Any criteria on bypass capacitor selection?

Dear all,

I am now checking on a PCBA on which the power rail for digital circuit (microcontroller and its peripherals) uses many 0.1uF, Y5V,

0805 capactiors in parallel between VCC and Gnd.

I wonder why 0.1uF capacitor was used there?

I could also see 4.7 uF used together with 0.1uF capacitors.

I would also like to know the criteria to choose voltage ratings on bypass capacitors.

Is it safe to use 0.1uF, 50V bypass capacitor for 24V supply rails which drive inductive loads?

Regards.

Reply to
Myauk
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You are after the lowest HF impedance between VCC and Gnd. Each capacitor has an inductance. Parallel capacitors increases capacitance and reduces inductance.

As a matter of principle I don't use Y5V, it may be OK in this application but they have high voltage coefficients and temperature coefficients of capacitiance. So the capacitance is always changing dramatically. I would treat a 0.1uF Y5V as a 10nF.

I would fit at least a 100V caps on the 24V rails unless I had reason to know a lower voltage would be OK.

Reply to
Raveninghorde

Please let me know what type of dielectric material would be the best for this application.

The reason that I am looking into the use of this capacitor is because one of the 0.1uF, 50V, 0805, at 24V rails is found severely burnt.

The average current drawn in the circuit is around 3 to 4 Amperes.

Although 0.1uF and 4.7uF capacitors (50V rating) are placed in parallel, only one particular 0.1uF is burnt, the rest of the capacitors are still OK.

Please help me to give some suggestions on this issue.

As I am a newbie in this issue, I am totally lost. At least I would like to request some help for the start. Any article discussing on such kind of issue will be of great help for me.

Thanks and Best Regards

Reply to
Myauk

Ive seen high ripple current burn .1uf ceramic caps. This was from a power opamp oscillating and dumining 4 amps at 500Khz through the cap. The ESR is low on these caps but not zero.

Also take a look at the location of the cap relative to the load. If the one closest to the load got popped there is likely inductance between that cap and the other ones so it was taking more than its fair share of the ripple current.

I think X7Rs are less lossy than Y5Vs nd can handle more ripple current.

If they are SMT caps you could have had one cracked during soldering. The crack eventially turns into a short and releases the magic smoke.

Reply to
mook johnson

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Nice article with several worked out examples:

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-- Joe

Reply to
J.A. Legris

These might help...

Choosing and Using Bypass Capacitors:

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A bypass-capacitor dialogue peels back the layers:

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Guy Macon
Reply to
Guy Macon

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