Q&A(FAQ) What is the adverse effect of input line impedance?
1. Summary
If the input line impedance of a DC-DC converter is high, the input voltage may fluctuate, which can cause instability in the output voltage.
2. Input voltage fluctuation
If the inductance value is too high (see Fig. 2.1), a resonant circuit is formed by the inductor (L) and capacitor (C), which can cause the input voltage to fluctuate.
Please refer to the following explanation regarding the magnitude of the input voltage variation.
In the circuit shown in Fig. 2.1, the energy stored in the inductor and capacitor is expressed as follows.
Energy of inductor El : 1 / 2 x L x ΔI2
Energy of capacitor Ec : 1 / 2 x C x ΔV2
In the resonance circuit, energy of inductor and energy of capacitor are the same. Therefore, the input voltage fluctuation (ΔV) is given by the following expression.
ΔV = √(L x ΔI2 / C) -------(1)
3. Proposed measures for the input voltage fluctuation
Please refer to Table 3.1 for the measures to suppress the resulting input voltage fluctuation.
| No. | Measure | Recommended actions |
|---|---|---|
| 1 | Decrease L | Shorten the input line and use a thicker cable |
| 2 | Increase C | Add a capacitor to the DC/DC converter input |
| 3 | Reduce I | Reduce the output current |