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Class-D topologies - why most modern amps are half-bridge? Why not multilevel?

EB1000

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Joined
Jan 8, 2020
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Location
Israel
#1
Hi

I've noticed that Hypex, Icepower and Purifi, are all half-bridge based.

Yes, half bridge are the simplest topologies and most easy to control. But still, since a class-D amp is essentially a DC to AC converter (Inventer), an a full-bridge inverter outperforms an half-bridge, and similarly, a multilevel inverter, such as a cascaded H-bridge (series connection of many full bridge topologies), will ensure the lowest THD per switching frequency, as well as lower device stress, lower EMI (dv/dt), higher efficiency and reduced or even eliminated output filter.

Also, full bridge topology is capable of absorbing power from the load back to the DC rail, thus reducing pumping effect of the DC supply, when proper control is utilized. So why most modern class-D amps are based on half bridge? The only full bridge topologies are not really full-bridge but a common mode based BTL connection of independently controlled two half bridge topologies. A true full bridge BTL should be differentially connected, because that why the effective output switching frequency can be doubled.

Attached are different possibilities of output filter configurations for a full bridge topology. Type 1 seems to ensure lowest THD for the same PWM frequency and filter cutoff frequency (BD modulation).

Does it have to do with reducing costs? Are there modern full bridge or multilevel class-D amps out there?
class-D2.jpg
 
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