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Why after serving well for years, a smps controller ic fails without apparent reason?

My Sharp TV (LC46LE835X) worked well for 37 months but suddenly while being watched in the middle of calm night, it blacked out with small explosion.

I managed to repair it after changed a 7-pin dip ic and a 2.2ohm fusible resistor. No other components (SMTs) appear abnormal. TV works well now.

So I'm really wondering what could have caused this ic to fail. If it had manufacturing defect why it worked well for 37 months?

Since the root cause is not known, the failure will repeat? enter image description here

soosai steven
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  • An IC cracking like that would lead me to think of mechanical loading. Thermal stresses and fatigue would eventually cause it to break, and thus breaking open. It could then cause some sort of short which could damage the resistor (drawing too much current would burn it out). – Puffafish Oct 31 '16 at 14:13
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    @Puffafish, the thermal stress is more likely from the silicon die suffering a catastrophic failure (the OP describes an explosion). The device often stinks of evaporated epoxy. – glen_geek Oct 31 '16 at 14:44

2 Answers2

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The TNY264 is a highly (electrically) stressed IC which sees mains voltage, probably part of a low wattage auxiliary power supply that is always present waiting for the soft power switch to be pressed.

The typical root cause would be a mains transient causing the internal MOSFET to avalanche, resulting in massive current and blowing the wires off the chip, cracking the package etc. Once the MOSFET fails short, the fusible resistor is next, protecting the transformer and wiring.

Since it worked for a long time you may not be too concerned, but if there is something new you've added to the electrical load nearby (perhaps a refrigerator) you could consider adding a surge absorbing device. There is no reason why this would not happen again.

Spehro Pefhany
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  • Yes, its part of standby power supply. Transient is a suspect too, but why it affect only this ic but not the six other SMPSs on other appliances? – soosai steven Oct 31 '16 at 16:43
  • The actual tolerance of that part to transients may be less than the others. The internal MOSFET sees voltages well above the bus voltage when it switches. Also, as the weak link, once it failed, perhaps it helped protect the others. – Spehro Pefhany Oct 31 '16 at 16:47
  • Thus replacing the same part number will cause future failure, right? – soosai steven Oct 31 '16 at 16:51
  • If the same transient comes along, and the part is no better, then yes. I can't judge the source of the transient though you seem to have ruled out a thunderstorm. Of course it **could** be a manufacturing defect of some kind that weakened that particular device. Nobody can answer the exact cause without spending huge amounts of money doing a failure analysis. I would not fool around trying to reverse engineer this- either add a surge arrestor or don't. If it happens again, rethink. – Spehro Pefhany Oct 31 '16 at 17:04
  • I believe thermal stress must be the cause of this. Plastic DIP having high thermal resistance thus may have heat stressed to this failure. Thus, I'm thinking of improving this by copper extensions on each leads of the ic with proper allowance for voltage insulation. Any suggestions? – soosai steven Oct 31 '16 at 18:05
  • My suggestion is to forget about it. It failed from voltage stress which caused it to short and the insides to explode. – Spehro Pefhany Oct 31 '16 at 18:11
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Well I had a similar problem caused by me not plugging the mains plug in all the way , my tny 264 did not explode so looked fine , TV would work for a minute then stop, then next day only click the relay immediatly on to off - replaced IC TNY264 problem fixed - board was rdenka 340 on Sharp 42 inch , same board as 42 inch Samsung also with fluro backlights I believe.

Keith Brown
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