Please explain the difference between Class D current and voltage mode

[Chris Wilson]

I have Googled but still unclear, could someone explain in simple terms how to tell from a schematic or otherwise if a low frequency RF amplifier is operating in voltage or current mode. I can cite an example, my confusion here is it is said to be voltage mode, yet the LPF seems one I normally see for current mode... very confused Thanks

[T3sl4co1l]

Because it's using both.  Or neither.

Within a cycle, L3 maintains fairly constant supply current.  This feeds TR1/2 and C9-C11 capacitances without extreme peak currents (well, give or take how TR1/2 switching goes; when tuned for an inductive or resistive filter input impedance, it should be alternating flats and sine humps, as a traditional Baxandall oscillator has, and the current will be steady).

Over many cycles, L3's DC voltage drop is almost zero, so it tends to enforce a constant output amplitude (at the inverter, not necessarily so after the filter).  So if there is suddenly a heavy load (due to some sort of mismatch, I suppose), current ramps up and up and up to maintain that voltage.  Conversely, if load drops suddenly (opposite sort of mismatch?), the voltage on TR1/2 shoots way up.  Both can easily result in blown transistors.

It's a wide open loop, with neither current nor voltage protection -- not even having a current limited supply or TVS voltage clamps!

So, for purposes of filter design, it is current mode.

For purposes of amplitude, it is voltage mode.

For purposes of switching and safety, it is open loop.

Tim

[Chris Wilson]

Thanks Tim, I agree protection is not its strong point, but my limited experience with Class D amps with protection is it can be a PITA at a simple level, in that the current inrush when a signal is initially implemented trips a current limiter, and setting it to such a level the inrush does NOT trip it leaves the protection marginal or none existent in the occurrence of a genuine issue. At my level I view FET's as sacrificial although of late I have developed the rest of the system to the extent where I no longer buy them in bulk

[T3sl4co1l]

It's only an LM393 and 74HC74 away, but suit yourself.

Tim