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TPA3255: Power Amp Failure

Jim Hartman
Prodigy 70 points
Part Number: TPA3255

I've been working on an amp for a powered speaker using the TPA3255. I have it configured in 2xBTL mode, one output for the woofer and one for the tweeter. We are limiting the outputs using a DSP preamp well below what the amp is capable of. We are almost done with the engineering stage and nearing production, but still with some long term testing to do. Over that time I've probably tested 10 units or more and never had any power amp failures. I started some long term testing yesterday and one of the units *that had been working" failed quickly. The supply still works perfectly and there are no damage to any other part on the pcba.  The supply (+50V) pins are burned at the IC and all of the outputs are shorted to ground. I don't see any cause for this kind of failure and that scares me this late in the design cycle.

Any ideas?

  • 0
    TI__Mastermind 32030 points

    Hi Jim

        Seems very serious short circuit happens. Can you confirm all the other devices on the PCB is no problem? If you exchange an TPA3255 on the PCB, can the board work normally? Also is the speaker or output LC filter damaged?

  • 0 in reply to Shadow He
    Prodigy 70 points

    I checked the drivers this morning, no problems found. I don't see any other damage on the circuit board...no other damaged parts. I will change the part later today and see if the board works...I suspect it will.

  • 0
    Prodigy 70 points

    Ok, I was wrong about that...and I did find more damaged parts. The 3.3 ohm in series with the +12V to pin 1 (GVDD_AB) and pin 22 (GVDD_CD) are both higher impedance than they are supposed to be. One is 86 ohms and the other is 32 ohms. After replacing the two resistors, the amp worked great. But I assume they were casualties of the catastrophic event...not the cause.

  • 0 in reply to Jim Hartman
    TI__Mastermind 32030 points

    Hi Jim

        Yes, I think the resistor is damaged by over current. These two pins are the supply for the output. Since nothing found at the output side and no OC or OT error triggered by our device, but break down directly. I think this device is damaged before the test, ESD is one of the possible reason. As the information we have, this should be an individual case. You can continue the long term testing with the other board, and check the results.

  • 0
    Prodigy 70 points

    We tried a second speaker/amp module, it played for a few hours then the amp latched off. I didn't get a chance to see the fault LED's before the power got cycled. The amp appears to be working after the power was cycled. We are going to restart the test with this module in the morning.

    We also tried the first module again, after repairing it. It blew up again in a similar fashion as the first time. 3 of the 4 outputs are shorted to ground, Pin 11 is shorted to ground, pins 29, 30, 31, 36,37 and 38 are burned off the board.

    I only have two sample boards, so I need to get this resolved without causing more damage.

  • 0 in reply to Jim Hartman
    TI__Mastermind 32030 points

    Hi Jim

        With your second board, you can try testing what is the case temperature when you doing the long term test. I think maybe OT triggered. Usually there would be heat sink, you can consider drill a whole on the heat sink to put the thermocouple in it. Better leave some margin to the temperature, 120 °C on the top case is high enough.

        It is possible that your first board is not somewhere damaged. Another possibility is there's short circuit risk on your PCB, the risk may comes true when temperature goes high enough and longer enough. For example, the heat sink foot usually mounted on PCB, also some electrolytic capacitor also have metal bottom, they are all connected to GND. Sometimes there maybe trace pass below these component, and only use Solder Mask to do the isolation. Normal temperature will works fine, but when temperature goes higher, there's risk. So maybe you can have some check.

        Our device all went through reliability test under extreme conditions, shouldn't break down so easily.