The Mystery of the Exploding Caps - Page 1
Forum section: electronics / projects

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 30, 2014, 05:34:35 pm
content: So a couple years ago I worked at EVGA, a company that normally makes graphics cards and motherboards. I was tasked with helping on the technical end for bringing their new ATX PSU line to market. Before 2012 EVGA only had one power supply, the SR-2 1200W, which was a rebranded Antec TPQ-1200, which was an OEM product made by Enhance. I was working in their Taipei office, where the bulk of their engineering is.While we were mainly focused on the new 1500W PSU, and the new 550-750W units, the SR-2 1200W was still a current product. One day in the lab, one of my coworkers was running some tests on a unit, and when he was done he turned off the DC load on the +12V rail. However, he left on the load on the +3.3V rail, which was set to 1A. That was the only load on the whole 1200W power supply: 1A @+3.3V.It ran for an unknown amount of time, at least half an hour. Then,BANG.The fan blew chunks of gunk out the back of the unit and the PSU shutoff. The whole lab filled with the stink of capacitor electrolyte. We immediately shut down all the equipment, aired out the lab, and notified the CEO.When we opened the unit we found that both of the primary bulk capacitors, Nippon Chemi-Con 450V 560uF 105C rated components, had exploded into confetti. A quick probing showed that the PWM and PFC fets still tested good, though they were tested in-circuit. We discussed with the CEO, and he said since this was the only reported instance, that the usage scenario was unlikely to occur in the field, and since the product would soon be EOL, that we wouldn't waste time trying to diagnose the cause of failure, and instead focus on the new products. An assessment I feel was fair.However, it still bugs me to this day. What could cause a 1200W PSU under a load of 1A @ 3.3V, to explode that way? And it appeared that the primary caps were the only major component damaged. I've never been able to figure it out. Maybe someone here might have a clue?I don't have a schematic, but there is a teardown and performance report available:http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=177The EVGA one is identical except for a few extra output cables and the label.Topology is a full bridge design with interleaved PFC running on an Intersil ISL6752AAZ and a UCC28070 respectively. +5V and +3.3V rail are generated by buck regulators from the +12V, industry calls this "DC-DC".

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 30, 2014, 06:32:47 pm
content: PFC lost feedback and had no backup overvoltage protection. If it's a unit strictly for home use in the US, PFC just decreases efficiency and adds cost while adding more possible failure modes. It's just too bad that consumers expect PFC nowadays in any good PSU.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 30, 2014, 07:11:40 pm
content: FCC doesn't care about conducted emissions until it gets above 150kHz. The mains frequency is far away from that, but PFC operates at a frequency where the low harmonics would fall right into the conducted EMI region.Even in a commercial environment, PFC is sometimes not needed as the UPS does the PFC. I worked on some Dell Poweredge servers that had the option to disable PFC in the BIOS. It's also possible to play some tricks like switching off the boost when the instantaneous input voltage is below a threshold (which yields a low instantaneous efficiency) to reduce the efficiency impact of PFC and still have a pretty good power factor. Marketing just needs to get a hold of that and somehow justify the increased parts cost. Might be a bit hard to disable PFC entirely on 120V input and still get a universal input range, but overseas, this "partial PFC" can make a lot of sense.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 30, 2014, 07:44:03 pm
content: Cheap ATX PSUs need to work in both home and commercial environments; while expensive ATX PSUs need to work in both home and server environments. So in both cases PFC is required. As well, PFC helps with EMI, allows for easier logistics selling to multiple countries (no voltage switch which needs to be set to the correct setting for each country or risk astronomical returns), it provides additional protection from input surges and transients, etc. It's a needed feature on any PSU over US$25.So you're saying the PFC lost feedback, voltage rose and exceeded the capacitor ratings? I guess that's possible.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 31, 2014, 04:49:46 am
content: What about a switch to enable and disable PFC? Ship the units with the switch on and make a note in the user manual to turn it off for home use. On the really fancy units with USB link, make it a software option just like the Dells I mentioned earlier.

author: calexanian
author_rank: Super Contributor
timestamp: March 31, 2014, 06:19:39 am
content: Yeah, sounds like loss of feedback loop. If its an opto type then that could be your problem.. Chemi Cons don't go bang unless they have a good reason to.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 31, 2014, 06:43:29 am
content: it provides additional protection from input surges and transients.My experiance indicates that the PFC MOSFET upfront is very susceptable to mains surges. From a long term reliability point of view I would definately not go for ACTIVE PFC if I had the choice.I also don't agree with the EMI argument regarding active PFC. I have found that active PFC creates terrible additional EMI. This is due to the reverse recovery time of the boost diode. When the PFC stage is operated in continuous current mode (as most are) the boost diode has to be very high speed in order not to cause a substantial reverse recovery current through the MOSFET on turn on. Even the highest speed FREDs struggle to meet this requirement. The resulting momentary reverse current creates terrible EMI. I have had to slow down the MOSFET turn on in order to reduce the EMI. I think SiC diodes should prove much better but I haven't yet tried them.Dick

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 31, 2014, 08:06:01 am
content: Quote from: NiHaoMike on March 31, 2014, 04:49:46 amDoesn't work for us. The PWM stage is usually designed for work at around 300-400VDC, the output of the boost stage. If we allowed PFC to turn off that means it the PWM section would to be able to operate at as low as 88VDC (minimum rating 90VAC minus bridge rectifier voltage drop). Designing the main PWM stage to function with that wide of a voltage range would be extremely difficult, especially for some of the more advanced topologies like ZVS phase-shift full bridge, used on the top tier units. It just makes everything massively more complicated than it needs to be and probably wouldn't improve efficiency or reliability much.PFC is fine.Anyway, I guess loss of feedback was the culprit. Maybe we can work in some protections against that in our newer units...

author: peter.mitchell
author_rank: Super Contributor
timestamp: March 31, 2014, 08:51:04 am
content: Quote from: Phaedrus on March 31, 2014, 08:06:01 amIt may not be very hard, look at the common failure modes for optos ect, design that the output goes low when that failure mode occurs rather than going high.That or use 500v rated caps (probably already are), make PFC boost boost to 400V~, slap 450-500V MOVs across the cap, heat shrinked to a thermistor driving the enable pin of the controller/supervisor.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 31, 2014, 01:23:52 pm
content: Quote from: Phaedrus on March 31, 2014, 08:06:01 amThe Dell PSUs are universal range and allow disabling PFC. Not sure what it would do if you disabled it on a 120V input. Maybe something inside reconfigures itself, or it just switches the PFC from high power factor mode to high efficiency mode. On 240V input, it should be indeed possible to disable PFC at least at part load. (Maybe mark the switch as "auto PFC" and "PFC on".)

author: Kjelt
author_rank: Super Contributor
timestamp: March 31, 2014, 02:01:08 pm
content: This is totally not my expertise but I can imagine that there needs to be a minimum load of at least a couple of % to make the PSU work within its design?I mean 3,3W on a 1200W PSU that is a 0.275% load.Maybe some expert can answer if there is a minimum required load necessary for SMPS to operate ok or not?

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 31, 2014, 02:04:30 pm
content: Any good ATX PSU will operate with no load. Nowadays, that's actually tested to ensure compatibility with Haswell systems.