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enter image description hereI'm using a AC-DC power supply to provide 24VDC input to a PCB. A 100uF,35V tantalum is used as one of the filter capacitors for the 24V(same from supply) input of DC-DC regulator on PCB. In some cases, this 35V tantalum capacitor goes bad and gets shorted and damages the PCB. I'm trying to find the root cause for this.

When looked at the datasheet of AC-DC power supply, one of the output parameter is as below

"Transient Response Recovery -<6% or 300mV of set voltage for 50% load change (above 25% load),500μs for recovery to 1% or 100mV of set voltage"

What does this specification mean?

Can the 24VDC output from power supply damage the 35V rated tantalum capacitor?

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  • \$\begingroup\$ Depends on how much ripple you have. Post a schematic and use space after each comma. \$\endgroup\$ – winny Jul 22 '16 at 8:35
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    \$\begingroup\$ Added schematics \$\endgroup\$ – Saranraj Annamalai Jul 22 '16 at 9:28
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    \$\begingroup\$ Good. How much ripple do you have on the DC output from the power supply? \$\endgroup\$ – winny Jul 22 '16 at 9:53
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    \$\begingroup\$ The answer given on the transient response recovery is correct, but really the spec means nothing without specifying the di/dt of the load change. If the 50% load change happens over 1 minute it will be a much different answer then if the load step happens at 80A/us. \$\endgroup\$ – John D Jul 22 '16 at 11:34
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    \$\begingroup\$ Tantalums fail most often for 2 reasons: Excessive ripple current and overvoltage. It's unlikely that the transient load release overshoot on your supply is >10V, so I suspect the ripple current in your cap may be too high. You could measure it, or pick a different cap like a polymer tantalum, ceramic or aluminum electrolytic. \$\endgroup\$ – John D Jul 22 '16 at 11:39
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Transient or step response means how quickly a power supply reacts to changes in load. In your case it may or may not matter depending what you're driving with that 5V. If you've got something like a motor or other power electronics, the load can change suddely.

enter image description here

With regards to the tantalum packing it in, it's most likely because you're simply overstressing it. I can't tell from the image the part number but if it's an SMPS, you can have significant input ripple current depending on load.

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    \$\begingroup\$ The part is a Linear Tech Micromodule (I use this particular one in some peojects) and it is a switching supply. linear.com/product/LTM4612 \$\endgroup\$ – Peter Smith Jul 22 '16 at 14:07
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    \$\begingroup\$ @PeterSmith 5A output? Well then definitely there's a good chance input ripple current fried your tantalum cap. Linear is an unfriendly company but they in fact do have design tools available that will let you enter your parameters and it'll tell you the nominal input/output ripple currents among other things. Last time I dealt with time it's just a simple excel sheet. But hey won't give it to you on their Web page because it would confuse customers... You might need electrolytic can for this or maybe a battery of High cap ceramics. I see you already have a 10uF cap there. Probably mlcc. \$\endgroup\$ – Barleyman Jul 23 '16 at 0:58
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    \$\begingroup\$ The thing to remember with ceramics is that the "high capacity" dielectrics have DC voltage bias effect meaning your 10uF 50V cap might be only 2.5uF at 24V. \$\endgroup\$ – Barleyman Jul 23 '16 at 1:00

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