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© 2022 ROHM Co., Ltd.
65UG028E Rev.001
2022.7
User's Guide
BD7F105EFJ-EVK-001
Application Design Examples-continued
5.
Secondary output diode
A Schottky barrier diode or a fast recovery diode with low V
F
is recommended because the forward voltage V
F
of the
secondary output diode causes an error in the output voltage. When selecting a secondary output diode, the peak of
the secondary reverse voltage must not exceed the rating of the diode. The secondary RMS current I
SRMS
must also
be set so that it does not exceed the rating.
Generally, 30 % or more of the reverse-direction breakdown voltage V
R
is recommended.
𝑉
𝑅
= (𝑉
𝐼𝑁(𝑀𝑎𝑥)
×
𝑁
𝑆
𝑁
𝑃
+ 𝑉
𝑂𝑈𝑇
) × 1.3 + 𝑉
𝑆𝑈𝑅𝐺𝐸
𝑉
𝑅
: Reverse voltage of the secondary output diode
𝑉
𝐼𝑁(𝑀𝑎𝑥)
: VIN pin maximum voltage
𝑁
𝑃
: No. of primary transformer turns
𝑁
𝑆
: Secondary transformer turns
𝑉
𝑂𝑈𝑇
: Output voltage
𝑉
𝑆𝑈𝑅𝐺𝐸
: Transformer surge voltage generated in the diode
It is recommended that the rated current of the secondary output diode be at least twice that of I
SRMS
.
6.
Output Resistance and Zener Diode (Minimum Load Current)
The output voltage rises when no load is applied or when light load is applied. The reason for this is that the MAXIMUM
is in the OFF-time t
OFF_MAX
when the IC is under light load.
This is because switching is always performed at the minimum frequency determined by the minimum ON-time t
ON_MIN
.
For the power P
O_MIN
determined by the switching frequency of this lowest frequency, when the secondary load is lighter
than this, the output voltage is
It moves up. P
O_MIN
is calculated by the following equation:
𝑃
𝑂_𝑀𝐼𝑁
=
𝑉
𝐼𝑁(𝑀𝑎𝑥)
2
2×𝐿
𝑃
× 𝑡
𝑂𝑁_𝑀𝐼𝑁(𝑀𝑎𝑥)
2
×
1
𝑡
𝑂𝑁_𝑀𝐼𝑁(𝑀𝑎𝑥)
+𝑡
𝑂𝐹𝐹_𝑀𝐴𝑋(𝑀𝑖𝑛)
𝐼
𝑂𝑈𝑇_𝑀𝐼𝑁
=
𝑃
𝑂_𝑀𝐼𝑁
𝑉
𝑂𝑈𝑇
Because it is an expression, it can also be obtained from I
OUT_MIN
.
If the rise in the secondary output voltage becomes a problem, connect a secondary output zener diode to suppress
the rise in the voltage. It is also necessary to suppress the rise in the output voltage by adding a resistor to the
secondary output to provide a constant loss. The output resistor R
OUT
to be connected to the secondary side should
be as follows. The resistor-loss P
LOSS
is calculated as follows.
𝑃
𝑙𝑜𝑠𝑠
=
𝑉
𝑂𝑈𝑇
2
𝑅
𝑂𝑈𝑇
𝑅
𝑜𝑢𝑡
≦
𝑉
𝑂𝑈𝑇
2
𝑃
𝑂_𝑀𝐼𝑁
=
𝑉
𝑂𝑈𝑇
2
𝑉𝐼𝑁(𝑀𝑎𝑥)
2
2×𝐿𝑃
×𝑡
𝑂𝑁_𝑀𝐼𝑁(𝑀𝑎𝑥)
2
×
１
𝑡𝑂𝑁_𝑀𝐼𝑁(𝑀𝑎𝑥)+𝑡𝑂𝐹𝐹_𝑀𝐴𝑋(𝑀𝑖𝑛)
In practice, even if R
OUT
loads calculated by the above equation are used, the output voltage rises transiently during
secondary discharging. Therefore, it should be set lower enough than this R
OUT
. Adjust this resistance value in the
actual evaluation. When selecting a resistor, pay attention to the rated power of the resistor.
[W]
[V]
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