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Transformer isolated output in class D?

Fred Stokes
Prodigy 40 points
Other Parts Discussed in Thread: TPA3007D1

I am designing a galvanically isolated output stage for a TPA3007D1.

Traditional audio output power transformers are bulky and expensive, so could the isolation be provided with an HF tranformer before the filter stage?

This might have been done for a 100V line output amplifier...

Has anyone done this or have any comments on the feasibility?

 

  • 0
    Prodigy 60 points

    I have actually done something that might be up your alley.  If you can pull up patent 6922101 from the US patent office it may be useful to you.  The patent has expired so it is on the public domain.  The only problem that I did encounter with the design is that the parasitic inductances in the transformer made the efficiency pretty low due to the size of the snubber components required to damp the ringing.  Let me know if this helps.

  • 0 in reply to Alan Bayko
    Prodigy 40 points

    Your work certainly indicates that the transformer is a key component in this sort of design.

    I am realy after some feedback from anyone that has used the TI TPA3007D1 or similar devices with a transformer load.

    I am aso particularily interested in the transformer design at these frequencies (250Khz) and how to obtain the best power transfer from the device.

    Thanks for your input

  • 0 in reply to Fred Stokes
    Prodigy 60 points

    Dear Fred:

    The transformer is a key component but there is also the requirement that if you want to reduce size, you cannot have the fundamental audio frequency going through the transformer.  Since this is only 6.5W, you might be able to get away with either an audio transformer with enough resistance in the primary such that the current caused by the duty cycle imbalance between the 2 sides of the bridge are low enough or by putting a capacitor in series with the transformer primary.  If you want to avoid the audio transformer and use a switching transformer, then you will need to encode the audio so it is not at baseband and then decode at the other side.  I used the phase shift in the patent but you can likely use some other mixing such as multiplication by 1/2f at the PWM and then again after the transformer to restore the baseband component.  As for the design of the high frequency transformer, this could easily fill a few books.  A good place to start is the Ferroxcube web site and the Micrometals web site.

  • 0 in reply to Alan Bayko
    Prodigy 40 points

    Dear Alan,

    I think I understand how the baseband will influence the design of the transformer but surely those transformers that are used in SMPS have to cope with widely varying load currents. They don't have to be linear mind you.

    I am only after a low power 6.5W solutions and I had my heart set on using a planar design transformer that would be part of the PCB. My audio bandwidth is only 300Hz to 5Khz and I am not too worried about distortion as this is a communications application.

    Incidentally I did come accross a similar design to yours at http://www.elkraft.ntnu.no/norpie/10956873/Final%20Papers/023.pdf

    There is a lot for me to learn about.

     

  • 0 in reply to Fred Stokes
    Prodigy 60 points

    It definitely is a big field. 

    With the SMPS power supply transformers, they always are running at a higher switching frequency.  The primary is typically a semiconductor switch and the demodulator is a rectifier arrangement of some sort.  The primary switch modulates the DC to a higher frequency, it goes through the transformer and is then rectified back to DC power.  This keeps the low frequency components from being lost due to the transformer inductance or core saturation.

    The planar transformers are ferrite with limited ability to create a lot of turns that would be necessary to pass the 300Hz signal.  You would likely need many turns on an iron core to get the inductance you need.  With the frequency range and signal quality you need, you could probably get away with a 12VAC->12VAC power transformer rated for 1A.