ncp1250 ON Semiconductor, ncp1250 Datasheet - Page 15

• Manufacturer Part Number: ncp1250
• Description: Ncp1250 Current-mode Pwm Controller For Off-line Power Supplies
• Manufacturer: ON Semiconductor

characteristics:
V
V
N
N
Given the turns ratio between the primary and the auxiliary
windings, the on−time voltage at high line (265 Vac) on the
auxiliary winding swings down to:
install a divider featuring the following ratio:
protect the pin against ESD pulses. These diodes accept
some peak current in the avalanche mode and are designed
to sustain a certain amount of energy. On the other side,
negative injection into these diodes (or forward bias) can
cause substrate injection which can lead to an erratic circuit
behavior. To avoid this problem, the pin is internally
Figure 41. The OPP Circuitry Affects the Maximum Peak Current Set Point by Summing a Negative Voltage to the
out
in
1
2
V
Let us assume that we have the following converter
To obtain a level as imposed by Equation 6, we need to
The OPP pin is surrounded by Zener diodes stacked to
aux
= N
= N
= 85 to 265 V
= 19 V
+ −N
p
p
:N
:N
s
aux
= 1:0.25
2
V
= 1:0.18
in,max
rms
+ −0.18
Figure 42. The Peak Current Regularly Reduces Down to 20% at 375 Vdc
100%
Io p p
R oppL
80%
Peak current
setpoint
This p oin t will
be adjusted to
reduce the ref
at hi line to the
desired level.
375 + −67.5 V
RoppU
OPP
Internal Voltage Reference
VDD
(eq. 7)
http://onsemi.com
VCC
$5%
0.8 V
aux
K2
(V O P P is negativ e)
K1
ref = 0.8 V + VOPP
15
SUM2
then the upper resistor can be obtained by:
implementing the recommended resistor values, we obtain
the following curve (Figure 42):
clamped slightly below –300 mV which means that if more
current is injected before reaching the ESD forward drop,
then the maximum peak reduction is kept to 40%. If the
voltage finally forward biases the internal zener diode, then
care must be taken to avoid injecting a current beyond
–2 mA. Given the value of R
present example.
ref
If we arbitrarily fix the pull−down resistor R
If we now plot the peak current set point obtained by
from FB
375
R
−N V in during ton
swings to:
Vout during toff
OPPU
−
+
CS
Div +
+
reset
67.5 * 0.16
0.16 1k
0.16
67.5
V
bulk
OPPU
[ 2.4m
, there is no risk in the
[ 421 kW
OPPL
to 1 kW,
(eq. 8)
(eq. 9)