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TAS5611A evaluation for a transducer amplifier

Manjula Ellepola
Expert 1335 points
Other Parts Discussed in Thread: TAS5611A

Hi,

I bought the EVM for the above device to evaluate it's suitability for a transducer amplifier for underwater acoustics. The frequency band of operation is from 16khz to 35khz. According to the data sheets the device is capable up to 80 khz. I coupled the evm to the transducer via a 1:20 isolation transformer to match the impedance between the amp and the transducer. The transducer has an impedance of about 2koms.

I was expecting plenty of acoustic power out but I was dissapointed to find out that I can only increase the input amplitude from 0 to 10% of full amplitude and the power output is no more than a couple of watts. Any more increase in the output power puts the device into fault condition and has to be reset. The transducer element is a piezo ceramic ring with around 9nf capacitance.

We have been using the TPA032D01 for years in this application successfully. The TPA is a much smaller device that the TAS5611A therefore I expected much greater power output at least up to 50-60W. I also looked at waveform on the secondary of the transducer by connecting it to an oscilloscope. I found that as the input voltage was increased to the proximity of the threshold where the device would cut off, there were discontinuties in the output waveform with large ringing in the out put.

Can you provide some advice on what I am doing wrong? What is the cause of the ringing? Can this amplifier be used in this type of application? It would be great if we could. We are planning to sell many of these  products using the TAS if it can be successfully implemented in the design.

I await your valuable comments.

Best regards

Manjula

CTO

  • 0
    Expert 1335 points

    Hi

    I did further research on this and found the following paper on connecting a transformer load to a class D power amp: http://www.ti.com/lit/an/sloa133/sloa133.pdf which more or less seems to be the problem that I am experiencing. The paper explains the transformer going into saturation during the first half of the cycle of a sine wave thereby triggering the short circuit protection (SCP) of the amplifier. This might be a drawback in the TI class D amp design for this type of application as the device might be too quick to press the button with SCP. SCP would be mainly there to protect the device from overheating causing damage to the chip. However the high current would have to flow for some time before there is damage to the chip. If the SCP triggering time can be extended this would allow the transformer to recover as the problem happens only in the first cycle during which time there wouldn't be damage to the chip from high current? Any comments from TI regarding if this is the likely cause would be appreciated. Also any advice on how to limit the SCP triggering with a transformer load would be appreciated as well. I know of other high power discrete MOSFET amplifier designs that work with transformer loads without a problem. So this would be the same for the H-bridge design as well?

    Thanks and regards

    Manjula

    CTO

  • 0 in reply to Manjula Ellepola
    Expert 1335 points

    Hello

    Can I get some attention to this problem please? If D2 is too busy to give some feedback can you R2 respond please?

    Need a little help from my friends.....

  • 0 in reply to Manjula Ellepola
    TI__Guru**** 189401 points

    Hi, Manjula,

    Sorry for the delay.

    Steve made several suggestions to avoid this problem, did you try implementing any of those?

    -d2

  • 0 in reply to Don Dapkus
    TI__Mastermind 18510 points

    Manjula, I expect you are seeing effects of over current protection (OCP) in TAS5611A, as you suggested.  But SLOA133 deals with transformer saturation during the first cycle or 2 at startup.  It seems you are able to start the amplifier running and then you see it shut down, so this may be a different problem.

    I wonder if the problem lies in high frequency impedance of the transformer-coupled load.  Maybe there is something unexpected in that impedance, like a resonance or rolloff.  Maybe the combination of transducer capacitance and transformer capacitance and inductance is not what is expected.

    I have a few questions.

    How are you coupling the amplifiers to the load transformer - are you using standard EVMs and LC filters for both TPA032D01 (15uH and 220nF) and TAS5611A (10uH and 680nF)?

    Can you measure impedance of your transformer coupled load at frequencies up to about 420kHz and see if there is any unexpected resonance or rolloff?

    Best regards,

    Steve.