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Ceramic capacitor is blowing at random time

Ilgaz

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Dear All;

I have a problem about a ceramic capacitor in my design.

Adsız.png



The problem occurs at C6. Note : C2 and C5 is same capacitor but no error occurs at them.
C6 is 1uF, 50VDC, X5R, 0603 MLCC type capacitor (Mfg P/N : Samsung CL10A105KB8NNNC)

Let me define the board and problem.
- This design is low-power RGB LED driver which has LED Driver IC and LDO.
- Design can work at 19~28VDC nom. at 24VDC (VIN power port)
- Design has resettable fuse and TVS diode(SMAJ26A) on LDO power input.
- LDO just fed LED driver IC, not LEDs, LEDs anodes are directly connected to 24VDC, LEDs cathode are connected to LED Driver IC.
- This sheet is called as a one pixel of linear luminarie. One luminarie has 8 pixels(8 pcs of given sheet).
- My power supply can fed up to 30 meters of linear luminarie and it means 240 pcs of pixels.(One pixel is given sheet as specified)
Please note that, 240 pcs of given design is connected as parallel to one power supply.
- 24VDC / GND distributed parallel with help of extra cabling . Data signals (SDI / SDO) are serially connected to each pixels, as in and out fashion.

The problem is;
C6 capacitor blows at a random time and at a random pixel, C6 become short-circuit.
So, LDO stops functioning, F4 fuse protects C6 to fire up. Then, remaining portion of cascaded connected pixels can't work, because data can't go through this defective pixel.
When we remove C6 from blown pixel, system / all pixels resumes to functioning.
Somehow, this capacitor blows and short-circuited, so it malfunctions of this pixel line.

What we try (All our changes and attempts doesn't give any result). Given attempts are done step-by-step and make observations for sufficient time.
- We decrease the power of pixels to reduce total current flowing through VIN line.
- We decrease the voltage of power supply to 21VDC.
- We place bulk capacitors (680uF, 100VDC, Aluminium Electrolytic) at VIN line to reduce overshoots and possible spikes that caps are at start, 10th, 20th, 30th meters of pixel line.
- We made observation that issue is happening at power on : Absolutely NOT. Cap may take the knock at power-on, but we oftenly observe total malfunctioning of pixel while system is working.
- We get measurements from start and end of power line(VIN/24VDC) of luminaries with scope. Power seems very very clear at power-on, continous and power-off times, no overshoot/no spikes, voltage rise to 24VDC very soft (5ms)
As seen here, we do most of known modifications and observations.

Pleased your help about what can be the reason of specified blowing cap issue.

Best Regards,
 

treez

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Is this cap getting some bending stress, or a connecter, or the enclosure is pressing down on it...or is it near the edge of the board, where depanelisation stress may crack it. Is its long axis transverse to the long axis of the board. Is it a "flexiterm" cap?
 

KlausST

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Hi,

Yes I agree with treez.
Show a photo of the board where the defective capacitor is marked.

Klaus
 

FvM

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Electrical stress is effectively prevented by the circuit, TVS protects against overvoltage, PTC series resistance against excessive ripple, if possible at all. Thus it's clearly a mechanical or component quality problem.
 

danadakk

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You triggering scope on some excessive level, one shot, to capture any input
transient on power on ? Say 30V as trigger point. DSO scope can handle this easy.

You can leave scope running overnight, several days, to see if a trigger is generated.
And what transient looks like.


Regards, Dana.
 
Last edited:

Ilgaz

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Dear All,

Thanks for your replies.
Here is pictures of board.


1618831443408.png

1618831472695.png


1618831518559.png


- One pixel is 12.5cm and whole board is 50cm. Board is flexible because it has 2cm x 50cm dimensions, tiny and long board.
- To be honest, We depanelize boards by trivial and un-professional ways.
- F4 is fuse and D9 is TVS diode.

** Cap is not flexiterm. But, after looking at PCB, direction of given cap seems OK for me and C7 (0,1uF, 50VDC, X7R, 0603) and C5 (same with C6) never have any defect, just have problem about C6 at all of defective pixel items.
It may give you some clue that we produce more than 40.000 pixels and we have 200 pcs defective pixels till now.

As I told, I do technically everything as my knowledge but error appears really too random, sometimes we get no errors for a week, sometimes we get two defective pixels in a day. We make many many power-on / off to whole system, nothing happens. It's not a electrical overstress issue, I guess.

Best Regards,
--- Updated ---

You triggering scope on some excessive level, one shot, to capture any input
transient on power on ? Say 30V as trigger point. DSO scope can handle this easy.

You can leave scope running overnight, several days, to see if a trigger is generated.
And what transient looks like.


Regards, Dana.
Dear Danadakk;

I have done this measurement from several points of daisy-chained connected luminaries.
Output voltage of supply is 24VDC and I didn't get any spike or transient for a day, trigger is set to 24.4V.
I get measurements from zero point and 30-meter of luminaries.
 
Last edited:

danadakk

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You contacted Samsung Rel department to see if there is a lot issue with those MLCCs.

Would not be the first time a vendor has had issues with MLCC, or tantalums.....


