# Aluminium electrolytic capacitors in an AC circuit

Looking through the datasheet for the TPA3123D2, I have noticed some confusing symbol usage and positioning. From my understanding, aluminium electrolytic capacitors can only be 'charged' on the positive terminal, while 'charging' it from the negative terminal will dissolve the oxide layer, resulting in a short circuit and subsequent failure of the capacitor. With the other capacitors in the schematic, generic film capacitors can be used (avoiding ceramic due to electromechanical vibration,) but the two 470uF capacitors (excluding PSU decoupling) are too big to be replaced with a film/polyester type capacitor.

The notion of only 'charging' a polarized capacitor from its positive terminal holds true for the two 470uF capacitors just before the outputs, but isn't an audio signal inherently AC? Why isn't the capacitor damaged when the output reverses polarity?

Additionally, regarding capacitor voltage ratings, should an AC VRMS(max) value be used instead of its maximum voltage rating marked on the body of the capacitor?

The output does not reverse in polarity so there will be no capacitor damage.

That amplifier works on single supply.

It means that if you feed on 12V supply, the output from the chip will idle at 6V. So there will be a 6VDC bias always over the capacitor, as the speaker sets the other capacitor terminal DC bias to 0VDC.

And that's why the capacitor is even there, to remove the 6V DC bias of the chip output.

So the AC RMS rating of the electrolytic is not important.

It is also a Class D amplifier, so the speaker output on the chip is digital and will either be supply voltage or ground, and so silence will actually be a 50% duty square wave, and it will only on average, be equal to idling at half of the supply voltage.

The speaker can be damaged by a constant DC voltage, so the DC should be removed. The capacitor will quickly charge to the DC average of the source.

The simulation confirms that the cap never sees a negative voltage.

simulate this circuit – Schematic created using CircuitLab