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© 2019 ROHM Co., Ltd.
No. 61UG024E Rev.002
Jul. 2021
User’s Guide
BM2P141X-EVK-001
Design Overview
– Continued
4 Capacitor Selection
4.1 Input Capacitor : C4
The input capacitor is determined by input voltage VI and output power POUT. As a guide, for an input voltage of 90 to 264 Vac,
2 x P
OUT
[W] μF. For 176 to 264 Vac, set 1 x P
OUT
[W] μF. Since the output power P
OUT
= 10
W, 22 μF / 450 V is selected at 20
μF or more.
4.2 VCC Capacitor : C6
The VCC capacitor CVCC is required for stable operation of the device and stable feedback of the output voltage. A withstand
voltage of 25 V or more is required, and 1.0 μF to 4.7 μF is recommended. 2.2 μF / 50 V is selected.
4.3 Output Capacitor : C7, C8
For the output capacitor, select output voltage V
O
of 25 V or more in consideration of derating. For C7 electrolytic capacitors,
capacitance, impedance and rated ripple current must be taken into consideration.
The output ripple voltage is a composite waveform generated by electrostatic capacity: Cout, impedance: ESR when the ripple
component of inductor current: ΔI
L
flows into the output capacitor and is expressed by the following formula.
∆𝑉𝑟𝑖𝑝𝑝𝑙𝑒 = ∆𝐼
𝐿
× (
1
8 × 𝐶𝑜𝑢𝑡 × 𝑓
𝑠𝑤
) + 𝐸𝑆𝑅
The inductor ripple current,
∆𝐼
𝐿
= 2 × {𝐼𝑝 − 𝐼
𝑂𝑈𝑇
(𝑚𝑎𝑥)} = 2 × (1.52 − 0.715) = 1.61
[A]
For this EVK, we use electrostatic capacity: 680 μF, ESR: 0.049 Ω, and the design value of output ripple voltage is less than
100 mV.
∆𝑉𝑟𝑖𝑝𝑝𝑙𝑒 = ∆𝐼
𝐿
× {(
1
8×𝐶𝑜𝑢𝑡×𝑓
𝑠𝑤
) + 𝐸𝑆𝑅} = 1.61 × {(
1
8×680𝜇×65𝑘
) + 0.049} = 83.4
[mV]
Next, check whether the ripple current of the capacitor satisfies the rated ripple current.
Inductor ripple current RMS conversion,
𝐼
𝐿
[𝑟𝑚𝑠] = ∆𝐼
𝐿
× √
1
3
= 0.93
[A]
The ripple current of the capacitor,
𝐼
𝐶
[𝑟𝑚𝑠] = √𝐼
𝐿
2
− 𝐼
𝑂𝑈𝑇
2
= √0.93
2
− 0.715
2
= 0.59
[A]