Regards, Dana.
 

mtwieg

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Agreeing with others that the failures are likely mechanical in origin. But I think the failure can be explained without assuming some flaw in the capacitor itself.

C6 in particular is vulnerable because of its position on this long, thin board. Large components nearby (D9, U2, and D4) act as board stiffeners, and thus components next to them (in the left-right direction) don't see much stress. But that means that stress will be increased at the gaps between those large components. C6 lies right along one such gap, see my dashed line added below. That's probably why it fails as opposed to C7, which is right next to it. The stress that would affect C7 is instead taken by D9.
1618921371039.png


So moving C6 a bit might resolve the issue. Using flex terminations wouldn't hurt either. But IMO C5 and F4 are also going to see increased stress. A rigid flex pcb would be a better resolution.
 

Ilgaz

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Hi All;

I have issue at ceramic capacitor in my design.
The capacitor is, 1uF, 50VDC, 0603, X5R Samsung capacitor (CL10A105KB8NNNC)

Let me clarify issue.
- I have a power supply, 24VDC output, max current capacity is 15A.
- I have loads which are LED Drivers that can be controlled by a sytem. Each load is max. 2W, total max. qty of load is 150 qty. Loads are connected as parallel to power supply.
- Loads's can be controlled by system, thus a load can be 2W, 1.7W, 1.2W, 0.8W, 0.3W at any time.
- My capacitor is placed as decoupling cap(MLCC) at VCC-GND terminals at input side of loads. ,
- Distance between LED Drivers and power supply has 50 meters.

When I set load power to 0.5W, everything is fine. Total power is max 75W.
When I set load power to 2W, input capacitor blows and it becomes short-circuited. Total max power is 300W

So, when total power consumption of load is low, everything is fine.
However, when total power consumption becomes higler as drive with 2W, the cap at input side of LED driver very randomly.

What can cause this? And do you have any solution to eliminate of given issue.

Best Regards;

B
 

KlausST

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Hi,

Use a scope and measure the capacitor voltage and waveform.

Possible issues:
* too high ripple current
* or too high (peak) voltage
Maybe caused by ringing / oscillations.

Klaus
 

BradtheRad

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Picture a little 1uF capacitor, its thin plates must endure 15 Amperes of current rushing across them in one direction, then the other, thousands of times per second.

It may be necessary to split the burden across several capacitors in parallel. Say ten 100 nF.
 

Easy peasy

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pretty obviously the small sized capacitor is getting overheated by the ripple current, have you measured the temp as it operates?

either a bigger part or parts in parallel, and bigger pads and tracks ...
--- Updated ---

the tan-delta is 0.1, so Vrms x Irms x tan-delta = dissipation.

for 0603, dissipation should be < 20mW, so for 1Vrms x 0.5A rms x 0.1 = 50mW

what is the worst case Vrms AC across the cap - and the Irms in the cap .... ?
 

KlausST

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Hi,

so I have to say: if both describing the same problem, neither your first post here gives a complete description, nor post1 of the other thread.

I´m even not sure how many of these "1uF" capacitors are installed in the complete system.

Because you started 2 threads about the same issue (read forum rules) ... I´ve merged them.

Klaus
 

Ilgaz

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Hi,

Use a scope and measure the capacitor voltage and waveform.

Possible issues:
* too high ripple current
* or too high (peak) voltage
Maybe caused by ringing / oscillations.

Klaus
Dear,

I got many measurements from scope, everything is clear about high / peak voltages.

Let me ask you that are there anyway to reduce ripple current on this cap.
Such as connecting serial resistor on the main input voltage line (24VDC).

Best Regards,
 

mtwieg

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KlausST

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Hi,
I'm confused, your link leads to this same thread....?
from post #14: --> "Because you started 2 threads about the same issue (read forum rules) ... I´ve merged them."

Klaus
 

andre_teprom

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Did you try replacing the 24v power supply by another one, prefereably from another brand ?
And, are you also driving any kind of inductive load (e.g motor) with this power supply ?

BTW, what did you mean by that ?

I get measurements from zero point and 30-meter of luminaries.
 

Ilgaz

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Did you try replacing the 24v power supply by another one, prefereably from another brand ?
And, are you also driving any kind of inductive load (e.g motor) with this power supply ?

BTW, what did you mean by that ?
I get measurements from zero point and 30-meter of luminaries.

Dear andre;

I didn't try any brand, but I tried same brand's another model PSU.
There is no inductive loads, just LED lights, but these lights are controllable lights and they are controlling by computer and change their states hundred times in a second. You can imagine them as outdoor LED display.Thus, current demand from PSU changes hundred times in a second.

As specified before, from PSU to load, there is 50 meters cable approx for transmitting 24VDC.
After 50 meters cable, load is connected and my load is daisy-chained connected LED lights(approx 30 meters).

I got measurements from output of PSU, after 50 meters of cable(it means start point of LED lights), and end of the LED lights(this is like 80 meters away from PSU)
 

FvM

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I got measurements from output of PSU, after 50 meters of cable(it means start point of LED lights), and end of the LED lights(this is like 80 meters away from PSU)
You didn't show it yet.
 

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