Poly-modding - Can ESR Be Too Low - Page 1
Forum section: electronics / projects

author: zape
author_rank: Contributor
timestamp: February 15, 2018, 02:50:19 am
content: I have a super socket 7 motherboard with some bulging capacitors so I decided to recap it completely.It still works but shows artifacts on screen.To improve reliability and reduce ripple (that potentially causes that artifacts) I decided to use ultra-low esr polymer capacitors instead of electrolytic.I've already ordered it on but then I came around this paper (attached) that says that TOO LOW ESR MAY BE BADNow I'm not sure about this project and risks.The document is a bit too technical for me but I found the datasheet of the VRM used on the board LX8383A (attached)According to the datasheet I have to use caps 3x 1500?F, 6.3V 6MV1500GX from Sanyo MV-GX SeriesI looked up the specs and 6MV1500GX ESR is 44mOhmThe polymer caps I ordered ESR of 5mOhm which is more than8 times less.The question is: Will it work?If not, will it burn the motherboard? what are the risks?

author: amyk
author_rank: Super Contributor
timestamp: February 15, 2018, 02:56:28 am
content: Think of the ESR as damping the rate at which the caps can charge and discharge. To use an analogy, too low ESR would be like having overly sensitive steering in a car --- the VRM control loop can become unstable and oscillate, and that definitely won't be good for the CPU.

author: BradC
author_rank: Super Contributor
timestamp: February 15, 2018, 03:22:25 am
content: Did you read the data sheet ?Specifically :n order to ensure good transient response from the power supplysystem under rapidly changing current load conditions, designersgenerally use several output capacitors connected in parallel. Suchan arrangement serves to minimize the effects of the parasiticresistance (ESR) and inductance (ESL) that are present in allcapacitors. Cost-effective solutions that sufficiently limit ESR andESL effects generally result in total capacitance values in the rangeof hundreds to thousands of microfarads, which is more thanadequate to meet regulator output capacitor specifications. Outputcapacitance values may be increased without limit.You'll be fine. I've repaired a few 7/Super7 boards with failed caps that generally led to smoked inductors and blown switching fets. I just ordered what was the best (highest temperature, lowest ESR vs highest lifetime) replacement part I could find and called it good. I binned the last of them late last year after running 24/7 since about 1999 (ish).

author: zape
author_rank: Contributor
timestamp: February 15, 2018, 03:32:52 am
content: Quote from: BradC on February 15, 2018, 03:22:25 amDid you use Electrolytic caps?Lowest Electrolytics I found were around 20mOhmThis is like 2 times less the original. Mine are 5mOhms. This is 8 times less.I was not concerned before I found the paper I attached next to the datasheet.

author: BradC
author_rank: Super Contributor
timestamp: February 15, 2018, 03:48:06 am
content: Quote from: zape on February 15, 2018, 03:32:52 amYes I did use Electros. Polymers we're not really a big thing back when I did those.I read the paper and the datasheet before I replied.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2018, 04:54:34 am
content: Depends on the controller type.A voltage mode controller depends on ESR for compensation. Too low and it oscillates.Tim

author: BradC
author_rank: Super Contributor
timestamp: February 15, 2018, 09:20:56 am
content: Quote from: T3sl4co1l on February 15, 2018, 04:54:34 amLinear regulator?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2018, 02:55:54 pm
content: Quote from: BradC on February 15, 2018, 09:20:56 amLinear or switching.Tim

author: zape
author_rank: Contributor
timestamp: February 15, 2018, 06:09:49 pm
content: Well I guess I will just try and see if it oscillates.Do you think oscillation may cause any damage to the CPU or motherboard?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2018, 09:38:45 pm
content: Yes, or instability. You'd need an oscilloscope to verify, unless you're lucky enough to have it oscillating at an audible frequency (which is fairly possible, that type of control usually has a cutoff in the 5-10kHz range).Tim

author: zape
author_rank: Contributor
timestamp: February 26, 2018, 07:55:52 am
content: Replaced the caps. It works.