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© 2018 ROHM Co., Ltd.
No. 60UG092E Rev.001
JULY.2018
User’s Guide
BM2P0161-EVK-003
1-3. Design of transformer inductances
Assuming condition BCM point is 260VDC
、
65kHz 12V/2A load.
Calculation for secondary and primary inductance, peak current.
Vo=12V
、
secondary rectifier diode VF
＝
1V
Po = 12V + 1V × 2A = 26W
Duty =
+
=
70
260 + 70 = 0.212
Ls =
Vo × 1 − 4* 5
6
2 ×
max × 7 = 12V + 1V ×
1 − 0.212
6
2 × 28 × 659 = 28.555:;
Lp = Ls ×
6
= 0.28555:; × 5.38
6
= 0.83=;
Ispk = @
2 × A max
B × 7 = 5.38
Ippk = @
2 × A max
B × 7 = 0.982A
Ipav = Ippk ×
4* 5
√3
= 0.982 ×
0.212
√3
= 0.128
1-4
．
Selecting transformer core size
Accordingly Po (max) =26W, Core is selected EE25.
Table 5. Output Power vs transformer core size
（＊）
above table is one of reference, please confirmation to transformer manufacturer.
1-5
．
Calculate of primary wiring turn : Np
The maximum rating of magnetic flux density of standard ferrite core is
0.4T@100
℃
. Accordingly Bsat=0.266T.
Np G
B ×
9
8H × I J =
830:; × 0.9828
40==
6
× 0.266K = 76.6turns
Np makes 77 turns.
Prevent form Magnetic saturation.
Check transformer AL-Value
－
NI value table
。
When Np=77 turns,
AL − Value =
6
=
830:;
77 *O
6
= 139 ;/ *O
6
NI = Np × Ippk = 77turns × 0.982A = 75.62A ∙ turns
Confirm by core specification whether or not it falls into this saturation region.
Output Power Po(W)
Core Size
Core Area Ae
（
mm
2
）
~30
EI25/EE25
41
~60
EI28/EE28/EER28
84
~80
EI33/EER35
107
Figure 13. TDK PC47EE25/19
AL-Value to NI Limit
