Wall chargers and ceramic caps team up to ruin your circuit

LT AN88Linear Tech has written an application note to warn warn of potential hazard when using ceramic caps for equipment input decoupling if long supply leads are involved.

Danger comes from the wonderful low resistance and inductance of these capacitors, which allows the capacitance to resonate at high frequency with power lead inductance.

If a voltage step is applied – for example when plugging an already-energised wall adapter into the circuit – a spike can be generated with far more amplitude than the voltage applied, and with enough energy to take out parts of the decoupled circuit.

Read Linear Tech app note 88, “Ceramic input capacitors can cause overvoltage transients”.

Moving to a capacitor with a higher ESR, or adding a resistor of a fraction of an ohm in series with the input, is the answer.

Tags: amplitude, application note, capacitors, decoupling, impedance

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1 Comment

  1. Steve Kurt
    August 07, 2013 20:41

    Another case of unintended consequences, eh? Use a nice ceramic as the input cap instead of that lousy electrolytic, and now the circuit dies because the ceramic cap helped create a big transient. Somedays you just can’t win. :)

    I like the idea of adding a small resistance in line with the power wire. It provides a bit more filtering, and doesn’t degrade the cap’s functionality.

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