VIPER37HE datasheet(20/35 Pages) STMICROELECTRONICS

20 / 35 page

Operation description

VIPER37

20/35

Doc ID 022218 Rev 1

8.3

Power-up and soft-start

If the input voltage rises up to the device start threshold VDRAIN_START, the VDD voltage

begins to grow due to the IDDch current (see Table 7) coming from the internal high voltage

startup circuit. If the VDD voltage reaches the VDDon threshold (see Table 7), the Power

MOSFET starts switching and the HV current generator is turned off (see

Figure 25).

The IC is powered by the energy stored in the capacitor on the VDD pin, CVDD, until the self-

supply circuit (typically an auxiliary winding of the transformer and a steering diode)

develops a voltage high enough to sustain the operation.

The CVDD capacitor must be sized correctly in order to avoid fast discharge and keep the

needed voltage value higher than the 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

The tSSaux is the time needed for the steady-state of the auxiliary voltage. This time is

estimated by the applicator according to the output stage configurations (transformer, output

capacitances, etc.).

During the converter startup 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 the startup converter or after a fault.

VDD

DDch

SSaux

DDon

DDoff

–

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