Your Bench Power Supply

Lack of Bulk Capacitance and/or Too Much ESR Can Play Havoc 

We know that a O.ljiF input capacitor takes care of the (high-frequency) noise. But it neither can nor do almost anything to smooth out the (low-frequency) ripple. However, we are now in a position to start calculating how much bulk capacitance we really need to ensure trouble-free performance (for typical ICs ). 

Example A Buck IC switching at 2MHz with 9V input is to provide 5V input bulk capacitance is required  

We pick the OFF time for the following calculation since the capacitor current is relatively fixed during this interval (and so we can therefore truly apply the equation I = CdV/dr). 

You may need to parallel several aluminum electrolytics to lower the ESR sufficiently, and you may also have to substantially increase the capacitance just to stay within the total 1% limit somehow. Also remember that the ESR of aluminum electrolytic capacitors gets significantly worse over time. So if you have a customer return after several months in the field, it may well be because of the aging of the electrolytic bulk capacitor] Try replacing the capacitor and then recheck. 

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Your bench power supply is set fairly close to the maximum input voltage rating of your IC. You have just changed the input ceramic capacitor of the converter from 22 0,F to 10 iF, and the supply line still looks very clean (under steady conditions). But the part gets damaged almost every time you connect the red lead coming from your bench power supply directly to your board. Why ... 

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