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參數資料
| 型號： | NCP1215SNT1 |
| 廠商： | ON SEMICONDUCTOR |
| 元件分類： | 穩壓器 |
| 英文描述： | Low Cost Variable OFF Time Switched Mode Power Supply Controller |
| 中文描述： | SWITCHING CONTROLLER, PDSO6 |
| 封裝： | SOT-23, SC-59, TSOP-6 |
| 文件頁數： | 10/16頁 |
| 文件大小： | 148K |
| 代理商： | NCP1215SNT1 |
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
ICCstart
(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
I
pk
Using equation for peak primary current estimation the
switchon time is:
Rshift
Rcs· Vbulk
Minimum switchon time occurs at maximum input
voltage:
Rshift
Rcs· Vbulkmax
As it can be seen from the above equation, the switchon
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 switchoff time is determined by the charge of an
external capacitor connected to the CT pin. The minimum
Toff value can be computed by:
Voffset
Transformer primary inductance
Peak primary current
ton
Lp
50 · 106
(eq. 7)
tonmin
Lp
50 · 106
(eq. 8)
toffmin
CT
ICt
CT
1.2
105
(eq. 9)
0.12 · 106CT
Where:
V
offset
I
Ct
The maximum switching frequency then can be evaluated
by:
1
tonmin
toffmin
Offset voltage (see spec.)
CT pin source current (see spec.)
(eq. 10)
fswmax
1
Lp· Rshift
Vbulk· Rcs
· 50 · 106
0.12 · 106· CT
As output power diminishes, the switching frequency
decreases because the switchoff 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)
Vbulkmin
VACmin 2
90 ·
2
127 VDC
(eq. 12)
Vbulkmax
VACmax 2
265 ·
2
375 VDC
The requested output power is 5.2 Watts.
Assuming 80% efficiency the input power is equal to:
Pout
(eq. 13)
Pin
5.2
0.8
6.5 W
The average value of input current at minimum input
voltage is:
Pin
Vbulkmin
The suitable reflected primary winding voltage for 600 V
rated MOSFET switch is:
Vflbk
600 V
Vbulkmax
600
375
Using calculated flyback voltage the maximum duty cycle
can be calculated:
Vflbk
Vflbk
Vbulkmin
125
125
127
Following equation determines peak primary current:
2 · Iinavg
max
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. 14)
Iinavg
6.5
127
51.2 mA
(eq. 15)
Vspike
125 V
100
(eq. 16)
max
0.496
0.5
(eq. 17)
Ippk
2 · 51.2 · 103
0.5
(eq. 18)
fmaxhigh
fmaxlow
Vbulkmax
Vbulkmin
max
75 · 1031270.5
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:
Vbulkmin·
Ippk· fswmax
(eq. 19)
Lp
max
127 · 0.5
0.2047 · 75 · 103
4.14 mH
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