VIPER26HD datasheet(18/25 Pages) STMICROELECTRONICS

18 / 25 page

Open loop failure protection

VIPER26

18/25

Doc ID 17736 Rev 2

Open loop failure protection

In case the power supply is built in fly-back topology and the VIPer26 is supplied by an

auxiliary winding, as shown in

Figure 27 on page 18 and Figure 28 on page 19, 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 on page 18 and

Figure 28 on page 19, respectively the non-isolated fly-back and the isolated fly-back.

If RH is opened or RL is shorted, the VIPer26 works at its drain current limitation. The output

voltage, VOUT, will increase and so 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

reported on

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

latch threshold, IDDol (the value is reported on Table 8 on page 7), 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 VDD clamp) have to be verified to reveal the fault.

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

voltage VREF_FB (in case of non isolated fly-back) or to the external TL voltage reference (in

case of isolated fly-back).

Figure 27.

FB pin connection for non-isolated fly-back

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