UPS cooling Mod for continuous full load usage? - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 11, 2017, 07:45:01 am
content: So I want to use my UPS as an inverter using a 120 AH car battery.It is rated at 600VA or 300W assuming a mediocre power factor of 0.5. So Theoretically connected to a computer it should provide at least 300W even though It shut down with a 250W load during a graphics benchmark session.Even when loaded with only 150W and connected to a 12 V power supply set to 13.6V, to remove the case of having the UPS shutdown due to voltage sag, it only manages to run for a couple of minutes.Upon further inspection, I noticed the two mosfets mounted to tiny heatsinks were heating up to around 150C so that could be the issue and the UPS is shutting down to prevent damage to the components.I also realized there was a capacitor in between the two heatinks that was also getting very hot and started venting. I removed it from the circuit and the UPS was still working.Would relocating the mosfets on a 100W TDP CPU heatsink with a fan be the way to go for constant use under full load or do I need to cool other components as well?

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2017, 11:17:22 am
content: If it does not already have forced air cooling, then just adding a fan and replacing the capacitor with a better one should be enough.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 11, 2017, 11:21:13 am
content: Sure I can add a fan since it has no forced air cooling.How can I tell if the replacing capacitor is better? It's a 4400uF 16V Electrolytic cap.I don't think I can find a polymer with the same capacitance as that would qualify as better in my book...

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: May 11, 2017, 11:40:34 am
content: Quote from: drummerdimitri on May 11, 2017, 11:21:13 amLower ESR, higher temperature rating, from a reputable brand. Many would suggest Panasonic, Nichicon, Vishay, ...

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2017, 11:59:32 am
content: Quote from: drummerdimitri on May 11, 2017, 11:21:13 amUse an aluminum electrolytic capacitor which has a higher ripple current rating and maximum operating temperature. This is also associated with physically larger capacitors so use a higher voltage part that still fits in the original spot if possible.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: May 11, 2017, 12:46:57 pm
content: I will try to find a better replacement then.Would it be a problem to relocate the capacitor elsewhere on the board if it is too large to fit in the original one's place by wiring it up with speaker cable?

author: X
author_rank: Regular Contributor
timestamp: May 11, 2017, 01:47:55 pm
content: Quote from: drummerdimitri on May 11, 2017, 12:46:57 pmAssuming this is the huge reservoir cap used to buffer for the backup source switch time, I would take this one step at a time. Start with the extra cooling first. Much better than going through the rigmarole of changing the capacitor when you never had to do it in the first place.In your application, this capacitor will most likely always remain charged anyway because your battery is providing the power.If this is going to be used anywhere other than in a motor vehicle with its engine running, I would also consider using a different sort of battery if possible, as many car batteries cannot be discharged too deeply without significantly reducing their lifespan. These batteries are designed for starting the engine and providing a reservoir for the sparks and accessories while being charged by an alternator. If you can, I would strongly advise a deep-cycle lead-acid battery (even a gel cell or AGM type) or LiFePO4 or something similar.

author: David Hess
author_rank: Super Contributor
timestamp: May 11, 2017, 04:20:07 pm
content: Quote from: drummerdimitri on May 11, 2017, 12:46:57 pmIf you use heavy wires and keep them short this should be acceptable. It depends on exactly what the capacitor is doing.The fact that the capacitor was running hot and venting implies that it is supporting a high ripple current. I doubt a bulk input decoupling capacitor would have that high a ripple current unless a large input inductor was used for filtering but the output coupling capacitor in a half bridge sure would because all of the output current flows through it and failure of an aluminum electrolytic coupling capacitor in a half bridge circuit do to excessive ripple current is common.For reliable operation, it might be cheaper to use an inverter designed for continuous operation, an online UPS, or an offline UPS which supports external battery cabinets. These can be found used for reasonable prices and usually the only problem they have if any is bad batteries and maybe a worn out fan.

author: james_s
author_rank: Super Contributor
timestamp: May 11, 2017, 07:30:26 pm
content: I would probably add additional heatsinking in addition to forced air cooling. Many UPS's take the thermal mass approach, the heatsink is relatively thick aluminum without a lot of surface area. Works fine for short periods but it doesn't dissipate the heat very well.

author: X
author_rank: Regular Contributor
timestamp: May 12, 2017, 02:32:13 pm
content: Quote from: David Hess on May 11, 2017, 04:20:07 pmThe OP mentioned the cap is in between the two heatsinks, and if he is running the UPS to the point where the heatsinks are hitting 150degC, this could be more than enough to heat the capacitor well beyond its maximum operating temperature if it is close enough to at least one of the heatsinks.

author: madires
author_rank: Super Contributor
timestamp: May 12, 2017, 03:03:44 pm
content: Small UPS' are built to be cheap and to run only a few minutes. You mght have to beef up some traces on the PCB, larger caps were already mentioned, and maybe better or multiple MOSFETs. The transformer also needs proper ventilation.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 13, 2017, 01:49:30 am
content: 300W inverters are cheap. Just buy one.Here's the biggest collection of inverter reviews/teardowns/repairs I know of:https://www.youtube.com/user/knurlgnar24/videosThere are also a few UPS mods but that only makes sense if you managed to get one of the big ones for free or cheap.

author: calmtron
author_rank: Regular Contributor
timestamp: May 13, 2017, 08:09:00 am
content: The transformer is usually not rated for continuous operation either. I'm a bit suspicious of these 600VA jobbies after seeing a particulary cheap one catch fire after a short power outtage.