Datasheet

Open-loop failure protection VIPER06
20/28 Doc ID 022794 Rev 1
15 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 R
H
is open or R
L
is shorted, the VIPER06 works at its drain current limitation. The output
voltage, V
OUT
, will increase as does the auxiliary voltage, V
AUX
, which is coupled with the
output through the secondary-to-auxiliary turns ratio.
As the auxiliary voltage increases up to the internal V
DD
active clamp, V
DDclamp
(the value is
given in Table 7 on page 8) and the clamp current injected on the VDD pin exceeds the latch
threshold, I
DDol
(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
I
DDol
through the VDD clamp) have to be verified to reveal the fault.
If R
L
is open or R
H
is shorted, the output voltage, V
OUT
, will be clamped to the reference
voltage V
REF_FB
(for non-isolated flyback) or to the external TL voltage reference (for
isolated flyback).
Figure 27. FB pin connection for non-isolated flyback
V
COMPL
D
AUX
nR
FB
VDD
V
AUX
COMP
+
-
to PWM
R
L
+
-
E/A
R
H
R
R
AUX
C
VDD
V
OUT
R
S
V
REF_FB
from R
SENSE
BUS
+
-
PWM stop
C
S
C
P