- - L
2
DD
instantaneous
the maximum limitation current
P
is no longer correct. For specific
I
S
2
STBY F SW
reaches V
anomalous
S
COMP
correction
proportional to this
or
(the amplified
STBY
premature
to
given
line
F
I
corresponding to the minimum on time that the
device is able to provide in normal operation. This
current can be computed as:
t
propagation time of the internal current sense and
comparator, and roughly represents the minimum
on time of the device. Note that P
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 (V
the shutdown mode where the power switch is
maintained in the off state, resulting in missing
cycles and zero duty cycle. As soon as V
back to the regulation level and the V
threshold is reached, the device operates again.
The above cycle repeats itself indefinitely,
providing a burst mode of which the effective duty
cycle is much lower than the minimum one when in
normal
frequency is also lower than the normal one,
leading to a reduced consumption on the input
mains lines. This mode of operation allows the
VIPer20/20A 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 because of the low output
current drawn in such conditions. The normal
operation resumes automatically when the power
gets back levels which are higher than P
HIGH
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 consumption and are
also provided to the external capacitor connected
to the V
reaches the high voltage threshold V
I STBY
STBY
b
SW
+ t
is the normal switching frequency.
d
VIPer20/SP/DIP - VIPer20A/ASP/ADIP
is the sum of the blanking time and of the
is the minimum controllable current,
DD
COMP
=
operation. The equivalent switching
VOLTAGE
--------------------------------
pin. As soon as the voltage on this pin
t
b
< V
+
L
t
d
P
COMPth
V
IN
START-UP
). This situation leads to
STBY
DDon
CURRENT
STBY
may be
DD
COMPth
of the
12/25
.
gets