SMPS in a Samsung Blu-ray

[tombi]

Hello,

My Samsung Blu-ray (BD-C5900) died (no lights - no response) and I've been trying to repair it. I'm pretty new to this and even newer at repairing SMPS supplies.

There was no output at all from the PSU. The rectifiers output looked ok and the bulk capacitor had 312v on it.

The thing is managed by one of these Infineon chips and the circuit diagram of the primary looks to be straight out of the manual.

http://www.infineon.com/dgdl/Datasheet_ICE3BR1765J_v2_3_19Nov2012.pdf?folderId=db3a304412b407950112b408e8c90004&fileId=db3a30431a5c32f2011a7a6a5b73688b

I found the secondary winding that provides power to the chip and I found a spot where it appears to have arced over from the 312v track from the rectifier!. They are right next to each other! There is a corresponding burn mark on the case.

I guessed that this arc killed the chip, I ordered a replacement and replaced it.

So now I am getting secondary voltages but they come and go. If I scope the secondary windings I can see the high-speed switching come on then the period get longer and then shut down. Then it repeats. The 3.3v line comes up, the 12v doesn't seem to always get to 12 before it shuts down. There is a VFD- VFD+ and -VP line that seems to get to around 20v occasionally before cutting out again.

I saw somewhere that this can be caused by a cap drying out in the secondary side but none have bulges or any obvious signs of damage.

I am trying to decide if I should just replace the opto-coupler as it may have also been killed by whatever killed the Infineon chip.

I tried using the math function on my scope to make a differential probe and look at the output of the opto (the primary side) but it isn't clear if it is working or not.

I thought it might be in some power saving mode as it is out of circuit but when I put it back in the Blu-ray I get no activity on the front panel.

Any ideas for what else to check? I've gone through and checked all the diodes, inductors and transformer windings for continuity.

Tom

[wraper]

That rather could be small cap on the primary side dying (C_vcc). Actually capacitors bulge less 50% of the time when they are dead. Also if you power SMPS without any load attached, it could shut down even if it is completely OK.

[tom66]

Agreed on primary start-up cap going bad - also the high voltage arc could have damaged it in the first instance. If there are any zeners or diodes in-line with the Vcc winding, check them for shorts and opens.

The Vcc circuit is pretty sensitive to leakage; more than about 500uA is likely to stop it from properly starting.

[tombi]

For a moment I thought you guys nailed it! I de-soldered the cap and my cheapo ESR meter said it was around 2-3 ohms. I replaced it with another one I have and which had a higher voltage rating but same nominal value and an ESR of more like 0.6 (again this could be my ESR meter).

Powered it on and... same result. Plugged it into the rest of the system and I noticed the power light will come on briefly and then go off and then repeat.

Interestingly I noticed a few differences from the reference circuit like there is no zener on the Vcc circuit. Also there is no diode across the Rsense resistor (which is apparently required for lightning/surge protection!). It doesn't have anything close to the 2.5mm clearance they talk about in the manual between the high voltage traces and the rest.

Also they have a resistor between the opto and the FB line and a zener between FB and ground. So they are clamping the opto - but why?

I'm trying to figure out what the voltage over Rsense should be to see if it is cutting out due to an over-current condition. I read the bands as grey, red silver gold so 0.082 ohms. I'm seeing 160mv so 2 amps. Hmm maybe it is going into overload?

The only secondary voltage that looks suspicious is the 12v which is at more like 10v

Thanks for your help so far!

Tom

[wraper]

2-3 ohms may be completely OK for such capacitor. At least few tens of ohms is needed so such circuit stops working normally.

I'm trying to figure out what the voltage over Rsense should be to see if it is cutting out due to an over-current condition. I read the bands as grey, red silver gold so 0.082 ohms. I'm seeing 160mv so 2 amps. Hmm maybe it is going into overload?

Check the diodes on the secondary side then.

[tombi]

Found it!

Again you guys were on the right track in the first place, There was a 28ohm resistor on the Vcc circuit that measured 300K. The trouble is all the voltages were bouncing around so much I couldn't see what was what. I looked at it under a magnifying glass and noticed a tiny crack and when I poked it the whole thing fell apart.

I replaced it and the unit is now back to fully operational!

Thanks again! That was fun!

Tom

[tombi]

Hmm perhaps I was too quick to declare success. Left the thing plugged in overnight (not smart) and found it in the morning with the fuse blown.

Found a burn between the two diodes leading onto the positive track (before the NTC). Tested the diodes in the bridge rectifier (removed them from the circuit) and 3 of 4 were dead shorts. Taking them out of circuit I no longer have a short either before or after the diode bridge.

Not sure why they suddenly died. Did they kill the fuse or did something else kill both the diodes and blow the fuse?

I thought maybe the snubber might be bad but the diode in there looks Ok.

Hmm. Weirdly this burn is within milimeters of the previous one that looked like a short between the 312v track and the SMPS chip Vcc line.

I'll replace the diodes and fuse later today.

Tom

[wraper]

Probably this device experienced transient from lightning or something like this. Therefore some diode might be half dead and just died when voltage was applied for longer time period. Could be viable explanation as you already have seen some arching sign on the PCB.

[tombi]

Lightning is likely - I live in Sydney and we've had a few good storms over this summer. In fact we noticed it had stopped working around the time a storm rolled through.

Would the lightning have partially killed the rectifier diodes or do you think it is something else that then killed the diodes? I thought 1N4007s would be very hard to kill (1000v rating).

I suppose I'll find out when I replace them. Its tempting to replace the diode in the snubber even though it looks Ok.

Thanks again,

Tom

[wraper]

IMO it would be not so easy to kill snubber diode  by overvoltage. Likely it's not some jellybean part too.

[tombi]

Just to finish this off - I replaced the diodes and fuse and tested it again.

Seconds later it blew with a loud crack. The track between the two diodes on the positive rail got even more damaged and it blew two diodes again (short).

I found a small dot of solder (from sucking out a component) that might have been the culprit. I replaced the diodes, bridged the burned track with some wire and replaced the fuse. I cleaned the board more thoroughly and put it back together.

Has been running for about 4 or 5 days now without issue.

Oops.

Again thanks for the help and advice.

Tom

[tautech]

Build yourself a dim bulb tester, ie. an incandesant bulb in series with the mains input.
It helps keep the magic smoke in after repairs. 

Google it.