NCP1251_11 Datasheet, PDF (23/24 Pages) ON Semiconductor – Current-Mode PWM Controller for Off-line Power Supplies

D3
15V
NT C
11
ROPPU
841k
OPP
10
4
ROPPL
2.5k
5
Vlatch
NCP1251
D2
1N4148
1
OVP
OPP
VCC
9
8
au x.
winding
Figure 50. With the NTC Back in Place, the Circuit Nicely Combines OVP, OTP and OPP on the Same Pin
In nominal V_CC / output conditions, when the Zener is not
activated, the NTC can drive the OPP pin and trigger the
adapter in case of an over temperature. During nominal
temperature if the loop is broken, the voltage runaway will
be detected and the controller will shut down the converter.
In case the OPP pin is not used for either OPP or OVP, it
can simply be grounded.
Filtering the Spikes
The auxiliary winding is the seat of spikes that can couple
to the OPP pin via the parasitic capacitances exhibited by the
Zener diode and the series diode. To prevent an adverse
triggering of the Over Voltage Protection circuitry, it is
possible to install a small RC filter before the detection
D3
15V
network. Typical values are those given in Figure 51 and
must be selected to provide the adequate filtering function
without degrading the stand−by power by an excessive
current circulation.
Latched OVP on V_CC
The V_CC pin is permanently monitored by a comparator.
When the V_CC exceeds 25.5 V (typical), all pulses are
immediately stopped and the V_CC falls to the SCR
latched-level around 7 V typical. The controller remains in
this state as long as a sufficient current flows in the SCR, at
least 30 mA. We recommend to put a design margin there,
with a minimum current around 60 mA at the lowest input
line.
ad d ition al fil ter
NT C
11
ROPPU
841k
D2
1N4148
2
C1
330pF
R3
220
OP P
10
4
ROPPL
2.5k
5
Vlatch
1
OVP
OPP
VCC
9
3
aux.
winding
Figure 51. A Small RC Filter Avoids the Fast Rising Spikes from Reaching the Protection Pin of the NCP1251 in
Presence of Energetic Perturbations Superimposed on the Input Line
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