VIPER26

Manufacturer Part NumberVIPER26
DescriptionVIPerPlus family: fixed frequency high performance off line high voltage converter
ManufacturerSTMicroelectronics
VIPER26 datasheet
 
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Oscillator
9
Oscillator
The switching frequency is internally fixed at 60 kHz (VIPER26LN or LD) or 115 kHz
(VIPER26HN or HD).
In both cases the switching frequency is modulated by approximately ±4 kHz (60 kHz
version) or ±8 kHz (115 kHz version) at 230 Hz (typical) rate, so that the resulting spread-
spectrum action distributes the energy of each harmonic of the switching frequency over a
number of sideband harmonics having the same energy on the whole but smaller
amplitudes.
10
Soft start-up
During the converters' start-up phase, the soft-start function progressively increases the
cycle-by-cycle drain current limit, up to the default value I
further limited and the output voltage is progressively increased reducing the stress on the
secondary diode. The soft-start time is internally fixed to t
page
7, and the function is activated for any attempt of converter start-up and after a fault
event.
This function helps prevent transformers' saturation during start-up and short-circuit.
11
Adjustable current limit set point
The VIPer26 includes a current mode PWM controller: cycle by cycle the drain current is
sensed through the integrated resistor R
inverting input of the PWM comparator, see
voltage is equal to the voltage derived from the COMP pin, the power MOSFET is switched
OFF.
In parallel with the PWM operations, the comparator OCP, see
the level of the drain current and switch OFF the power MOSFET in case the current is
higher than the threshold I
The level of the drain current limit, I
from the pin LIM. The resistor R
than the level of the current limit, I
When the LIM pin is left open or if the R
is fixed to its default value, I
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Dlim
and the voltage is applied to the non
SENSE
Figure 2 on page
Table 8 on page
7.
, see
Dlim
, can be reduced depending from the sunk current
Dlim
, between LIM and GND pins, fixes the current sunk and
LIM
Figure 13 on page
, see
Dlim
has an high value (i.e. > 80 kΩ) the current limit
LIM
Table 8 on page
, as reported on
Dlim
Doc ID 17736 Rev 2
VIPER26
. By this way the drain current is
Table 8 on
, see typical value on
SS
3. As soon as the sensed
Figure 2 on page
3, checks
9.
7.