The source impedance of an input filter should be at least 6dB less than the input impedance of a switching regulator as a safety margin to minimize the chance for oscillation.
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In Power Tip #3, we discussed how the source impedance of an input filter can turn resistive, and how it can interact with the negative input impedance of a switching regulator. At worst case, these impedances can be equal in magnitude, but opposite in sign to make an oscillator. A general criterion has been established that the source impedance of an input filter should be at least 6dB less than the input
impedance of a switching regulator as a safety margin to minimize the chance for oscillation.
The design of an input filter usually begins with selection of an input capacitor (CO of Figure 1) based on ripple current rating or hold-up requirements. The next step usually involves selecting an inductor (LO) based on the system's EMI requirements. As we saw last month, near resonance the source impedance of these two elements can be quite high, leading to an unstable system. Figure 1 presents a method to control this impedance by placing a series resistor (RD) and capacitor (CD) in parallel with the input filter. The filter could be damped with just a resistor across CO. However, in most cases the power loss would be unacceptable. An alternative method is to add a series connection of an inductor and resistor across the filter inductor.
Figure 1: CD and RD damp the output filter source impedance.
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