Driving high-power Class-D audio to new heights

In the 100-400W per channel range, class-D is a technology used in markets for consumer products such as soundbars, subwoofers, home theatre, and mini component systems. The compant claim that it is also preferred in professional applications, including active speakers, active studio monitors, guitar amplifiers, as well as aftermarket car and marine audio systems. Semiconductor development has played a key role in enabling class-D amplifiers to meet the demands of these markets. Successive generations of chips continue to unlock more potential, enabling improved thermal performance, easing circuit layout, strengthening system protection, increasing integration to help simplify design and enhance reliability, and further improving audio quality.

New power audio semiconductor families continue to power innovations in class-D product development. Key ingredients include the latest power MOSFETs with characteristics that are highly optimised for class-D audio-amplifier applications. Infineon has several families of discrete power MOSFETs that are ready to pair with various PWM drivers. The power family also includes multi-chip modules (MCMs) that integrate the analogue input stage, PWM driver, and power MOSFET in the same device. Also, MCMs integrate additional protection features to simplify circuit design, save board space and bill of materials costs, and permit greater reliability.

MOSFETs and modules move forward

The Infineon MERUS MA5332MS is an example of the latest generation of class-D audio MCM. The company claim that as a richly featured module, the device enables higher power density than a discrete solution of comparable power. At the same time, it allows a higher operating voltage than earlier MCMs such as the IR4302M, IR4312M, and IR4322M, and hence can handle more power.

MOSFET Technology

The OptiMOS 5 technology integrated into the MERUS MA5332MS has Figure of Merit (FoM), which permits lower switching and conduction loss. On top of that, the MOSFET breakdown voltages are increased to 100V and 150V compared to the 40V, 60V, and 80V ratings of preceding families with reduced RDS(on). This improvement allows the amplifier to deliver greater output power while keeping the same package size 7x7mm².

Infineon’s 100V MOSFET, contained in the new MCM, has on-resistance RDS(on) of just 2.4mΩ, enabling heatsink-free operation that monolithic devices cannot achieve at the same power level. Indeed, the module can drive 2x100W into 4Ω speakers with no heatsink or 2x200W with a small 8C/W heatsink.

The company claim that the new modules enable high power single-ended (SE) designs with reduced bus capacitors and output filters compared to a low-voltage bridge tied load (BTL) approach. Using a single-ended topology as an alternative to BTL can allow the sizes of bus capacitors and output low-pass filters to be reduced.

Protection on-chip

Highly-integrated MCMs, like the MERUS MA5332MS, integrate protection circuitry for functions such as over-current, over-temperature, and under-voltage protection with self-reset capability. This on-chip protection circuitry saves the time to design protection using external components and the board space and bill of material costs associated with general-purpose gate driver external protection circuitry. On top of that, there is also a new internal logic scheme to control soft-start operation that delivers a further reduction in click- and pop noise.

Leveraging these improved characteristics, and in a small 7x7mm² package that reduces footprint by 80% compared to alternative solutions, the latest modules can operate heatsink-free or benefit from greatly reduced heatsink size. On the other hand, they can deliver more than 50% extra power with the same package size as the preceding IR43x2M MCMs.

Multi-Chip Module in action

The EVAL_AUDAMP25 evaluation board lets designers try out the MERUS MA5332MS amplifier module in various configurations based on single-ended (SE) and bridge-tied load (BTL) output circuitry. Infineon has tested a dual SE amplifier to be used with a split power supply, typically used in applications such as audio/video receivers, high-end consumer distribution amplifiers, automotive aftermarket equipment, and professional active speakers. It is capable of driving 2Ω - 6Ω speakers and delivers 2x200W into a 4Ω load. With the output filter provided on-board, harmonic distortion and noise (THD+N) is about 0.01% in the 10-100W range, depending on impedance.

A second configuration with BTL output,designed for a split power supply, models the type of circuit used in high-power subwoofers and professional active speakers. Suitable for 4-8Ω loads, it delivers 400W into 8Ω and exhibits THD+N similar to that of the 2x 200W dual SE amplifier described earlier.

A parallel single-ended (PSE) configuration with a split power supply has also been tested. The PSE configuration is often used in applications such as subwoofers and aftermarket car audio. Good for 2Ω high-power applications above 300W, it has demonstrated low distortion driving 400W into a 2Ω load.

A reference design, REF_MA5332BTLSPS, has also been created based on the MERUS MA5332MS. The reference design has a BTL output with a single power supply, as often used in products such as woofers, subwoofers, soundbars, aftermarket automotive audio, and marine active speakers. As the figure below shows, this reference design has demonstrated THD+N better than 0.01 over a wide frequency range, which has decent overall performance for such a compact design.

Conclusion

Markets for power audio equipment constantly demand improvements in size, thermal performance, reliability, and sound quality. Technological advancement in semiconductors is essential to meet these needs. 

The latest, best-performing MOSFETs and integrated multi-chip modules (MCMs), such as MERUS MA5332MS audio MCM, deliver superior efficiency, low output distortion, robust protection, and higher power density, that help customers bring better audio products faster to the market. 

For more information on Infineon’s portfolio of fully integrated single-chip audio solutions and discrete audio solutions with highly scalable amplifier driver IC and power MOSFET combinations, please visit the MERUS website.