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The EF16 core for transformer was selected. It has
cross−section area A
= 20.1 mm
e
allows
to
use
maximum
B
= 0.28 Tesla.
max
The number of turns of the primary winding is:
L p · I ppk
n p +
B max · A e
−3 · 0.2047
+ 4.14 · 10
0.28 · 20.1 · 10 −6
The A
factor of the transformer's core can be calculated:
L
L p
+ 4.14 · 10
A L +
(n p ) 2
For an adapter output voltage of 6.5 V, the number of turns
of the secondary winding can be calculated accounting
Schottky diode for output rectifier as follows:
(V s ) V fwd )(1 * d max )n p
n s +
d max · V bulk− min
(6.5 ) 0.7)(1 * 0.5)150
+
0.5 · 127
The number of turns for auxiliary winding can be
calculated similarly:
(V s ) V fwd )(1 * d max )n p
n s +
d max · V bulk− min
(12 ) 1)(1 * 0.5)150
+
0.5 · 127
The peak primary current is known from initial
calculations. The current sense method allows choosing the
voltage drop across the current sense resistor. Let's use a
value of 0.5 V. The value of the current sense resistor can
then be evaluated as follows:
2
. The N67 magnetic
operating
flux
density
(eq. 20)
+ 150 turns
−3 ·
+ 184 nH
(eq. 21)
(150) 2
(eq. 22)
+ 8.5 + 9 turns
(eq. 23)
+ 15.35 + 15 turns
http://onsemi.com
NCP1215
V CS
R CS +
+
I ppk
The voltage drop across the sense resistor needs to be
recalculated:
V CS + R CS · I ppk + 2.7 · 0.2047 + 0.553 V
Using the above results the value of the shift resistor is:
V CS
R shift +
I CS
The value of timing capacitor for the off time control has
to be calculated for minimum bulk capacitor voltage since
at these conditions the converter should be able to deliver
specified maximum output power. The value of the timing
capacitor is then given by the following equation:
Lp · I ppk
1
*
f sw
V bulk− min
C T +
1.2 · 10 6
1
*
75 · 10 3
+
0.12 · 10 6
The value of the startup resistor for startup time of 200 ms
and Vcc capacitor of 200 nF is following:
R startup +
C Vcc
The result of all the calculations is the application
schematic depicted in Figure 21.
11
0.5
+ 2.442 W + 2.7 W
0.2047
0.553
+
+ 11.06 kW + 11 kW
50 · 10 −6
4.14 · 10 *3 · 0.2047
+ 55.5 pF + 56 pF
127
V bulk− min
V startup
) I CC−start MAX
t startup
127
+
) 10 · 10 −6
200 · 10 −9
12
0.2
+ 5.77 MW + 5.6 MW
(eq. 24)
(eq. 25)
(eq. 26)
(eq. 27)
(eq. 28)
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