TPA3156D2: Class-D Amplifier Output for a 100V audio transformer

Hi Ivan,

You can refer the below APP note:

https://www.ti.com/lit/an/sloa133/sloa133.pdf?ts=1646874804409&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253DSLOA133%2526nr%253D24

Regards

Derek

Hello Derek,

Thank you but I am still unsure about how to desing the circuit. In this image from the other post you can see that even after filtered, they have the signal go through a capacitor-resistor additional filter for which they give no further explanation. From simulating it I can't find how that's helpfull in any way.

I've also seen other examples of this in other evaluation boards like the TPA3244 that I'm showing in the following picture, I believe the C21 capacitor have the same function as the other two in the previous image but I can't see why they are there and how they work, also why is there no resistor now.

Hope you can give any tips on the topic.

Regards

Ivan

Hi Ivan

The capacitor is usually for blocking low frequency audio. 

Regards,

Derek

Hi Derek,

I know that capacitor is a high-pass filter, but do you have any desing guide or document about this circuit? I need to know what to connect between the amplifier output and the 100-V Transformer input. 

Regards,

Ivan

Hi Ivan

I think you did not get what I mean.

The LC filter design is same, you can refer datasheet typical design. But with transformer load, AMP has overcurrent shutdown risk. It is related to transformer. You should evaluate according to your application.

Regards,

Derek  

Hi Derek

I get what you are saying, but I have no experience in this field and have no idea what I have to connect between the transformer and the amplifier. Can I find a complete example somewhere?

Regards,

Ivan

Hi Ivan,

You can add one resistor(s4.7ohm or 10ohm) between inductor(LC filter) and transformer to avoid OC risk.

Regards,

Derek

Hi Derek,

By adding that resistor right beffore the transformer I lose half of the amplifier's power there. Is there any way I can have more efficiency? Also, Why 4.7-10 Ohm?

Regards,

Ivan

Hi Ivan

Yes, the resistor has power lost.

Do you have EVM board in your hands? I suggest you can connect transformer to EVM board, it can evaluate your application.

Regards,

Derek

Hi Derek,

I will use the EVM board for testing but do you have any guide on any other ways to connect the transformer which are more power efficient. I am no expert in this topic and my knowledge is very limited so the more explanation you can give me the better.

Regards,

Ivan

Hi Ivan,

The APP note I provided to you includes the root cause and solutions. 

The OC shutdown issue root cause with transformer load is saturation current of the transformer core. "The reason for transformer core saturation during the first quarter to half cycle is that the transformer is exposed to unipolar volt-seconds during that interval, so magnetizing current builds up to a level above its steady-state value. In steady-state operation the input is bipolar, and magnetizing current lags primary voltage and avoids this effect."

And the final part gives the solutions. "A third alternative is to design the transformer to avoid saturation under these conditions."

So the risk happen or not depends your transformer load. The LC design is same. But the rise is related to your transformer load. So I told you can evaluate it on EVM board before your actual design. 

TPA3156 OC shutdown current threshold is 10A typical. So you should select one Isat <10A transformer during the first start-up.

Regards,

Derek 

Thank you for the quick response,

I understood the "A third alternative is to design the transformer to avoid saturation under these conditions." but there is no guide on how to design it. Do you know any transformers who are fit for this application? I will have a 4 Ohm load on the other end.

And I just need to have and LC filter with a resistor following it up?

Regards,

Ivan