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VIPER25HD Datasheet(PDF) 23 Page - STMicroelectronics

Part No. VIPER25HD
Description  Off-line high voltage converters
Download  40 Pages
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER25HD Datasheet(HTML) 23 Page - STMicroelectronics

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VIPER25
Operation description
Doc ID 15585 Rev 4
23/40
7.8
Double blanking time
The blanking time, TBLANK, can have two different values: the lower one is 2,5 μs (typical
value) and the higher one is 6,3
μs (typical value). The value is linked to the voltage V
ZCD,
sampled during the time TSTROBE defined as for the over voltage protection (see the
relevant Section 7.11 on page 26). The time TBLANK has the lower value if is detected VZCD
< 1V or it has the higher value if is detected VZCD > 1V, refer to Table 8 on page 8 and
Figure 30 on page 23
.
The higher value of the blanking time is normally activated during the start-up phase or in
case of output short circuit; when the output voltage of the converter is quite lower than the
regulated value. In this condition can happens that during the demagnetization of the
transformer, the VZCD is very close to the arming and triggering thresholds (VZCDAth and
VZCDTth) and the ZCD circuit can be erroneously trigged, leading the system to work at
higher frequency and in continuous mode. This false trigger is inhibited by the selection of
the higher value of TBLANK when VZCD is lower than 1 V.
During the normal operation, in steady state condition, the voltage VZCD during the
demagnetization is higher than 1V and the selected TBLANK value is the lower one. The
Figure 30
shows the typical waveforms during the power up and the linked TBLANK selection.
Figure 30.
Double TBLANK timing diagram
Vaux
1V
t
0
ZCD
(pin 3)
1. 5
0. 5
T
BL ANK
C
t
t
t
t
t
t
D
A
T
ST RO B E
6.3μs
2.5μs
F
Delay
Mosfet switched on by the starter
Quasi Resonant Operation
lim
OSC
F
4
t
0.8V
0. 6V


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