A similar question has been asked, but I wanted to get to a more fundamental understanding of when, in general, power supply pumping occurs in a class D amplifier.

In general terms "power supply pumping" is the return of energy from a load to a power supply via output switches so that the power supply is forced to accept energy via an unintended path.

This quote was from an answer that was given before. How is the energy being supplied back? Is it mainly due to the inductive voltage spikes from the abrupt current switches? If I was driving something that was not a speaker, and say, purely resistive, would this still happen, or is it irrespective of the load type?

I was told that using a full bridge output (4 driving FETs) would help alleviate this problem versus using just a half bridge topology, but I do not see how. Any links to outside reading would be useful.

EDIT: I don’t have a specific schematic issue, I just wanted to know more about this specific issue that occurs.

  • 1
    Could you include a schematic illustrating the problem? – Dmitry Grigoryev Oct 11 at 7:24
  • It does depend what the amp is driving. Some loads (like electric motors) also act as generators. – Brian Drummond Oct 11 at 7:27
  • For the load to be able to deliver power back into the amplifier it cannot be purely resistive. So for a purely resistive load: supply pumping cannot occur. I think the main source of energy coming back is mechanical/acoustic energy from the loudspeaker. A loudspeaker can also act as a generator when the diaphragm is moved (or it moves back after being moved by the amplifier) and that creates EMF in the loudspeaker coil. – Bimpelrekkie Oct 11 at 7:32
  • There's a decent explanation of supply pumping in the datasheet of the MAX9742: datasheets.maximintegrated.com/en/ds/MAX9742.pdf page 25. Unfortunately it does not explain why this less of an issue in bridged configurations. My guess: in a bridged config. the grounded side of the speaker can be connected to the supply so that the pumping current cannot charge the supply anymore. – Bimpelrekkie Oct 11 at 8:09

A highly simplified view of the problem is as follows. A class-D amplifier PWM modulates 2 output states. The first, sourcing power to the load. The second, commutating the load and reconstruction filter current.

In a split supply half-bridge configuration the amplifier sources power from one supply rail and commutates through the other. At low loads or short-durations (high-frequency audio) the commutation current is easily absorbed into the bulk capacitance on the rail.

In a full-bridge configuration the same supply rail that is sourcing power to the load is being commuated through. Hence the same supply that is sourcing load current also ends up being regenerated on the opposite PWM state.

Additionally, some full-bridge class-d ampifiers will also commutuate the load through the top 2 or bottom 2 switching elements and not pump the supply at all during this 3rd PWM state.

If you were to operating a half-bridge amplifier without an LC reconstruction filter into a resistive load, there would be no "supply pumping". However the output waveform would not be continuous.

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