Author Topic: Switched power supply carnage  (Read 10582 times)

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Offline massterTopic starter

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Switched power supply carnage
« on: May 24, 2014, 08:46:12 pm »
hi
I need your advice on how a switched power supply from a DVD player got burned, almost literally. In other words, what was the chain of events that 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.








« Last Edit: May 25, 2014, 06:04:55 am by masster »
 

Offline FrankenPC

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Re: Switched power supply carnage
« Reply #1 on: May 24, 2014, 09:09:58 pm »
Considering the fuse literally exploded, I'd say the PS suffered a catastrophic surge.  Difficult to protect against that kind of thing.
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Offline Paul Price

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Re: Switched power supply carnage
« Reply #2 on: May 25, 2014, 01:12:48 am »
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.
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #3 on: May 25, 2014, 06:03:16 am »
The power supply died when 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.
« Last Edit: May 25, 2014, 06:10:03 am by masster »
 

Offline Rerouter

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Re: Switched power supply carnage
« Reply #4 on: May 26, 2014, 10:51:08 am »
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,
 

Offline Psi

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Re: Switched power supply carnage
« Reply #5 on: May 26, 2014, 10:57:27 am »
was it a 110V unit plugged into 240V, because that would do it  :-DD

Edit: na looks like its designed to handle 240V
« Last Edit: May 26, 2014, 10:59:03 am by Psi »
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Offline DrGeoff

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Re: Switched power supply carnage
« Reply #6 on: May 26, 2014, 11:05:57 am »
Check C5 and D8 for short circuit.
Was it really supposed to do that?
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #7 on: May 26, 2014, 11:28:07 am »
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,
By D8 'failing' you mean shorted? Because C5-R2-R3-D8 in parallel with transformer primary is only acting as a spike protection during shutdown.

Update
I 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?
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #8 on: May 26, 2014, 11:37:38 am »
Check C5 and D8 for short circuit.
I 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.
 

Offline Jay_Diddy_B

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Re: Switched power supply carnage
« Reply #9 on: May 26, 2014, 11:51:48 am »
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


 

Offline madires

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Re: Switched power supply carnage
« Reply #10 on: May 26, 2014, 12:03:11 pm »
I 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?

Please check the resistance of the primary coil of the flyback transformer.
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #11 on: May 26, 2014, 01:10:35 pm »
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.
NTC 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)

Please check the resistance of the primary coil of the flyback transformer.
It is 0.5 ohms.
 

Offline ResR

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Re: Switched power supply carnage
« Reply #12 on: May 26, 2014, 04:06:50 pm »
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?
« Last Edit: May 26, 2014, 04:13:52 pm by ResR »
 

Offline madires

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Re: Switched power supply carnage
« Reply #13 on: May 26, 2014, 04:15:20 pm »
It's a switching power supply that is designed to work with 110-240VAC 50/60Hz. Maybe the optocoupler (U2) is dead?

Yep, the next step is to check if the opto coupler's collector and emitter are shorted.
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #14 on: May 26, 2014, 04:53:27 pm »
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?
I 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.
 

Offline Zero999

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Re: Switched power supply carnage
« Reply #15 on: May 26, 2014, 06:57:39 pm »
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)
That shouldn't be a problem. If it's designed correctly, the input should be able to handle 250VAC or more.
 

Offline Bryan

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Re: Switched power supply carnage
« Reply #16 on: May 26, 2014, 08:16:52 pm »
This link may be of some assistance.

http://www.repairfaq.org/sam/smpsfaq.htm#smpstslbt
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Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #17 on: May 26, 2014, 09:13:35 pm »
Thank you, Bryan.
 

Offline Riotpack

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Re: Switched power supply carnage
« Reply #18 on: May 27, 2014, 04:39:24 am »
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.
 

Offline Riotpack

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Re: Switched power supply carnage
« Reply #19 on: May 27, 2014, 04:43:24 am »
How is C5 looking?
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #20 on: May 27, 2014, 06:24:01 am »
How is C5 looking?
C5 is looking good, no short.
 

Offline madires

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Re: Switched power supply carnage
« Reply #21 on: May 27, 2014, 11:42:24 am »
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.

That would be R11 (100k) and R13 (680) in series which is fine. Could you please also check C9.
 

Offline tautech

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Re: Switched power supply carnage
« Reply #22 on: May 27, 2014, 11:59:52 am »
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.pdf

You will see the Enable/Undervoltage pin biased to rectified mains for assured startup as is normally done.
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Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #23 on: May 27, 2014, 05:13:59 pm »
That would be R11 (100k) and R13 (680) in series which is fine. Could you please also check C9.
I 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.

Check this datasheet:
http://www.powerint.com/sites/default/files/product-docs/tny274-280.pdf

You will see the Enable/Undervoltage pin biased to rectified mains for assured startup as is normally done.
Yeah, 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."
 

Offline massterTopic starter

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Re: Switched power supply carnage
« Reply #24 on: May 27, 2014, 05:48:30 pm »
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 is I don't have any TNY IC in libraries  and I'm not sure about the Transformer too. Do you have any suggestion?
« Last Edit: May 27, 2014, 05:56:12 pm by masster »
 


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