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Negative input resistance of switching regulators

ANP008

Developers of switching regulators and switch mode power supplies attach great importance to the efficiency of their circuits. However, at the end of their development phase, they encounter unpleasant effects such as unwanted oscillations at the input of the switching regulator and these although the switching regulator produces a constant output voltage under all conditions. But why does the input of the switching regulator tend to oscillation under certain circumstances?

Summary

After extensive examination of the "negative" resistance, we come to the understanding that the sign is based on a behaviour which can be attributed to the drop of the input current during increase of the input voltage of a switching regulator – transients. Due to the conducted interference, an input filter is absolutely required; however it should be attenuated so that the negative resistance is overcompensated and further prevents any oscillation. Finally, it is recommended not to use ceramic capacitors and to use electrolyte capacitors to prevent unwanted oscillations at the switching regulator input.

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