Help, after replacing MLCC capacitors with electrolytic capacitors, an abnormali - Page 1
Forum section: electronics / projects

author: daisizhou
author_rank: Super Contributor
timestamp: October 12, 2025, 01:56:16 am
content: I made a DCDC power conversion circuit board using MP2315GJ。The original author recommended using MLCC. I used MLCC as required by the original author and found that the output was normal.Since I don't know the withstand voltage of MLCC, in order to ensure reliability, I use 35v 10uf~22uf to replace MLCCAfter replacement, the circuit output voltage fluctuates,8.7v~6.4v fluctuates irregularly,So I changed back to MLCC and the voltage output returned to normal 5.41vI don't know why,If anyone knows please tell me, thanksMP2315GJ Datasheet:https://www.alldatasheet.com/datasheet-pdf/view/1035057/MPS/MP2315GJ.html

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 02:05:05 am
content: What did you effing expect from 20uF of electrolytic capacitors? Most likely there is 1+ ohm of ESR at 100kHz. Not to say it uses 500kHz switching frequency, those capacitors are not even really capacitors at such frequency.QuoteWhat is it even supposed to mean? If you buy a particular part number, you know the spec.

author: daisizhou
author_rank: Super Contributor
timestamp: October 12, 2025, 02:07:04 am
content: Quote from: wraper on October 12, 2025, 02:05:05 amI removed the old MLCC from the material board, so I don't know the exact withstand voltage and capacity values.

author: Magnethicc
author_rank: Contributor
timestamp: October 12, 2025, 05:33:39 am
content: by changing the MLCC to an electrolytic you are introducing very high ESR to the output filter which can cause significant voltage ripple on the output. this voltage ripple is then fed into the control by the resistor divider and to make things worst, the 39pF capacitor.Also I doubt there was 40uF of capacitance to begin with so you also changed the output capacitance which changed the whole dynamics of the system.I dont think you necessarily need MLCC to make the supply stable but you do however need to stabilize it with the new electrolytic.Try removing the 39pF from the top resistor in the resistor divider and see if it helps, also you could increase the resistance of the 20kohm that goes into the FB pin.

author: daisizhou
author_rank: Super Contributor
timestamp: October 12, 2025, 08:24:46 am
content: Quote from: Magnethicc on October 12, 2025, 05:33:39 amThanksFrankly speaking, I really don’t know the withstand voltage and capacitance values ​​of MLCC No. 1 and No. 2. I only roughly know that MLCC No. 2 has a darker color and looks like a larger capacity MLCC, while No. 1 has a smaller capacity MLCC.I have a voltage withstand tester. Can I test the voltage withstand value of MLCC?

author: makebit
author_rank: Unknown
timestamp: October 12, 2025, 11:02:34 am
content: MLCC can withstand voltages much higher than there rating.Obviously their capacitance goes down a lot as their voltage goes up.But yes, a 500khz switcher shouldnt really be having electro caps at its output....well it can do...as long as there are enough mlcc's there to soak up the high frequency stuff.

