VIPER25 STMicroelectronics, VIPER25 Datasheet - Page 24

• Manufacturer Part Number: VIPER25
• Description: VIPerPlus family: quasi-resonant high performance off line high voltage converter
• Manufacturer: STMicroelectronics

Operation description
7.9
7.10
24/40
Starter
If the amplitude of the voltage on ZCD pin at the end of one oscillator cycle is smaller than
the V
system would stop.
This is what normally happens during converter’s power-up or under overload/short circuit
conditions.
During the converter’s startup phase, the voltage on ZCD pin is not high enough to arm the
triggering circuit. Thus, the converter operates at a fixed frequency, F
on page 8
arm the ZCD circuit, MOSFET's turn-ON is locked to transformer demagnetization, hence
setting up quasi-resonant operation.
As protection, in case the ZCD voltage is permanently above the threshold V
switching frequency is reduced to the minimum value, F
page 8
Current limit set point and feed-forward option
The VIPER25 is a current mode converter and the drain current is limited cycle by cycle
according to the FB pin voltage value that is related with the feedback loop response and
the load. When the drain current, sensed by the integrated Sense-FET, reaches the current
limitation, after the internal propagation delay, the MOSFET is switched OFF. The current
limitation cannot exceed a certain value, I
sunk from the ZCD pin during MOSFET’s ON-time.
Usually a resistor, R
and then the peak drain current set-point: the lower the resistor is, the lower I
For a QR fly-back converter the power capability strongly depends on the input voltage. In
wide-range applications at maximum line the power capability can be more than twice the
value at minimum line, as shown by the upper curve in the diagram of
To reduce this dependence, the current limit I
increment of the input voltage, implementing the so called line feed-forward. It’s realized with
a resistor, R
on page 26
proportional to the input voltage through the auxiliary-to-primary turns ratio N
current proportional to the input voltage is sunk from the ZCD pin, thus lowering the over
current set point.
ZCDAth
.
). As the voltage developed across the auxiliary winding becomes high enough to
. Since the voltage across the auxiliary winding during MOSFET’s on-time is
FF
arming threshold, in which case MOSFET's turn-ON could not be triggered, the
, connected between the ZCD pin and the auxiliary winding, see the
LIM
, connected from ZCD pin to ground is used to fix this sunk current
Doc ID 15585 Rev 4
Dlim
, that can be adjusted acting on the current
Dlim
has to be reduced according to the
OSCmin
, reported on
STARTER
Figure 31 on page 25
Table 8 on
ZCDAth
Dlim
AUX
(see
will be.
/N
Figure 32
VIPER25
Table 8
, the
P
, a
.