Advice on LG DVD player blown power supply
Good day folks. I wasn't sure whether this goes in the audio equipment or power supply section, but I chose the latter, since it's rather irrelevant what this supply is powering, since it could be anything really. I've got an LG DVD/home theater player which was damaged by a power surge (some branches fell on some power lines and caused a spike in the area which resulted in several blown appliances, including this thing, according to the guy who brought it in).
A quick look at the power supply and I was presented with a bulged reservoir cap, but that was pretty much it visually speaking. Measuring all critical components in the primary, revealed a shorted bridge rectifier and, curiously, just ONE shorted FET (there's two of them and the other was fine)....sounded like a very simple and actually enjoyable job to do. I replaced all faulty parts, which was all fine and dandy until I got to those FETs: they're 2SK3530 (datasheet here) and I could not find these exact ones at my local shop, so the chap there, who knows his stuff and whom I trust, gave me a pair of 2SK2645 instead (datasheet here)....sounded like a good plan...if anything, the specs of the replacement FETs looked even better than the originals, so I installed them, ran the lightbulb test which was a pass, before going for a full live test: plugged the thing straight into the mains, the blue standby light DID come on, but as soon as I tried powering it on, there was a massive flash and the obligatory bang, then I saw part of the circuit board going up in flames before the main breaker tripped...damn :|
As I inspected the damage, I noticed the fuse didn't open as it was supposed to, hence adding to the damage and also the rectifier and one of the new FETs had shorted, again, like before, just ONE of them. What actually caught fire were two filter chokes located on the AC side...they're fried and no longer conduct now, so this PSU looks beyond repair at this point, but I must wonder why the hell this happened :| Standby was fine, but as soon as I tried powering the thing on, it went boom....WTH ? I think I may have an explanation: see the power rating of the replacement transistors is only 50w, whereas the originals could take up to 70w, so if the originals were already pushed pretty close to that 70w rating, it's no wonder the 50w ones died as soon as they were subjected to the slightest load - the PSU IS driving a power amplifier after all, so I can imagine that when those large filter caps charge up they induce an enormous spike on that primary, so if the PD figure of the transistors is lower, we get unwanted results like these....facepalm if that's the case.
On that topic, what exactly do I need to look out for when replacing FETs with non-identical parts ? The max voltage and current ratings are no-brainers, but what about other parameters like the RDSon or the gate threshold ? Do those make so much of a difference that they can't be just a fraction different ? Please compare the two and tell me if that's indeed the reason it failed or there's some other fault I should be looking out for. Also, anything I can do about those fried coils ? How about feeding AC straight to the rectifier ?
I shall post pictures of the board tomorrow if I find the time. Cheers and thanks for any input.
A quick look at the power supply and I was presented with a bulged reservoir cap, but that was pretty much it visually speaking. Measuring all critical components in the primary, revealed a shorted bridge rectifier and, curiously, just ONE shorted FET (there's two of them and the other was fine)....sounded like a very simple and actually enjoyable job to do. I replaced all faulty parts, which was all fine and dandy until I got to those FETs: they're 2SK3530 (datasheet here) and I could not find these exact ones at my local shop, so the chap there, who knows his stuff and whom I trust, gave me a pair of 2SK2645 instead (datasheet here)....sounded like a good plan...if anything, the specs of the replacement FETs looked even better than the originals, so I installed them, ran the lightbulb test which was a pass, before going for a full live test: plugged the thing straight into the mains, the blue standby light DID come on, but as soon as I tried powering it on, there was a massive flash and the obligatory bang, then I saw part of the circuit board going up in flames before the main breaker tripped...damn :|
As I inspected the damage, I noticed the fuse didn't open as it was supposed to, hence adding to the damage and also the rectifier and one of the new FETs had shorted, again, like before, just ONE of them. What actually caught fire were two filter chokes located on the AC side...they're fried and no longer conduct now, so this PSU looks beyond repair at this point, but I must wonder why the hell this happened :| Standby was fine, but as soon as I tried powering the thing on, it went boom....WTH ? I think I may have an explanation: see the power rating of the replacement transistors is only 50w, whereas the originals could take up to 70w, so if the originals were already pushed pretty close to that 70w rating, it's no wonder the 50w ones died as soon as they were subjected to the slightest load - the PSU IS driving a power amplifier after all, so I can imagine that when those large filter caps charge up they induce an enormous spike on that primary, so if the PD figure of the transistors is lower, we get unwanted results like these....facepalm if that's the case.
On that topic, what exactly do I need to look out for when replacing FETs with non-identical parts ? The max voltage and current ratings are no-brainers, but what about other parameters like the RDSon or the gate threshold ? Do those make so much of a difference that they can't be just a fraction different ? Please compare the two and tell me if that's indeed the reason it failed or there's some other fault I should be looking out for. Also, anything I can do about those fried coils ? How about feeding AC straight to the rectifier ?
I shall post pictures of the board tomorrow if I find the time. Cheers and thanks for any input.
....that may explain the fire, since it didn't rupture fast enough and kept sending power to the faulty circuit, but doesn't explain the failure to begin with, because a functional PSU should work even with a shunted fuse IMO, though it's obviously a stupid and unrecommended thing to do.
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