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 11:14:45 am
content: 20uF of electrolytic is not enough for any converter running at any frequency, unless it supplies like 50 mA max.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 12, 2025, 02:04:19 pm
content: Quote from: daisizhou on October 12, 2025, 01:56:16 amHow is that possible? Unless you maybe used parts from a drawer for which you had lost the specs?For MLCCs, the max rated voltage depends mainly on the package, although it varies depending on the manufacturer. But the package size will give you at least an indication.So what's the package for these caps you used, for which you don't mention neither the package nor the actual capacitance? (We can guess 10 µF to 22 µF as that's what you used for the replacement.)As an example, a 10 µF MLCC in 0805 package has a typical max voltage of 16 V to 25 V depending on the manufacturer. You can usually safely bet on 16 V if you have no datasheet, but using parts with actual specs is usually preferable. Also note that MLCCs tend to have a capacitance which varies with the operating voltage. If you're running one close to its max rated voltage, its actual capacitance has a high probability of being significantly lower than the rated capacitance. So take that into account when designing.Regarding the use of electrolytic caps for your use case, everything has been said already above.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:07:36 pm
content: Maybe I missed it, but I think the OP forgot to mention which specific capacitors were replaced. None of the photos show electrolytics on the boards. I'm guessing it's C1, C6, or C7.C6 or C7 being electrolytic will affect loop stability a lot. I agree with Magnethicc's suggestion of removing C9, that might be sufficient to make it stable again.Substituting C1 with electrolytic might be ok, though depending on the load current it might exceed the ripple current rating.Quote from: makebit on October 12, 2025, 11:02:34 amThis is terrible advice.Quote from: SiliconWizard on October 12, 2025, 02:04:19 pmPretty sure they mentioned they just pulled it off another PCB. Guessing they measured capacitance with an LCR meter but have no way of determining rated voltage.QuoteNo, I don't think you can get any meaningful estimate of voltage rating based on package and capacitance (especially if the dielectric is not known).QuoteAs a simple test, I asked digikey to show me all in-stock 10uF MLCCs in 0805 package. Voltage ratings span from 4V to 50V. Not useful. Though if I had to bet, I would be very surprised if the part is rated less than 10V.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: October 12, 2025, 02:20:01 pm
content: MLCCs are a dime a dozen, even from reputable vendors.Order some with the correct voltage rating. Add some additional voltage margin because for the X7R and other class-II ceramics, its effective capacitance is significantly reduced as the applied voltage approaches the rated voltage.EDIT; the schematic gives you clues for the voltage ratings. For the input caps use 25 volt rated ones, for the output caps, 10 volts should be plenty.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 12, 2025, 02:21:50 pm
content: Do not reuse desoldered MLCCs (especially large packages). They are more prone to cracking due to mechanical and thermal abuse.And as said by others, electrolytic-only is not going to work well with a switcher. Too much ESR and ESL.Just buy new MLCCs, in values/sizes recommended by the switcher IC manufacturer.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 12, 2025, 02:24:42 pm
content: Quote from: UnijunctionTransistor on October 12, 2025, 02:20:01 pmYou can hear that a lot, but it is incorrect advice, unfortunately. Derating voltage is very ineffective against DC bias capacitance reduction, because correlation is weak, almost nonexistent. X7R specimen A can be still 50% capacitance at full rated voltage, while specimen B can be down to 25% at 50% derated voltage!!If at all possible, instead try to find actual DC bias curves. If unable and in need of some rule-of-thumb, look at volumetric energy density: this means, large package (remember the contribution of height too!) tends to give more usable capacitance, than same uF and V ratings in a smaller package.

author: daisizhou
author_rank: Super Contributor
timestamp: October 12, 2025, 10:41:23 pm
content: Quote from: wraper on October 12, 2025, 11:14:45 amDo you mean I need to use a large capacity electrolytic capacitor, such as a 1000uf or 2000uf aluminum electrolytic capacitor?

author: daisizhou
author_rank: Super Contributor
timestamp: October 12, 2025, 10:58:57 pm
content: Thanks everyoneWhen no-load, the voltage output is 5.45V，At this time I am using MLCC capacitors.The input capacitor is 1206 package and the output capacitor is 0805 packageCan I use aluminum electrolytic capacitors instead of these MLCC capacitors, or is it that this circuit only supports MLCCs and aluminum electrolytic capacitors are not allowed?Are capacitors C1, C2, C6, and C7 used for filtering? If so, why can't aluminum electrolytic capacitors be used?

author: wraper
author_rank: Unknown
timestamp: October 12, 2025, 11:12:52 pm
content: QuoteESR and ripple current. In principle you could use electrolytic capacitors. But you'd need like 2000uF of LOW ESR ones.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 12, 2025, 11:42:05 pm
content: Quote from: daisizhou on October 12, 2025, 10:58:57 pmC6, C7 are part of feedback loop.If you change ESR form 10mΩ to 1Ω this has an impact on the stability of loop.