Replacing MLCCs with Polymer Capacitors -- The Learning Circuit 42

Hey! Thanks for a great video. I was wondering if I can replace electrolyte caps on my old Pentium4 motherboard with polymers. Do I use the same capacitance or what parameters should I focus on? I've come to several ideas, for, example here

https://www.eevblog.com/forum/beginners/can-be-simple-electrolytic-capacitor-replaced-by-polymer-capacitor/msg750423/?PH…

it is said that capacitance of the polymer should be like 1/3 of the original electrolyte cap. Is it like that?

Thank You!

Hey! Thanks for a great video. I was wondering if I can replace electrolyte caps on my old Pentium4 motherboard with polymers. Do I use the same capacitance or what parameters should I focus on? I've come to several ideas, for, example here

https://www.eevblog.com/forum/beginners/can-be-simple-electrolytic-capacitor-replaced-by-polymer-capacitor/msg750423/?PH…

it is said that capacitance of the polymer should be like 1/3 of the original electrolyte cap. Is it like that?

Thank You!

nmanjofo , sadly, that thread is a great example of why I avoid that forum. You have to be careful when changing out polymers in LDO-circuits and switching circuits. The significantly lower ESR can result in control loop stability issues that the original design did not consider. A Pentium4 design is on the verge of the timeframe where polymers were just entering the market. When I was at the capacitor company, we had issues where a supply chain manager swapped out a polymer for a traditional electrolytic in production, which resulted in failures in the field. (As engineers became aware of this issue, design rules were put into place to address that.)

So, for the big bulk electrolytics, you can probably safely replace them. They are usually input filter caps to other regulators and are less sensitive to ESR issues. If they are on the output of a regulator, there could be more issues, especially in the POL (point-of-load) converters that get closer to the processor.

Unfortunately, as that thread shows, there are many design engineers (or want to be engineers) who have never measured the control loop stability of a power supply design.

tldr; can you replace them in a P4 mobo? Maybe. I think the larger electrolytics are probably a safe bet. The smaller surface mounts near a switching regulator, I would be more cautious.

it is said that capacitance of the polymer should be like 1/3 of the original electrolyte cap. Is it like that?

Unfortunately, it is difficult to make a blanket statement like that. It comes down to the fact that the lower ESR results in better high-frequency performance. However, without characterizing how the control loop responds to that change in impedance is going to be difficult. When designing-in a polymer into a new design, this is one of the advantages. You can typically use a much smaller polymer because of the higher effective capactiance. But in that case, you're able to account for the difference in overall impedance.