NCP1215
http://onsemi.com
10
If the IC current consumption is assumed constant during
the startup phase, one can obtain resulting equation for
startup resistor calculation:
Rstartup
+
Vbulk
CVcc
Vstartup
tstartup
)
ICC−start
(eq. 5)
Switching Frequency
The switching frequency varies with the output load and
input voltage. The highest frequency appears at highest
input voltage and maximum output power.
Since the peak primary current is fixed, the on time
portion of the switching period can be calculated:
ton
+
Lp
Ipk
Vbulk
(eq. 6)
Where:
L
p
Transformer primary inductance
I
pk
Peak primary current
Using equation for peak primary current estimation the
switch−on time is:
ton
+
Lp
Rshift
Rcs · Vbulk
50 · 10−6
(eq. 7)
Minimum switch−on time occurs at maximum input
voltage:
ton− min
+
Lp
Rshift
Rcs · Vbulk− max
50 · 10−6
(eq. 8)
As it can be seen from the above equation, the switch−on
time linearly depends on the input bulk capacitor voltage.
Since this voltage has ripple due to AC input voltage and
input rectifier, it allows natural frequency dithering to
improve EMI signature of the SMPS.
The switch−off time is determined by the charge of an
external capacitor connected to the CT pin. The minimum
Toff value can be computed by:
toff− min
+
CT
Voffset
ICt
+
CT
1.2
10−5
(eq. 9)
+
0.12 · 106 CT
Where:
V
offset
Offset voltage (see spec.)
I
Ct
CT pin source current (see spec.)
The maximum switching frequency then can be evaluated
by:
(eq. 10)
fsw− max
+
1
ton− min
)
toff− min
+
1
Lp · Rshift
Vbulk · Rcs
· 50 · 10−6
)
0.12 · 106 · CT
As output power diminishes, the switching frequency
decreases because the switch−off time prolongs upon
feedback loop. The range of the frequency change is
sufficient to keep output voltage regulation in any light load
condition.
Application Design Example
An example of the typical wall adapter application is
described hereafter.
As a wall adapter it should be able to operate properly with
wide range of the input voltage from 90 VAC up to 265 VAC.
The bulk capacitor voltage then can be calculated:
(eq. 11)
Vbulk− min
+
VAC− min 2
Ǹ +
90 ·
2
Ǹ +
127 VDC
(eq. 12)
Vbulk− max
+
VAC− max 2
Ǹ +
265 ·
2
Ǹ +
375 VDC
The requested output power is 5.2 Watts.
Assuming 80% efficiency the input power is equal to:
(eq. 13)
Pin
+
Pout
h
+
5.2
0.8
+
6.5 W
The average value of input current at minimum input
voltage is:
(eq. 14)
Iin−avg
+
Pin
Vbulk− min
+
6.5
127
+
51.2 mA
The suitable reflected primary winding voltage for 600 V
rated MOSFET switch is:
(eq. 15)
Vflbk
+
600 V
*
Vbulk− max
*
Vspike
+
600
*
375
*
100
+
125 V
Using calculated flyback voltage the maximum duty cycle
can be calculated:
(eq. 16)
d
max
+
Vflbk
Vflbk
)
Vbulk− min
+
125
125
)
127
+
0.496
+
0.5
Following equation determines peak primary current:
(eq. 17)
Ippk
+
2 · Iin−avg
d
max
+
2 · 51.2 · 10−3
0.5
+
204.7 mA
The desired maximum switching frequency at minimum
input voltage is 75 kHz.
The highest switching frequency occurs at the highest
input voltage and its value can be estimated as follows:
(eq. 18)
f max −high
+
f max −low
Vbulk− max
Vbulk− min
d
max
+
75 · 103
375
127
0.5
+
110.7 kHz
This frequency is much below 150 kHz, so that the desired
operating frequency can be exploited for further calculation
of the primary inductance:
(eq. 19)
Lp
+
Vbulk− min ·
d
max
Ippk · fsw− max
+
127 · 0.5
0.2047 · 75 · 103
+
4.14 mH
