Class D SMPS...with coupled inductor output?

[eem2am]

Hi


I am enquiring about the use of coupled inductors on the outputs of SMPS’s which supply Class D amplifiers at 300W+.

How much improvement to the transient performance do they provide?

-and is it not viable to avoid the use of such coupled inductors.?



At 300W+ power levels, the SMPS would be of the form of a half-bridge converter.

-an alternative would be an LLC resonant converter, but then unless sales volumes are really large, the cost of the custom-built , sectioned coil former, (needed for an LLC resonant converter) would be too high.

Flyback is not viable because the peak currents become too high at 300W+……….it would be do-able if a power factor correction stage was used to provide a 400VDC input, but PFC costs too much so we wouldn’t use it.
-When mains input is 90VAC, even with a voltage doubler, the peak of the rectified DC bus is just 250V……..and that’s the peak…….so really this is not high enough voltage for 300W flyback, and the flyback currents become too large.

So we would be looking at a half-bridge smps.

Now, obviously this half-bridge is going to have two outputs (+-35V) , because Class D amplifiers require split rail inputs.

Another point about Class D amplifiers, is that they draw load from each of the split rails alternately……..in other words, the Class D amplifier loads either the “+” or “-“ rail alternately…….

-so what this means is that when one rail is on no-load, the other is on full-load..(unless the music has gone quiet and there is no-load on either rail)

-This uneven loading plays havoc with the cross-regulation of the dual rail outputs of the half-bridge smps.

-So a way round this is to make the output inductors of each rail coupled with each other.

So when one rail is fully loaded, it couples a field into the other output’s coupled inductor, such that that unloaded output sees a very high impedance at its output……….this high impedance seen from the half-bridge transformer, of the unloaded output , influences the half-bridge transformer to supply current mainly to the loaded output only….which is the desired effect.

So my question is, ……this coupled inductor technique is obviously more expensive than using separate off-the-shelf inductors , so is its performance really that much better?

One point is that there are times when both outputs of the Class D amplifier are pretty much simultaneously loaded……………….and this means that each output is flowing current through the coupled inductor………

…….in such cases, each output sees an unfortunately high output inductance of this coupled inductor, which means it has a worse transient response.

So, are coupled inductor outputs on half-bridge smps mandatory for Class D amplifier supply?

Is it still worth doing an smps cheaply, without such coupled inductors.