```AND9057/D
Temperature Protection Trip
Point in NCP1250-Based
http://onsemi.com
APPLICATION NOTE
resistor on the OPP pin in relationship with this NTC value.
This technique can lead to rather high pull−down resistors
on the OPP pin to ground. If this resistance is too high and
the converter noisy, the converter immunity can be reduced
with possible erratic operations. The solution to fix this kind
of problem is to reduce the pull−down resistance. Values
below 3 kW give good operating results. There are cases
where the noise in the adapter will require low values such
as 1 kW. To still meet the OTP trip point, you will need to
install a resistor in parallel with the existing NTC device.
The solution appears in Figure 1.
The NCP1250 features a multi−function pin in which the
user can implement Over Power Protection (OPP) and Over
Voltage Protection (OVP). If you add a Negative
Temperature Coefficient (NTC) resistor in parallel with the
OVP Zener diode, you have a means to protect the adapter
against thermal runaway. The operating principle is simple:
the latch detection is made by observing the OPP pin level
via a comparator featuring a 3 V reference voltage. When
the NTC resistance decreases as the temperature increases,
the off−time voltage on the OPP pin goes up. When it
touches the 3 V reference four consecutive times, the
controller permanently latches off.
Customers often re−use an NTC they have already
implemented in previous projects and define the pull−down
Aux
D5
1N4148
Aux
D5
1N4148
R2
X1
NTC
R2
ZD1
1N965
R3
X1
NTC
ZD1
1N965
OPP
C10
22p
OPP
C10
22p
R1
R1
Figure 1. This Picture Shows How to Adapt an Existing NTC Value to Adjust the OTP Trip Point
As a design example, let us assume that we have the
following component values around the NCP1250:
− R1 = 1 kW; the NCP1250 pull−down resistor on the
OPP pin.
− RNTC = 15 kW; the NTC value at the select OTP trip
point.
− Npa = 0.2; the turns ratio between the primary and
auxiliary windings.
© Semiconductor Components Industries, LLC, 2011
November, 2011 − Rev. 0
− Nps = 0.09; the turns ratio between the primary and
secondary windings.
− Vout = 5 V ; the converter output voltage.
− Vf = 0.6 V; the diodes forward drops in the auxiliary
and secondary windings.
At first, we need to know the voltage across the NTC in
fault mode, e.g. when the OPP pin level reaches 3 V. This
voltage is defined by the plateau voltage on the auxiliary
1
Publication Order Number:
AND9057/D
AND9057/D
winding during the off time. It also depends on the 0.6 V
forward drop of diode D1 and the 3 V OVP level:
N pa
V NTC + ǒV out ) V f Ǔ
The next step is to recalculate the OPP resistor (R2 in
Figure 1). For instance, if we need a 300 mV decrease from
the 0.8 V setpoint at high line, then the auxiliary diode anode
during the on−time swings to:
* V OVP * V f
N ps
(eq. 1)
V ANODE + −N pa
0.2
* 3 * 0.6 + 8.8 V
0.09
+ (5 ) 0.6)
I R1 +
3
+
+ 3 mA
1000
R1
V R2 + 74.7 * 0.3 + 74.4 V
R eq +
I R1
+
8.8
+ 2.9 kW
3m
(eq. 2)
I R1 +
R 3R NTC
R2 +
(eq. 3)
R eqR NTC
R NTC * R eq
+
2.9k
15k
15k * 2.9k
300m
1k
+ 300 mA
(eq. 8)
74.4
+ 248 kW
300m
(eq. 9)
This application note explains how we can easily adjust
the Over Temperature Protection trip point by paralleling a
resistor with an available NTC device. In case of noisy
layouts, it helps to decrease the OPP pin pull−down resistor
to a value close or below one kW, naturally improving the
converter robustness to external perturbations.
R3 +
(eq. 7)
Conclusion
(eq. 4)
R NTC ) R 3
(eq. 6)
The R2 value is therefore easily derived:
Knowing that the equivalent resistor is the NTC paralleled
with the added resistance (R3), we have:
R eq +
Ǹ2Ǔ + −74.7 V
The current flowing in the pull−down resistor R1 in this
condition will be:
From Equations 1 and 2, we can derive the equivalent
resistance made of the NTC in parallel with the resistor we
are looking for (R3):
V NTC
ǒ264
We can evaluate the voltage drop around R2 as:
Based on 1 kW pull−down OPP resistor, the current in fault
mode (VOVP = 3 V) is:
V OVP
V IN + −0.2
+ 3.7 kW (eq. 5)
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