NCP1251
www.onsemi.com
21
First, calculate the OPP network with the above equations.
Then, suppose we want to latch off our controller when V
out
exceeds 25 V. On the auxiliary winding, the plateau reflects
the output voltage by the turns ratio between the power and
the auxiliary winding. In case of voltage runaway for our
19 V adapter, the plateau will go up to:
V
aux,OVP
+ 25
0.18
0.25
+ 18 V
(eq. 15)
Since our OVP comparator trips at a 3 V level, across the
1 kW selected OPP pulldown resistor, it implies a 3 mA
current. From 3 V to go up to 18 V, we need an additional
15 V. Under 3 mA and neglecting the series diode forward
drop, it requires a series resistor of:
R
OVP
+
V
latch
* V
VOP
V
OVP
ńR
OPPL
+
18 * 3
3ń1k
+
15
3m
+ 5kW
(eq. 16)
In nominal conditions, the plateau establishes to around
14 V. Given the divide−by−6 ratio, the OPP pin will swing
to 14/6 = 2.3 V during normal conditions, leaving 700 mV
margin. A 100 pF capacitor can be added between the OPP
pin and GND to improve noise immunity and avoid erratic
trips in presence of external surges. Do not increase this
capacitor too much otherwise the OPP signal will be affected
by the integrating time constant.
A second solution for the OVP detection alone, is to use
a Zener diode wired as recommended by.
D3
15V
4
5
1
OPP
Vlatch
10
8
9
VCC
aux.
winding
OPP
ROPPL
1k
ROPPU
421k
11
D2
1N4148
C1
22pF
OVP
Figure 49. A Zener Diode in Series with a Diode Helps to Improve the Noise Immunity of the System
For this configuration to maintain an 18 V level, we have
selected a 15 V Zener diode. In nominal conditions, the
voltage on the OPP pin is almost 0 V during the off time as
the Zener is fully blocked. This technique clearly improves
the noise immunity of the system compared to that obtained
from a resistive string as in Figure 48. Please note the
reduction of the capacitor on the OPP pin to 10 pF − 22 pF.
This capacitor is necessary because of the potential spike
coupling through the Zener parasitic capacitance from the
bias winding due to the leakage inductance. Despite the 1 ms
blanking delay at turn off. This spike is energetic enough to
charge the added capacitor C
1
and given the time constant,
could make it discharge slower, potentially disturbing the
blanking circuit. When implementing the Zener option, it is
important to carefully observe the OPP pin voltage (short
probe connections!) and check that enough margin exists to
that respect.
Over Temperature Protection
In a lot of designs, the adapter must be protected against
thermal runaways, e.g. when the temperature inside the
adapter box increases above a certain value. Figure 50
shows how to implement a simple OTP using an external
NTC and a series diode. The principle remains the same:
make sure the OPP network is not affected by the additional
NTC hence the presence of this isolation diode. When the
NTC resistance decreases as the temperature increases, the
voltage on the OPP pin during the off time will slowly
increase and, once it passes 3 V for 4 consecutive clock
cycles, the controller will permanently latch off.
