Earth short protection TDA7498E

[just280]

Hi, any suggestions to protect the speaker outputs of the TDA7498 from a short to mains supply earth?

Thanks!

[Peter BENSCH]

Welcome @just280, to the community!

If the supply voltage of the TDA7498 is galvanically isolated from mains, there should be no problem if one of the speaker output wires is connected to mains supply earth.

Does it answer your question?

Regards
/Peter

[just280]

Not sure that's a practical solution on a 170VA linear supply?  

[Peter BENSCH]

Well, such a linear supply must also be galvanically isolated for safety reasons, or do you not have such isolation? Therefore the previous statement still applies.

[just280]

Apologies misunderstood what you said.

Yes the power supply has an AC transformer providing isolation.

We do however connect circuit ground to chassis ground via a 10 Ohm resistor, see schematic.

[Peter BENSCH]

I see no problem in connecting EARTH with PGND via the R74. However, you should clarify this again with the licensing authority if this is required in your region.

[just280]

Going back to my original issue though is what can I do to stop the TDA7498E exploding if one of its speaker outputs is shorted to earth?

[Peter BENSCH]

Ah, now I understand. The problem is that the TDA7498 is a bridge-tied load (BTL) amplifier in which one non-inverting and one inverting channel per speaker amplify the signal in antiphase. If you connect a speaker cable to EARTH, the corresponding channel works towards zero. However, the overcurrent protection only works for short circuits in the speaker and apparently no longer for one against EARTH, especially as the external resistor of 10ohms makes detection more difficult.

• Is there a reason why you are connecting EARTH to PGND?
• The TDA7498 should remain undamaged if you remove R74 and short the speaker connection to EARTH. It should then also remain intact in the event of a short to PGND. Can you verify this?

Regards
/Peter

[just280]

We have had customers that have inadvertently connected a speaker to the unit while it is live and I assume one of the speaker cables has touched the metal case (earthed) and caused the TDA7498 to fail.

We have also had technicians connect oscilloscope probes to the speaker outputs and caused the same issue as the probe is earthed. Just something I would like to prevent.

I will try what you said and remove R74.

Many Thanks

Justin