Datasheet

NCP1351
http://onsemi.com
13
Over power protection can be done without power
dissipation penalty by arranging components around the
auxiliary as suggested by Figure 14. On this schematic, the
diode anode swings negative during the on time. This
negative level directly depends on the input voltage and
offsets the current sense pin via the R
OPP
resistor. A small
integration is necessary to reduce the O
PP
action in light load
conditions. However, depending on the compensation level,
the standby power can be affected. Again, the resistor R
OPP
should be placed as close as possible to the CS pin. The
22 pF can help to circumvent any picked-up noise and D
2
prevents the positive loading of the 270 pF capacitor during
the flyback swing. We have put a typical 100 k O
PP
resistor but a tweak is required depending on your
application.
Figure 14. The OPP is Relatively Easy to Implement and It Does not Waste Power
C
Bulk
+
C3
270p
R1
150k
R
sense
DRV
I
Lp
V
CC
L
P
DRV
CS
R
offset
D2
1N4148
C4
22p
+
R
OPP
100k
CV
CC
L
aux
D
aux
Suppose you would need to reduce the peak current by
15% in high-line conditions. The turn-ratio between the
auxiliary winding and the primary winding is N
aux
. Assume
its value is 0.15. Thus, the voltage on D
aux
cathode swings
negative during the on time to a level of:
V
aux_peak
+ -V
in_max
N
aux
+ -375 0.15 + -56V
(eq. 8)
If we selected a 3.7 k resistor for R
offset
, then the
maximum sense voltage being developed is:
V
sense
+ 3.7k 270 + 1V
(eq. 9)
The small RC network made of R
1
and C
3
, purposely limits
the voltage excursion on D
2
anode. Assume the primary
inductance value gives an on time of 3 s at high-line. The
voltage across C
3
thus swings down to:
V
C
3
+
t
on
V
aux_peak
R
1
C
3
+ -
3 56
150k 270p
+ -4.2V
(eq.
10)
Typically, we measured around –4 V on our 50 W prototype.
By calculation, we want to decrease the peak current by
15%. Compared to the internal 270 A source, we need to
derive:
I
offset
+ -0.15 270 + -40.5A
(eq. 11)
Thus, from the –4 V excursion, the R
OPP
resistor is
derived by:
R
OPP
+
4
40.5
+ 98k
(eq. 12)
After experimental measurements, the resistor was
normalized down to 100 k.
Feedback
Unlike other controllers, the feedback in the NCP1351
works in current rather than voltage. Figure 15 details the
internal circuitry of this particular section. The optocoupler
injects a current into the FB pin in relationship with the
input/output conditions.