VIPER17 STMicroelectronics, VIPER17 Datasheet - Page 18

• Manufacturer Part Number: VIPER17
• Description: VIPerPlus family: fixed frequancy high performance off line high voltage converter
• Manufacturer: STMicroelectronics

Operation descriptions
7.6
7.7
7.8
18/31
Oscillator
The switching frequency is internally fixed to 60 kHz or 115 kHz. In both case the switching
frequency is modulated by approximately ±4 kHz (60 kHz version) or ±8 kHz
(115 kHz version) at 250 Hz (typical) rate, so that the resulting spread-spectrum action
distributes the energy of each harmonic of the switching frequency over a number of side-
band harmonics having the same energy on the whole but smaller amplitudes.
Current mode conversion with adjustable current limit set
point
The device is a current mode converter: the drain current is sensed and converted in voltage
that is applied to the non inverting pin of the PWM comparator. This voltage is compared
with the one on the feed-back pin through a voltage divider on cycle by cycle basis.
The VIPER17 has a default current limit value, I
the electrical specification, by the R
page
The CONT pin has a minimum current sunk needed to activate the I
R
Table 8 on page
Overvoltage protection (OVP)
The VIPER17 has integrated the logic for the monitor of the output voltage using as input
signal the voltage V
the voltage from the auxiliary winding tracks the output voltage, through the turn ratio
The CONT pin has to be connected to the auxiliary winding through the diode D
resistors R
the voltage V
on page
the auto-restart mode.
In order to bypass the noise immediately after the turn off of the power MOSFET, the voltage
V
the
digital signal and increments the internal counter. The same counter is reset every time the
signal OVP is not triggered in one oscillator cycle.
Referring to the
Equation 2
N
--------------
N
CONT
LIM
AUX
SEC
Figure 26 on page
or with high R
11.
is sampled inside a short window after the time T
7) the overvoltage protection will stop the power MOSFET and the converter enters
OVP
CONT
and R
Figure
7).
exceeds, four consecutive times, the reference voltage V
LIM
CONT
LIM
(i.e. 100 KΩ) the current limit is fixed to the default value (see I
19. The sampled signal, if higher than V
21, the resistors divider ratio k
as shows the
k
during the OFF time of the power MOSFET. This is the time when
OVP
=
Doc ID 14419 Rev 7
-------------------------------------------------------------------------------------------------- -
N
--------------
N
AUX
SEC
LIM
⋅
Figure 27 on page 20
resistor connected to the CONT see
(
V
OUTOVP
V
DLIM
OVP
+
V
, that the designer can adjust according
DSEC
OVP
STROBE
) V
will be given by:
–
When, during the OFF time,
DAUX
OVP
, see
, trigger the internal OVP
DLIM
Table 8 on page 7
adjustment: without
OVP
Figure 16 on
(see
OVP
VIPER17
and the
Table 8
DLIM
and
,