# Q&A（FAQ）What is harmful effect of input line impedance?

## 1. Summary

If the input line impedance for the DC/DC converter is high, the input voltage might get fluctuation, then it causes the output voltage unstable.

## 2. Input voltage fluctuation

If the inductance value is too much (see Fig. 2.1), the resonance circuit is composed by inductor L and capacitor C, and then it makes the input voltage change. Fig. 2.1 The input line circuit

Please refer to the following for the change level of the input voltage.
In the circuit as shown in Fig. 2.1, the energy of inductor and capacitor is shown as the following.

Energy of inductor El : 1 / 2 x L x rI2
Energy of capacitor Ec : 1 / 2 x C x rV2

In the resonance circuit, energy of inductor and energy of capacitor are same. Therefore, the input voltage fluctuation (rV) is shown as the following.

rV = √(L x rI2 / C) -------(1)

## 3. Proposed measures for the input voltage fluctuation

Please refer to Table 3.1 for the measures to inhibit the input voltage fluctuation derived by the formula (1).

Table 3.1 Proposed measures for the input voltage fluctuation
No. Measure Concrete measures
1 Decrease L Make input line short and use thick cable
2 Increase C Add capacitor to DC/DC converter input
3 Reduce I Reduce the output current

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