Help to identify switching transformer for replacement - Page 1
Forum section: electronics / repair

author: dnotq
author_rank: Contributor
timestamp: May 25, 2024, 07:02:53 am
content: This transformer (see image) is from an APC BR1000MS USP, and the secondary shorted. I cannot find any information on what this transformer is, other than EE19 seems to designate a pin-out and size specification (I'm not really up to speed on switching transformers)?Any insight as to what the actual specifications of this transformer are would be greatly appreciated. The UPS is dead without this since it powers the MCU and other control circuits, and of course I'm 2 months over the warranty. The unit just died under normal conditions.Text on the part from image in case there is a problem:CLASS 130(B) SBI4.2430-0843-Z BCK-EE19-1571BJingquanhua NERE E208707

author: wraper
author_rank: Unknown
timestamp: May 25, 2024, 07:08:48 am
content: Why do you think it's shorted? SMPS transformer secondary winding resistance should be near 0 Ohm, not to say it would be extremely unusual for secondary winding to fail. Also SMPS transformers are custom made in 99% of cases.

author: jonpaul
author_rank: Super Contributor
timestamp: May 25, 2024, 02:51:18 pm
content: "shorted"What instrument to measure, VOM? RLC meter? Exact reading?Most faults are due to shorted FETs or rectifiers.99.9999% it is NOT the transformerJon

author: dnotq
author_rank: Contributor
timestamp: May 26, 2024, 12:03:23 am
content: Meter is a Fluke 189, secondary is 0.008 ohms. But after posting I did check the secondary for frequency and it shows ~55KHz, same as the primary. DC average is 1V, AC average is 4V. Primary side driver is a TNY278PN.The resistance across the secondary rectifier diode (marked with "ON" in a circle (assume onsemi), M35 B320) is about 1.2 ohms in both directions, so I'll be removing it and confirm it has failed. I'm also going to replace all electrolytic caps on the secondary side.I will be very happy if it is not the transformer, since it seems that would be very difficult to find a suitable replacement.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2024, 01:03:31 am
content: Easier to take apart and rewind than find a replacement, I'd say. Magnet wire is widely available, and the yellow tape might be 3M #56 or similar.The TNY nearby suggests it's merely a flyback transformer for auxiliary supplies in the unit; tracing the circuit will yield what voltages it's producing.Tim

author: wraper
author_rank: Unknown
timestamp: May 26, 2024, 01:54:24 am
content: Quote from: dnotq on May 26, 2024, 12:03:23 amMost likely SB320, and certainly looks like it failed.

author: floobydust
author_rank: Super Contributor
timestamp: May 26, 2024, 02:09:17 am
content: I use a ring tester to find shorted SMPS transformers which is rare that happens. I have seen the primary get damaged, cooked with shorted-turns if the IC shorts, on Harman Kardon AVR3700 AV receivers it is a common happening.OP sounds like a shorted rectifier maybe. I would also replace any low value 22-47uF 50V feeding power to the TNY278, if that is used.

author: dnotq
author_rank: Contributor
timestamp: May 30, 2024, 04:47:06 am
content: Well, it is always a bummer when you replace parts you think failed, and it does not fix the problem... Replaced the B320 diode, and eight electrolytic caps on the secondary side, and same symptoms.The B320 I pulled out tested good, as well as all the pulled caps. With the diode out I measured the same 1.2 ohms across the pads where the diode sits.Without a schematic I'm not sure this is worth throwing more time and money into. Very frustrating though, that the problem is just the low-voltage regulator for the control circuit, rather than something in larger power output side that I would not want to mess with anyway. The battery is still good too, and there is a really big transformer in the unit.Bah! What ever happened to getting a schematic with your electronic stuff? (that is a rhetorical question, I'm just ranting at this point)

author: jonpaul
author_rank: Super Contributor
timestamp: May 30, 2024, 06:00:02 am
content: small UPS are consumer junk from China.Seldom worth the fix time, cost.We junk even 2..5 KVA APC.No schematics or parts avail.Junk it, new ones $50..100j

author: wraper
author_rank: Unknown
timestamp: May 30, 2024, 07:17:17 am
content: Quote from: dnotq on May 30, 2024, 04:47:06 amIt means that SMPS most likely is not the problem and and it's what it's powering that is shorted. Usually an easy find if you have a thermal camera and apply low voltage to the rail from a lab PSU.

author: fmashockie
author_rank: Frequent Contributor
timestamp: June 01, 2024, 05:53:34 am
content: These units are crap anyway. I'm surprised to see one with a SMPS failure though. I've repaired a few APC BR1000 units and usually the pure sine wave models have a problem with the large AVR/inverter transformer. They are destine to fail right from the start. I've done videos on my YT channel repairing these and HI-POT testing the large AVR transformers. For some reason, the BR1000MS (pure-sine wave models) are the worst. I've tested them brand new and they will fail HI-POT test. If you get one of these bad transformers (or if they go bad over time) it will eventually short the inverter MOSFETs - usually when you need it most such as when it is switching from mains to battery backup. It can fail in spectacular fashion, too. The 40A fuses that are there to protect the battery don't doing either to help either (never find them open after this failure). However, you can repair them and get them to last a good while as long as you replace that large inverter transformer with a different make. I've used ones from the APC simulated sine wave models and then it works just fine. For some reason they are made better.Failure likely isn't with the SMPS transformer. Take a look at the TNY278PN and its datasheet. If you have a scope you can check to make sure it is switching properly. You can also use your meter to check it as well. Check for ~5.85VDC on the BP/M pin. If it drops below, 4.9VDC the internal MOSFET is not switching.That SMPS outputs ~24VDC and then it gets stepped down to 5V and 3.3V for the microcontroller