VIPER06HSTR datasheet(20/28 Pages) STMICROELECTRONICS

20 / 28 page

Open-loop failure protection

VIPER06

20/28

Doc ID 022794 Rev 1

Open-loop failure protection

If the power supply has been designed using flyback topology and the VIPER06 is supplied

by an auxiliary winding, as shown in Figure 27 and Figure 28 on page 21, the converter is

protected against feedback loop failure or accidental disconnections of the winding.

The following description is applicable for the schematics of Figure 27 and Figure 28 on

page 21, respectively the non-isolated flyback and the isolated flyback.

If RH is open or RL is shorted, the VIPER06 works at its drain current limitation. The output

voltage, VOUT, will increase as does the auxiliary voltage, VAUX, which is coupled with the

output through the secondary-to-auxiliary turns ratio.

As the auxiliary voltage increases up to the internal VDD active clamp, VDDclamp (the value is

given in Table 7 on page 8) and the clamp current injected on the VDD pin exceeds the latch

threshold, IDDol (the value is given in Table 7 on page 8), a fault signal is internally

generated.

In order to distinguish an actual malfunction from a bad auxiliary winding design, both the

above conditions (drain current equal to the drain current limitation and current higher than

IDDol through the VDD clamp) have to be verified to reveal the fault.

If RL is open or RH is shorted, the output voltage, VOUT, will be clamped to the reference

voltage VREF_FB (for non-isolated flyback) or to the external TL voltage reference (for

isolated flyback).

Figure 27.

FB pin connection for non-isolated flyback

COMPL

AUX

VDD

AUX

COMP

to PWM

E/A

AUX

VDD

OUT

REF_FB

from R

SENSE

BUS

PWM stop

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