Industry-leading class D amplifiers at every power level: Part 4 of 4 Editorial Series sponsored by Infineon; “Looking to the future of class D audio amplifiers”

­The fourth and final article of this series about using class D amplifiers discusses the benefits that Infineon’s MERUS™ MA53x2MS series of multi-chip modules (MCM) brings to mid-power audio applications ranging from 50 to 200 W.

The benefits that Gallium Nitride (GaN) semiconductor technology could offer the class D audio market are also considered before outlining how Infineon is preparing to take advantage of this potentially exciting development.

Mid-power audio applications

In the 50-200 Watts-per-channel range, class-D amplifiers predominate in the market for consumer audio products like soundbars, subwoofers, home theatre, and mini-component systems. They are also the preferred choice for professional equipment like active speakers, active studio monitors, guitar amplifiers, and aftermarket automobile and marine audio systems.

The most common class D implementation uses a pulse width modulation controller (PWM) IC to drive switching MOSFETs with parameters tailored to audio applications. However, an alternative approach, which offers several additional advantages, is to use a multi-chip module (MCM) that integrates the main class D building blocks – the analog input stage, PWM driver, and power switch – into a single package, like Infineon’s MERUS™ MA5332MS (Figure 1).

MERUS™ MA5332MS MCM features integrated OptiMOS™ switches and protection

The integrated power switch in the MERUS™ MA5332MS features Infineon’s OptiMOS™ 5 technology, designed to reduce switching and conduction losses. The device combines high breakdown voltage (100 V) and low R_DS(on) (24 mΩ), enabling it to deliver high output power from a small package size measuring only 7mm x 7 mm without the requirement for an additional heatsink.

The MERUS™ MA5332MS can deliver 2x 200 W using only a tiny 8 C/W heatsink. Another advantage of using an MCM compared to a low-voltage bridge-tied load (BTL) approach is that it enables high-power single-ended designs with smaller bus capacitors and output filters. This module also integrates protection circuitry to monitor overcurrent, overtemperature, and undervoltage and has a self-reset capability. This saves audio engineers the time it would take to design external protection and reduces the amount of board space and bill of material costs required for external general-purpose gate driver protection circuitry. This results in an overall solution up to 80 percent smaller than other approaches.

Furthermore, the MERUS™ MA5332MS has an internal logic scheme that controls soft-start operation to further reduce click and pop noise. Other modules in this family include the MERUS™ MA5302MS for driving 2 Ω loads and the MERUS™ MA5342MS for 8 Ω loads[1]. Conveniently, these modules have a centrally located ground pin, making them exceptionally easy to design into a sound card and enabling them to deliver exceptional sound quality.

MERUS™ MA5332MS demonstrates excellent distortion performance

The EVAL_AUDAMP25 evaluation board makes it easy for designers to explore the many features and benefits of the MERUS™ MA5332MS module in single-ended and bridge-tied load (BTL) configurations. For example, Figure 2 shows the module’s harmonic distortion and noise (THD+N) is about 0.01 percent in the 10-100 W range for dual single-ended output.

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Figure 2. THD+N for the EVAL_AUDAMP25 evaluation board as dual single-ended output

The REF_MA5332BTLSPS reference design, featuring the MERUS™ MA5332MS, has a BTL output and operates from a single supply. It exhibits a THD+N better than 0.01 percent (Figure 3) over a wide frequency range, representing excellent performance from a compact amplifier design.

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Figure 3. THD+N performance of the REF_MA5332BTLSPS reference design

What will be the next chapter for class D audio?

Class D audio amplifier performance is closely tied to that of the switching devices which underly their operation. Continuous improvements in semiconductor process technology in recent years, which have seen the R_DS(on) of switching devices fall considerably, have delivered enormous benefits to audio applications. However, with the performance of silicon-based devices being pushed to its limits, the question arises as to where further improvements will come from.

GaN HEMTs look promising for class D audio

In terms of further improving switching performance, GaN (gallium nitride) high electron mobility transistors (HEMT) can potentially be the next significant development in class D audio applications.

A class D amplifier requires switching devices with low R_DS(on) and fast, clean transitions. These are traditionally opposing performance metrics for silicon MOSFETs, which must be traded against each other at high power ratings. However, this is not the case for GaN HEMTs. The reverse conduction mode from source to drain is essential for class D amplifiers to keep the switching output voltage within the power supply rails during dead time. GaN HEMTs are bidirectional devices and realize a reverse current as one of their on-states. When the drain voltage becomes lower than the source, the drain behaves as a source and turns on the device allowing reverse current flow.

Conversely, a silicon MOSFET is a unidirectional switch with an intrinsic PN junction body diode with reverse current flowing from the source to the drain when the device is turned off. The absence of the body diode in GaN HEMTs is another notable feature that eliminates the primary source of switching noise caused by the PN junction body diode in silicon MOSFETs. This enables GaN HEMTs to deliver cleaner switching in high applications operating with high voltages and currents at fast switching frequencies.

Infineon’s CoolGaN™ is a highly efficient GaN transistor technology for applications up to 600 V and offers the most reliable and best-performing GaN HEMT on the market in volume production. Figure 4 shows the superior switching performance of an Infineon CoolGaN HEMT compared to a silicon MOSFET.

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Figure 4. Infineon’s CoolGaN™ HEMTs exhibit faster and cleaner switching than a silicon MOSFET.

The IGT40R070D1 E8220 (Figure 5) is a class D audio amplifier reference design based on Infineon’s audio-dedicated 400 V, 70 mΩ R_DS(on),max CoolGaN™ HEMTs and its MERUS™ IRS20957S 200 V class D controller.

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Figure 5. Infineon’s IGT40R070D1 E8220 reference design using audio-dedicated CoolGaN™ HEMTs

GaN HEMTs require a different gate-driving scheme than silicon MOSFETs (which typically use 0 V or 10 V gate voltage to switch on and off). Therefore, Infineon’s CoolGaN™ gate-injection (GIT) HEMTs are controlled using a different gate-driving voltage and a sustaining DC gate bias current. An interface circuit level shifts the 0-10 V signal from the IRS20957S class D controller IC to drive the gate of the CoolGaN™ HEMTs with an output voltage varying between -1 V and +3 V.

In a 250W + 250 W example design, switching at 500 kHz into a 4 Ω load, there is no visible THD+N level transition bump in the transition from soft- to hard-switching (at the low Watt level) that can appear in some high-voltage class D amplifiers (Figure 6).

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Figure 6. THD+N vs. output power at 4 Ω load

Conclusion

This series of articles explored the requirements for class D amplifiers in low, medium, and high-power audio applications.

With everything from multilevel MERUS™ for miniature portable soundbars, to modules for mid-power speakers, to highly-integrated solutions for high-power stadium announcers, Infineon currently offers industry-leading class D amplifiers at every power level. It now aims to stay ahead of the competitors by exploring how the power efficiency and audio performance improvements of GaN technology can help to realize future audio solutions in even smaller form factors.

If you missed one of our four articles, you can find them on PSD’s website.

For more information, please visit our MERUS™ class D audio amplifier IC website. Click here.