VIPER17LN Ver la hoja de datos (PDF) - STMicroelectronics
Número de pieza
componentes Descripción
Fabricante
VIPER17LN Datasheet PDF : 33 Pages
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Operation descriptions
VIPER17
7.3
Power-up and soft-start up
If the input voltage rises up till the device start level (VDRAIN_START), the VDD voltage begins
to grow due to the IDD_ch current (see Table 7 on page 7) coming from the internal high
voltage start up circuit. If the VDD voltage reaches VDDon threshold (~14 V) the power
MOSFET starts switching and the HV current generator is turned OFF. See Figure 23 on
page 17.
The IC is powered by the energy stored in the capacitor on the VDD pin, CVDD, until when
the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode)
develops a voltage high enough to sustain the operation.
CVDD capacitor must be sized enough to avoid fast discharge and keep the needed voltage
value higher than VDDoff threshold. In fact, a too low capacitance value could terminate the
switching operation before the controller receives any energy from the auxiliary winding.
The following formula can be used for the VDD capacitor calculation:
Equation 1
CVDD=
--I--D----D---c---h----×-----t--S---S----a---u---x--
VDDon – VDDoff
The tSSaux is the time needed for the steady state of the auxiliary voltage. This time is
estimated by applicator according to the output stage configurations (transformer, output
capacitances, etc.).
During the converter start up time, the drain current limitation is progressively increased to
the maximum value. In this way the stress on the secondary diode is considerably reduced.
It also helps to prevent transformer saturation. The soft-start time lasts 8.5 ms and the
feature is implemented for every attempt of start up converter or after a fault.
Figure 22. Start up IDD current
IDD
2 mA
VDS = 120V
FSW = 0 kHz
AFTER FAULT
1 mA IDD0
IDD_FAULT
IDD_OFF
IDS_CH_FAULT
-1 mA
VDDrestart
-2 mA
-3 mA
IDS_CH
-4 mA
VDDoff
VDDon
VDD
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