Power Supply Tip 3: Damping Input Filter - Part 1 (including circuit diagram! See PDF!)

Filter 18.432M

description:

Switching regulators are generally preferred over linear regulators because they are more efficient, and switching topologies rely heavily on input filters. This circuit component, combined with the typical negative dynamic impedance of the power supply, can induce oscillation problems. This article will explain how to avoid such problems. In general, all power supplies maintain their efficiency over a given input range. Therefore, the input power is more or less constant with the input voltage level.

The voltage-current line exhibits a slope that essentially defines the dynamic impedance of the power supply. The slope of this line is equal to the negative input voltage divided by the input current. That is to say, from Pin = V ? I, it can be concluded that V = Pin / I; and thus dV / dI = - Pin / I2 or dV / dI ≈ - V / I can be obtained. The approximation is a bit too simple because the control loop affects the frequency response of the input impedance. But many times this simple approximation is sufficient when it comes to current mode control.