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© 2022 ROHM Co., Ltd.
65UG028E Rev.001
2022.7
User's Guide
BD7F105EFJ-EVK-001
Application Design Example
1.
Transformer design
1.1
Determining the volume ratio N
P
/N
S
The winding ratio is a parameter that sets the output voltage, maximum output power, duty, and SW terminal
voltage.
The duty of the flyback converter is calculated by the following equation:
𝐷𝑢𝑡𝑦 =
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
𝑉
𝐼𝑁
+
𝑁
𝑃
𝑁
𝑆
× (𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
)
𝑁
𝑃
: Primary transformer turns
𝑁
𝑆
: Secondary transformer turns
𝑉
𝑂𝑈𝑇
: Output voltage (Since there are 3 outputs, the design is based on output voltage 1)
𝑉
𝐹
: Forward voltage of the output diode on the secondary side
𝑉
𝐼𝑁
: VIN pin voltage
From the above formula, the winding ratio is calculated as follows.
𝑁
𝑃
𝑁
𝑆1
=
𝐷
𝑇𝑌𝑃
1 − 𝐷
𝑇𝑌𝑃
×
𝑉
𝐼𝑁
𝑉
𝑂𝑈𝑇
+ 𝑉
𝐹
𝐷
𝑡𝑦𝑝
: Duty at VIN Voltage (Typ)
It is recommended to set D
TYP
from 30% to 50% at the VIN voltage in the middle of the operating range.
Initially, set D
TYP
= 40 %. (This time, set the Duty to 35%)
In this case, the following formula is used.
𝑁
𝑃
𝑁
𝑆1
=
0.35
1 − 0.35
×
12𝑉
6.2𝑉 + 0.6𝑉
= 0.92
Therefore, we will proceed with designing with a Np/Ns1 of 0.92.
The turn ratio is also limited by the maximum duty D
MAX
determined from the minimum incoming voltage.
Make sure that D
MAX
given by the equation below does not exceed 70%. If this is the case, set D
TYP
so that it
becomes smaller. If it exceeds 70 %, the OFF time will be shortened. Therefore, the output voltage may deviate
due to deviations in the flyback voltage detection.
𝑁
𝑃
𝑁
𝑆1
=
𝐷
𝑀𝐴𝑋
1 − 𝐷
𝑀𝐴𝑋
×
𝑉
𝐼𝑁(𝑀𝑖𝑛)
𝑉
𝑂𝑈𝑇(𝑀𝑎𝑥)
+ 𝑉
𝐹(𝑀𝑎𝑥)
𝐷
𝑀𝐴𝑋
: Maximum duty of VIN voltage (Min) condition
𝑉
𝑂𝑈𝑇(𝑀𝑎𝑥)
: Maximum output voltage (Since there are 3 outputs, the design is based on output voltage 1)
𝑉
𝐹(𝑀𝑎𝑥)
: Forward voltage of secondary diode (Max)
𝐷
𝑀𝐴𝑋
=
0.92
8𝑉
6.2𝑉 + 0.6𝑉 + 0.5
= 0.44 < 0.70
For this reason, there is no problem in this design.
D
MAX
of this designer is 0.44 and 0.70 or less, so it is judged without any problem.
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