Switched power supply carnage - Page 1
Forum section: electronics / beginners

author: masster
author_rank: Regular Contributor
timestamp: May 24, 2014, 08:46:12 pm
content: hiI need your advice on how a switched power supply from a DVD player got burned, almost literally. In other words, what wasthe chain of eventsthat could have led to destruction of power supply. By helping me with this analysis, I will be able to understand better how a SPSU works and what are its weak points.Here is all necessary data:- power supply is from Philips DVP5990K DVD Player.- I've posted below the schematics and I've marked in red the components that are toasted (see also photos of carnage, chip exploded, fuse exploded, copper trace exploded). I have a hunch that the real culprit is still not discovered yet. My guess is C1 and/or C2, but I don't have an ESR/C meter.

author: FrankenPC
author_rank: Unknown
timestamp: May 24, 2014, 09:09:58 pm
content: Considering the fuse literally exploded, I'd say the PS suffered a catastrophic surge. Difficult to protect against that kind of thing.

author: Paul Price
author_rank: Super Contributor
timestamp: May 25, 2014, 01:12:48 am
content: The cause of failure is difficult to determine, but a voltage surge is certainly a best choice among possibilities.The "Chain of Events" is not a link to any sad story leading to the fireworks, so it tells nothing.You have me arriving at the scene of the crime, broken glass and blood are seen on the floor, nobody's talking, no one has seen anything.

author: masster
author_rank: Regular Contributor
timestamp: May 25, 2014, 06:03:16 am
content: The power supply diedwhen I switched on the DVD. So I guess it's quite improbable a power surge to occur exactly the same millisecond... Not to mention the TV was on and plugged into the same wall socket and is OK.What if C1 or C2 suddenly got shorted from heat/age/cheapness/whatever ? Would that explain all the mess? If yes, without using an ESR/C meter (I don't have one) is there any other basic test I can do on those caps? I took them out.

author: Rerouter
author_rank: Super Contributor
timestamp: May 26, 2014, 10:51:08 am
content: D8 being blown is the most interesting to me, as it seems the least likely to fail, but it failing would then, apply a negative voltage to the switcher, fry it then have it short, blowing the rectifier and fuse for a thought,

author: Psi
author_rank: Super Contributor
timestamp: May 26, 2014, 10:57:27 am
content: was it a 110V unit plugged into 240V, because that would do itEdit: na looks like its designed to handle 240V

author: DrGeoff
author_rank: Frequent Contributor
timestamp: May 26, 2014, 11:05:57 am
content: Check C5 and D8 for short circuit.

author: masster
author_rank: Regular Contributor
timestamp: May 26, 2014, 11:28:07 am
content: Quote from: Rerouter on May 26, 2014, 10:51:08 amBy D8 'failing' you mean shorted? Because C5-R2-R3-D8in parallel with transformer primaryis only acting as a spike protection during shutdown.UpdateI replaced all diodes fried (D1-D4, D8), fuse, C1, C2 (22uF/450V instead of 15uF/450V) and U1 (TNY276 instead of TNY176). Problem is that none of the output DC voltages are present. Outputs are not shorted, there is no AC voltage on transformer secondary. I am stuck. Any thoughts?

author: masster
author_rank: Regular Contributor
timestamp: May 26, 2014, 11:37:38 am
content: Quote from: DrGeoff on May 26, 2014, 11:05:57 amI replaced D8 and C5 is good.C5-R2-R3-D8 comprise the clamp circuit, limiting the leakage inductance turn-off voltage spike on the DRAIN pin to a safe value.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 26, 2014, 11:51:48 am
content: Hi,Check the NTC, TR1, it should have a fairly low resistance, less than 50 ohms.Voltage across C1 & C2 should be 1.41 x line voltage.Regards,Jay_Diddy_B

author: madires
author_rank: Super Contributor
timestamp: May 26, 2014, 12:03:11 pm
content: Quote from: masster on May 26, 2014, 11:28:07 amPlease check the resistance of the primary coil of the flyback transformer.

author: masster
author_rank: Regular Contributor
timestamp: May 26, 2014, 01:10:35 pm
content: Quote from: Jay_Diddy_B on May 26, 2014, 11:51:48 amNTC is 33 ohms.What bothers me is that instead of (220*1.41)=310V DC on C1 & C2, I measured 328V DC with no load (separated from U1 and Tr)Quote from: madires on May 26, 2014, 12:03:11 pmIt is 0.5 ohms.

author: ResR
author_rank: Regular Contributor
timestamp: May 26, 2014, 04:06:50 pm
content: It's a switching power supply that is designed to work with 110-240VAC 50/60Hz, your wall socket may not have exact 220VAC, specially if the 10/0,4kV transformer feeding your household is old - it tends to have higher output voltage with age (once there was 245VAC instead of 230VAC in a wall socket few months before the transformer died here in Estonia). Maybe the optocoupler (U2) is dead?

author: madires
author_rank: Super Contributor
timestamp: May 26, 2014, 04:15:20 pm
content: Quote from: ResR on May 26, 2014, 04:06:50 pmYep, the next step is to check if the opto coupler's collector and emitter are shorted.

