APC BN650M1 UPS repair - Page 1
Forum section: electronics / repair

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 31, 2020, 12:54:44 am
content: I've been repairing a broken APC BN650M1 UPS.It failed in a mode where the only thing it does when powered on is to emit a continuous beep after pressing its "on" button. There is no 120V AC output from it. Replacing the 12V battery made no difference. The original 12V battery was somewhat weak but not totally dead. I obtained a new 12V battery and I'm using it for testing while I do the repairs.This is a popular low-cost UPS model, often used to supply power to cable modems, small Ethernet switches, IP telephone sets, and USB charging devices for mobile phones and tablets. I suspect it is a bit too small to serve as a reliable UPS for a desktop PC. This model is still in current production. I saw a shelf full of them yesterday at a local office supply store. Street price is around $80.This is a "two stage" UPS:The first stage is a switching DC-DC converter, 12V DC input, 160V DC output.The second stage is an "H-bridge" of 4 MOSFETs which converts the 160V DC into a very rough approximation of a 60Hz AC output. The output waveshape is a "square wave" with dead zones (0V) between the upper and lower parts of the square wave.There are no large "iron core" transformers inside this unit.It does contain two small AC to DC switching power supplies. These take 120V AC input and generate 12V outputs. One of them operates directly from the incoming 120V AC line. The other (slightly larger) AC to DC supply operates only when the 120V output from the UPS is switched on. This larger AC to DC supply also powers the USB charging port.The following detailed teardown article from Tom's hardware guide is extremely informative:http://www.tomshardware.com/picturestory/732-apc-bn650m1-ca-ups-tear-down.html#s40What I found inside my failed unit:First of all, the PC board inside my unit looks ***exactly*** like the PC board shown in the photos of the Tom's hardware guide teardown.Next, this is what I found when I diagnosed it:The two 12V to 160V DC-DC step-up converter MOSFETs were both shorted. Part # is PSMN4R6-60PS. Rated for 100A at 60V. RDS(on) is 4.6 milliohms. Not bad for a MOSFET in a TO-220 package that costs only $ 1.40 each in small quantities.The foils on the PC board connected to the drains of these two MOSFETs had both vaporized right next to the drain terminal pads. No large burned areas, but just a small 1mm wide section that went "missing" from each foil. The 30A fuses on the PC board were still good. Oddly these 2 fuses are connected in parallel, which would enable 60A of battery current to flow into the DC-DC converter. So, in this case, the foils on the PC board vaporized in order to protect the fuses. I intend to remove one of these fuses, leaving only a single 30A fuse between the 12V battery and the PC board. I think that will provide enough power for the loads I intend to drive with this small UPS. I expect my total maximum load to be about 120W, which corresponds to <15A of battery current. A 30A fuse will hold that just fine.I replaced the vaporized foils with 16ga wires soldered to the bottom of the PC boardChecking the remainder of the power devices, I discovered 2 shorted "H-bridge" MOSFETs. The other 2 MOSFETs in the H-bridge were still good. The 4 devices in the H-bridge are IRF630. 200V, 9A, RDS(on) .4 ohm, 120W. As with the DC-DC MOSFETs, these are TO-220 packages. These cost about $ 0.80 each in small quantities.I replaced only the 2 that were shorted. I'll probably also replace the other 2 H-bridge MOSFETs before I button up this unit.At this time the UPS is fully functional again. It drives a 60W incandescent lamp (as a test load) without any problems. It charges the battery. And nothing runs hot with the 60W load.I will do plenty of additional checking and evaluation before I apply any heavier loads to it.Has anyone repaired one of this model, or a similar model?I'd like to hear your comments.-E

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 31, 2020, 09:02:57 pm
content: Follow-up: My APC BN650M1 is all finished and reassembled.There were no other faulty components except for the 4 shorted MOSFETs that I replaced. All electrolytic capacitors on the PC board had "in-spec" ESR values. All internal voltages and waveforms looked good. I did remove one of the two paralleled 30A fuses in order to provide a bit more protection from short circuits on the 12V battery rail.I tested it with various loads up to 100W. My application for it has a maximum load of <100W with an average of about 35W. So I think it will work fine.-E