ncp1250 ON Semiconductor, ncp1250 Datasheet - Page 13

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

capacitor which will supply the controller when it operates
until the auxiliary winding takes over. Experience shows
that this time t
consider we need at least an energy reservoir for a t
10 ms, the V
the laboratory will let us know if we were too optimistic for
the time t
evaluate the charging current we need to bring the V
voltage from 0 to the VCC
current has to be selected to ensure a start−up at the lowest
mains (85 V rms) to be less than 3 s (2.5 s for design margin):
controller, then the total charging current delivered by the
start−up resistor must be 49 mA. If we connect the start−up
network to the mains (half−wave connection then), we know
that the average current flowing into this start−up resistor
will be the smallest when V
controller:
then the minimum value for R
R
CV
I
start*up
The first step starts with the calculation of the V
Let us select a 4.7 mF capacitor at first and experiments in
If we account for the 15 mA that will flow inside the
To make sure this current is always greater than 49 mA,
Figure 39. The Startup Resistor Can Be Connected to the Input Mains for Further Power Dissipation Reduction
charge
CC
w
w
v
1
. The V
VCC
VCC
V
CC
ac,rms
I
CVCC,min
1
on
2.5
p
capacitor must be larger than:
on
I
I
CVCC,min
can be between 5 ms and 20 ms. If we
CC
C
* VCC
CC
input
mains
2
VCC
t
* VCC
1
capacitor being known, we can now
+
w
min
D4
1N4007
V
1N4007
18
on
ac,rms
on
CC
w
p
start−up
D2
of the IC, 18 V typical. This
2.5
v
R
3m
reaches the VCC
4.7m
start*up
2
85 1.414
12
* VCC
p
9
can be extracted:
w 34 mA
49m
10m
5
* 18
D3
1N4007
D1
1N4007
on
w 3.3 mF
v 413.5 kW
on
1
time of
(eq. 1)
(eq. 2)
(eq. 3)
(eq. 4)
http://onsemi.com
of the
Cbulk
22uF
CC
CC
13
3
VCC
R3
200k
constant charging current. In reality, the take over time can
be shorter (or longer!) and it can lead to a reduction of the
V
increase of the start−up resistor, thus reducing the standby
power. Laboratory experiments on the prototype are thus
mandatory to fine tune the converter. If we chose the 413 kW
resistor as suggested by Equation 4, the dissipated power at
high line amounts to:
must ensure that the self−supply does not disappear when in
no−load conditions. In this mode, the skip−cycle can be so
deep that refreshing pulses are likely to be widely spaced,
inducing a large ripple on the V
too large, chances exist to touch the VCC
controller into a new start−up sequence. A solution is to
grow this capacitor but it will obviously be detrimental to the
start−up time. The option offered in Figure 39 elegantly
solves this potential issue by adding an extra capacitor on the
auxiliary winding. However, this component is separated
from the V
ability to grow this capacitor as you need to ensure the
self−supply of the controller without jeopardizing the
start−up time and standby power. A capacitor ranging from
22 to 47 mF is the typical value for this device.
improve the standby power, its value cannot fall below a
certain level at the minimum input voltage. Failure to inject
P
CC
This calculation is purely theoretical, and assumes a
Now that the first V
One note on the start-up current. If reducing it helps to
Rstart*up
capacitor. Hence, a decrease in charging current and an
10
11
1
CC
C1
4.7uF
R2
200k
R1
200k
+
+
pin via a simple diode. You therefore have the
4R
0.827Meg
V
D6
1N4148
ac,peak
230
start*up
2
CC
2
+
+ 64 mW
2
capacitor has been selected, we
C3
47uF
D5
1N4935
230
4
CC
capacitor. If this ripple is
413k
4
aux.
2
2
min
and reset the
(eq. 5)