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Have you ever connected the input voltage to your power supply and found that it has failed? A short input voltage rise time and a high Q resonant circuit that can generate twice the input supply voltage can be a problem. A similar problem occurs if you quickly interrupt the current in the sensing element. Some situations where such problems can occur include hot-swappable circuits or when attempting to open an input to an electromagnetic interference (EMI) filter.
Things are not as bad as they seem. The inductor's current will saturate it, and the capacitor can be charged with a lower series inductance than expected. If the inductor is saturated, the filter characteristic impedance will drop like Q, reducing overshoot. To verify if this is the case in a high-Q system, divide the voltage step by the system characteristic impedance to calculate the peak current. After that, please refer to the inductor product manual to determine if it will saturate.
In summary, the filter ringing caused by the step input voltage can result in a voltage that has a damaging effect on downstream electronic components. Especially in systems such as PoE, where low-loss ceramic capacitors and some unsaturated inductors tend to be high Q, the above voltage becomes a problem. If the voltage reaches an unacceptable level, these systems will require more damping, current limiting, or an alternative charging method.
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