Can I substitute a capacitor with the same rated capacity and voltage but a lower inductance and higher rated ripple current for a mobo repair?

Question

I have an early to mid 2000's motherboard that I am trying to restore. It looks to be mostly in good condition but a blown radial electrolytic capacitor might be causing trouble. From the placement of the capacitor on the board between the ATX 20 pin power supply and the CPU, my guess is it is a filter capacitor for the CPU. I plan to replace the damaged device but the specific series KZG from Nippon Chemi-com does not seem to be made any more. I found a datasheet online for the capacitor. However on looking for a replacement, I have found no modern capacitor that fits every single specification criteria. I can find a modern one with the same voltage, temperature, and capacity but none with the same rated ripple current or inductance value. Several candidates for sale have a lower inductance (by .03 milliOhms) but a much higher rated ripple current.

What would the risks of replacing a capacitor with a higher rated ripple current and lower inductance be? Would I be able to get away with something close?

Answer 1

OK, the datasheet is pretty explicit about what type of capacitor this is...

First, the replacement needs to fit on the board, so it needs the same pin spacing. It can be taller or wider, if there is space available. Picking a 105°C cap is always a good idea, motherboards tend to be pretty hot.

Voltage and capacity of the replacement cap should be the same or higher. While a higher voltage rating won't cause trouble, a higher capacitance can cause some problems like higher inrush current.

Then, you have to check the impedance (not the inductance):

Impedance and ripple current are measured at 100kHz, which is a de facto standard for low-ESR caps meant to be used in switching power supplies.

Impedance is the sum of the capacitor's capacitive impedance, its internal resistance (ESR) and its inductive impedance, which depends on its construction. Since the value of interest for filtering a switching regulator output is the impedance, the manufacturers just list that, for convenience.

As for ripple current, it should be the same or higher.

The ripple current limit is due to self-heating. AC current in the cap dissipates power in its internal resistance (ESR) which heats the cap from the inside. When the ripple current spec is exceeded on a capacitor, it overheats and its life is shortened, or if you exceed it by a lot, sometimes it just pops. So, the replacement cap having higher ripple current rating is not a problem. It'll just heat less and last longer.

You should pick a similar or lower impedance/ESR. It's generally not recommended to overdo it, for example if you replace a "low ESR" electrolytic with an ESR in the tens of milliohms with a polymer cap in the single digit milliohms, you would have to check if it is at the output of a regulator, and then check the datasheet of this regulator for what kind of ESR it requires for stability. So it's best to pick something similar.

You should also check other caps of the same series on the board. If the caps were faulty, maybe there are other dead ones.

To clear up the confusion about inductance: this one depends pretty much only on the area of the loop taken by the current inside the cap. So, capacitors with wider pin spacing have a higher loop area, therefore they have more inductance and poorer high frequency performance. That's why your CPU VRM has many small caps in parallel instead of just a large one: physically smaller caps have lower inductance, and on top of that, putting several in parallel further improves the total impedance.

This also means you should not leave extra lead length when soldering the capacitor. The bottom of the cap should touch the board, it should not stand on its legs.

Answer 2

Lower inductance and ESR and larger ripple current handling means it's a better capacitor for a buck regulator like it most likely is on a motherboard.

Sometimes some regulators might require some specific range of ESR or impedance, but that is of course something we can't know.

If the motherboard is already broken, it will likely work fine with the caps you found, as long as you are not replacing a low-ESR cap with a standard electrolytic.

Answer 3

Capacitance and voltage are the primary specifications. You have matched those.

Residual inductance is a secondary specification, that is ideally zero. It's highly unlikely that an application will require a finite inductance from a large capacitor, though there are some regulators that require a finite ESR for stability.

Rated ripple current is a rating that represents a mix of ESR (how much heat is generated by the ripple current), the thermal conductivity of the capacitor, and how hot it can run for a given lifetime. If the ripple current is higher in the replacement than the original, then it should all be good.

That is, of course, assuming your replacement is from a reputable manufacturer. Unrealistically high ripple current ratings might be a warning that they are indulging in 'specification inflation', and that maybe you shouldn't trust any of the specifications. If you buy from Mouser/Digikey, you're less likely to run into this sort of problem than if you buy from AliBay or the like.

Answer 4

In many cases it is fine to replace a capacitor with another of the same type that has the same voltage and capacitance rating, but higher ripple current rating. HOWEVER...

Sometimes the parasitic features of a capacitor are just as much a part of the design as the ideal capacitance features.

For example, sometimes a capacitor used in a filter is chosen for it's Effective Series Resistance (ESR). The designer may have chosen a capacitor with high ESR to ensure proper damping of the filter, or to ensure a sufficiently wide bandwidth. Replacing the capacitor with "better" one, i.e. one with lower ESR could degrade the functioning of the circuit. In worst case, it could result in excessive ringing, and overheating of some other device leading to a more generalized failure.

So, the answer to your question is "It depends". If you understand the reason for the designer's choice of that particular capacitor, you can make an informed decision as to whether a substitute of the same type, same capacitance and voltage rating, but "better" ratings for something like ESR, would be advisable.

If you DON'T understand the reason for the designer's choice, the best course of action is to use an exact replacement.

If you are a hobbyist, and are not particularly perturbed by occasional failures, or damage, you might try something and see if it works. The more experience you get, the better you will become at judging when and where a substitution is likely to turn out well.