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Choosing the right input caps for your buck converter

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KentHuang
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re: Choosing the right input caps for your buck converter
KentHuang   9/10/2007 3:11:31 AM
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very useful.

Bob K.
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re: Choosing the right input caps for your buck converter
Bob K.   9/13/2007 3:32:57 PM
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What would be a big help is a discussion of how the ESR of the bulk capacitor can be use to control the Q of the input filter. The drawing show an input inductor with VIN on the input side. Since a buck converter exhibits negative resistance the input filter can easily "Ring". This frequency can be quite low, down to 10Ks of Hz and the VIN source can be low Z at low frequencies. Load power transients at the resonate frequency can produce high ringing voltages that can blow the power FET or IC. Worse yet many of these little converters are mounted on a PCB with plenty of bypass caps. That makes a the Z of VIN even lower, the Q higher, and ringing worse. Designing an input filter that has a low Q for any input Z is difficult. This task is impossible for "Digital Engineers" schooled only in digital design.

Finleythewombat
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re: Choosing the right input caps for your buck converter
Finleythewombat   9/2/2013 6:10:36 AM
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Bob K: indeed.  This actually happened to me recently: the (ceramic) input capacitor was ringing so badly due to the inductance of a long power supply cable that the switcher was experiencing a significant overvoltage at switch-on.  I have never seen this phenomenon given adequate treatment in data sheets or application notes.

 

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