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VIPER25HD Datasheet(PDF) 34 Page - STMicroelectronics

Part No. VIPER25HD
Description  Off-line high voltage converters
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER25HD Datasheet(HTML) 34 Page - STMicroelectronics

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Operation description
VIPER25
34/40
Doc ID 15585 Rev 4
enable the brown-out function the BR pin voltage has to be higher than the maximum of
VDIS threshold (150 mV, see Table 8).
7.16
2nd level over current protection and hiccup mode
The VIPER25 is protected against short circuit of the secondary rectifier, short circuit on the
secondary winding or a hard-saturation of fly-back transformer. Such as anomalous
condition is invoked when the drain current exceed the threshold IDMAX, see Table 8 on
page 8
.
To distinguish a real malfunction from a disturbance (e.g. induced during ESD tests) a
“warning state” is entered after the first signal trip. If in the subsequent switching cycle the
signal is not tripped, a temporary disturbance is assumed and the protection logic will be
reset in its idle state; otherwise if the IDMAX threshold is exceeded for two consecutive
switching cycles a real malfunction is assumed and the power MOSFET is turned OFF.
The shutdown condition is latched as long as the device is supplied. While it is disabled, no
energy is transferred from the auxiliary winding; hence the voltage on the VDD capacitor
decays till the VDD under voltage threshold (VDDoff), which clears the latch.
The start up HV current generator is still off, until VDD voltage goes below its restart voltage,
VDD(RESTART). After this condition the VDD capacitor is charged again by 600 µA current,
and the converter switching restarts if the VDDon occurs. If the fault condition is not removed
the device enters in auto-restart mode. This behavioral results in a low-frequency
intermittent operation (Hiccup-mode operation), with very low stress on the power circuit.
See the timing diagram of Figure 39.
Figure 39.
Hiccup-mode OCP: timing diagram
Vcc
VDRAIN
IDRAIN
Secondary diode is shorted here
t
t
t
DMAX
on
off
(RESTART)
Secondary diode is shorted here
t
t
t
I
VDD
VDD
VDD
VDD


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