author: masster
author_rank: Regular Contributor
timestamp: May 26, 2014, 04:53:27 pm
content: Quote from: ResR on May 26, 2014, 04:06:50 pmI thought everybody will assume the first thing I did was to measure mains voltage. It was 220V AC as in the last 28 years. So 220V AC in -> 328V DC out.I measured in circuit the optocoupler with an ohmmeter. Pin 1-2 measurement exhibits the normal characteristic of a diode. Pin 3-4 resistance is 100Kohm.

author: Zero999
author_rank: Super Contributor
timestamp: May 26, 2014, 06:57:39 pm
content: Quote from: masster on May 26, 2014, 01:10:35 pmThat shouldn't be a problem. If it's designed correctly, the input should be able to handle 250VAC or more.

author: Bryan
author_rank: Frequent Contributor
timestamp: May 26, 2014, 08:16:52 pm
content: This link may be of some assistance.http://www.repairfaq.org/sam/smpsfaq.htm#smpstslbt

author: masster
author_rank: Regular Contributor
timestamp: May 26, 2014, 09:13:35 pm
content: Thank you, Bryan.

author: Riotpack
author_rank: Regular Contributor
timestamp: May 27, 2014, 04:39:24 am
content: The FET in U1 went short circuit. Why? usually over voltage, anything from mains spike to inductive kickback from a bad power switch "Double bouncing" to a failed snubber component.

author: Riotpack
author_rank: Regular Contributor
timestamp: May 27, 2014, 04:43:24 am
content: How is C5 looking?

author: masster
author_rank: Regular Contributor
timestamp: May 27, 2014, 06:24:01 am
content: Quote from: Riotpack on May 27, 2014, 04:43:24 amC5 is looking good, no short.

author: madires
author_rank: Super Contributor
timestamp: May 27, 2014, 11:42:24 am
content: Quote from: masster on May 26, 2014, 04:53:27 pmThat would be R11 (100k) and R13 (680) in series which is fine. Could you please also check C9.

author: tautech
author_rank: Super Contributor
timestamp: May 27, 2014, 11:59:52 am
content: These switching supplies can be touchy to get going after a blow up.Often designs are minimized and that can cause problems when components are substituted.Check this datasheet:http://www.powerint.com/sites/default/files/product-docs/tny274-280.pdfYou will see the Enable/Undervoltage pin biased to rectified mains for assured startup as is normally done.

author: masster
author_rank: Regular Contributor
timestamp: May 27, 2014, 05:13:59 pm
content: Quote from: madires on May 27, 2014, 11:42:24 amI have no C/ESR meter, so I changed it with a new one. No success.But something caught my eye in the Tr. secondary side. If the +5V circuit is dead (D8 or maybe C7/C8) then there is no feedback whatsoever and there is no switching. The Schottky diode looks beefy enough and is acting normal when measured on diodes scale. But those caps... yeah... I definitely have to get a C/ESR meter.Quote from: tautech on May 27, 2014, 11:59:52 amYeah, but they also say: "If there is no external resistor connected to this pin, TinySwitch-III detects its absence and disables the line undervoltage function."

author: masster
author_rank: Regular Contributor
timestamp: May 27, 2014, 05:48:30 pm
content: I have an idea that may help me study and troubleshoot this dead SMPS. What if I simulate it in a simulation software and play with various parts to see what breaks? I own Proteus and Multisim. Problem isI don't have any TNY IC in librariesand I'm not sure about the Transformer too. Do you have any suggestion?

author: madires
author_rank: Super Contributor
timestamp: May 27, 2014, 06:36:00 pm
content: Quote from: masster on May 27, 2014, 05:13:59 pmIf the 5V rail is dead the TL431 doesn't sink current through the optocoupler's LED. That means, the optocoupler's transistor doesn't sink current. The TNY276's datasheet mentions something about auto-restart in case of an open loop, output overload or output short circuit. If the EN/UV pin isn't pulled down for 64ms, switching will be disabled for 2.5s. Checking the 5V secondary including the TL431 based feedback is a good idea.

author: masster
author_rank: Regular Contributor
timestamp: June 01, 2014, 03:46:17 pm
content: I give up. I changed the Schottky diode and both capacitors on 5V rail, then the optocoupler. NOTHING. Zero on all outputs.What have I learned from this?SMPSes are a POS. Ten times more unreliable and fussy than a linear PS. I wasted a lot of time with that little garbage PS. They work like standing in balance on the tip of a needle. I will build myself instead a nice external linear PS, put it in a small plastic box and use a 4 pins connector to feed 5V, 12V, and -22V to DVD. That will give me years of peace.\Thank you to all contributors.

author: nsayer
author_rank: Regular Contributor
timestamp: June 02, 2014, 10:22:32 pm
content: Oh, I dunno. If you're going to replace the whole thing, why not design your own switching supply? I'm new to it as well, but from what I see it's not to hard to do it properly.You might consider going ahead with a nice, beefy +5v supply, and then use switchers to make the other two voltages from that. Do you know what current requirements there are for the device?