Open loop failure protection
VIPER16
15
Open loop failure protection
In case the power supply is built in fly-back topology and the VIPer16 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 R is opened or R is shorted, the VIPer16 works at its drain current limitation. The output
H
L
voltage, V
, will increase and so the auxiliary voltage, V
, which is coupled with the
OUT
AUX
output through the secondary-to-auxiliary turns ratio.
As the auxiliary voltage increases up to the internal V active clamp, V
(the value is
DDclamp
DD
reported on Table 8 on page 7) and the clamp current injected on VDD pin exceeds the latch
threshold, I
generated.
(the value is reported on Table 8 on page 7), a fault signal is internally
DDol
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
through VDD clamp) have to be verified to reveal the fault.
DDol
If R is opened or R is shorted, the output voltage, V
, will be clamped to the reference
L
H
OUT
voltage V
(in case of non isolated fly-back) or to the external TL voltage reference (in
REF_FB
case of isolated fly-back).
Figure 27. FB pin connection for non-isolated fly-back
DAUX
RAUX
VAUX
CVDD
VDD
VOUT
VCOMPL
PWM stop
+
-
RH
BUS
FB
-
from RSENSE
E/A
+
VREF_FB
RL
+
-
to PWM
nR
R
COMP
RS
CS
CP
18/25
Doc ID 15232 Rev 5
STMICROELECTRONICS [ ST ]
