Industry-leading class D amplifiers at every power level: Part 2 of 4 Editorial Series sponsored by Infineon; “Multilevel Class D audio amplifiers for compact consumer application”

­The first article in this series discussed how class D amplifiers generally offer substantially higher efficiency levels than other amplifier classes in audio applications. Nonetheless, they still face significant design challenges in low and mid-power compact consumer applications like portable sound systems and smart speakers. These include lower efficiency at typical listening levels, poor heat dissipation, and the requirement for bulky and costly output filter components to reduce out-of-band noise and comply with electromagnetic compatibility (EMC) standards.

This secont article of our 4-part series explores the reasons behind these challenges before showing how Infineon’s range of MERUS™ class D multilevel amplifiers can be used to overcome them, making them an excellent choice for audio applications requiring small size, low heat dissipation, and have a typical output power below 100 Watts.

Class D amplifier operation

The principle of operation for class D amplifiers has been covered in depth in the previous article but is briefly revisited here for completeness.

As shown in Figure 1, two power MOSFETs are driven by a pulse-width-modulated (PWM) signal. An audio input modulates the duty cycle of the PWM signal. The switch node (VPWM) then transfers the amplified PWM signal to the output, where an LC filter recovers the audio signal to drive the speaker. Filters are not necessary for smaller applications such as mobile phones due to the low power output of the audio amplifier; however, applications with output levels higher than around 20 Watts typically do.

Limitation of conventional class D amplifiers

A significant drawback of traditional class D amplifiers is their low efficiency at typical listening levels. This is because they typically have a two-level power stage in which each half-bridge has a voltage potential of either 0 V or PVDD. Consequently, they have relatively high power consumption, even in (or close to) idle mode. This is partly due to the continuous rail-to-rail switching of each half-bridge (required to maximize dynamic performance).

Figure 2 plots the efficiency of a traditional class D amplifier on a logarithmic scale and shows that efficiency barely reaches 50 percent for typical listening levels of 0.1 to 1 W of output power per channel. The graph also shows that a traditional class D amplifier can achieve 90 percent efficiency, but only for power levels above 10 W. However, these are rarely used for extended time intervals with real-life audio signals. At lower output power levels, efficiency is negatively impacted by the amplifier’s power consumption in the idle condition.

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Figure 2. Efficiency curve for a traditional class D amplifier

Another limitation of traditional class D amplifiers is that they typically require a common-mode LC filter to reduce electromagnetic interference (EMI) and improve their electromagnetic compatibility (EMC). Sometimes two extra resistor-capacitor (RC) networks are also used to help dampen switching signal edges. Including these bulky filter networks increases the size and cost of the solution because they require a larger printed circuit board to be used.

Using MERUS™ multilevel class D amplifiers to improve efficiency and reduce solution size

Multilevel class D amplifiers offer several advantages to overcome these limitations, including:

›      Improved efficiency at typical listening levels,

›      Reduced heat dissipation brings the benefits of cooler operating conditions,

›      Superior audio quality through more accurate signal reproduction,

›      Smaller solution size and lower cost due to the reduced requirement for filtering.

All these benefits don’t sacrifice emissions, audio quality, or idle power consumption and can be combined simultaneously.

Infineon’s MERUS™ was the world’s first range of digital and analog input multilevel class D amplifiers designed for audio applications. They use a multilevel power stage (half-bridge) to achieve low power consumption (normalized to maximum output power) at idle or near-idle operating points.

MERUS™ class D multilevel amplifiers have a unique topology with four smaller switches (Figure 3) for each half-bridge power stage (instead of the two big switches used in traditional amplifiers).

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Figure 3. The output stage of a multilevel class D amplifier

The half-bridge establishes multiple PWM output levels from a single supply. Each half-bridge uses four MOSFETs, each driven by an individual PWM signal. The capacitor (CFLY) between the top and bottom MOSFETs is charged to a fixed voltage using separate circuitry, behaving like an extra supply rail. In this way, each half-bridge power stage establishes a three-level output signal at the output-switching node: 0 V, PVDD/2, and PVDD.

A full bridge-tied load (BTL) configuration (Figure 4) combines two three-level half-bridges. It uses a switching pattern in which each half-bridge switching pattern is modulated with a 90° phase shift relative to the other. The resulting power stage can provide a modulation of up to five levels to a differentially connected speaker load. In addition, since the resulting output frequency is higher and the individual voltage steps are smaller, this approximates the output audio signal much closer to the input waveform than in a conventional class D amplifier.

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Figure 4. The five-level modulation scheme in the BTL (full bridge) configuration of a multilevel amplifier

Improved efficiency at typical listening levels

Multilevel class D amplifiers reduce overall power consumption dominated by near-idle power loss. When a multilevel amplifier is in idle mode or playing at a very low output level, a dedicated 50 percent duty cycle PWM signal drives each MOSFET in the half-bridge output stage, meaning the actual output node contains only a direct current (DC) signal (1/2 PVDD). This means that in a differential speaker load, it becomes zero in idle mode in a combined BTL configuration. Hence, the switching activity is negligible.

Infineon’s MERUSTM MA2304DNS and MA2304PNS multilevel amplifier consumes only 52 mW (idle) and achieves an efficiency of approximately 80 percent at 1 W in an 8 Ω load in comparison to approximately 50 percent for traditional Class D competitors. Compared to most conventional class D amplifiers, the multilevel amplifiers demonstrate an overall power efficiency improvement factor of 4x (or more), as shown in Figure 5.

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Figure 5. Comparing the efficiency of Infineon’s MERUS™ MA2304xNS series to a traditional class D amplifier

Reduced heat dissipation brings the benefits of cooler operating conditions

Reduced power losses translate to less heat generation and cooler operating conditions in space-constrained audio applications. As a result, there is much greater scope for periods of higher output power, for example, during music playback at high volume levels. In addition, MERUS™ devices integrate several protection features, including on-chip temperature sensors ensuring that the amplifier and speakers are protected from potential damage if audio equipment experiences unexpected conditions. Lower operating temperatures also mean equipment is subjected to less thermal aging, improving reliability.

DSP capabilities of the MA2304DNS

Infineon provides audio amplifier configuration software, which can be used to explore the DSP capabilities of the MERUS™ MA2304DNS. This tool has multiple audio DSP flows to choose from, which can be programmed into the internal DSP of the amplifier. It contains a graphical view of the current DSP flow and a plot of the expected amplitude and phase responses with options for viewing single channels or combined responses (Figure 6).

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Figure 6. GUI of Infineon’s MERUS™ MA2304DNS DSP evaluation software

Smaller solution size and lower cost due to reduced filtering requirements

Multilevel modulation schemes have lower EMI signal content than traditional class D amplifiers, while EMC performance is also greatly improved. Infineon MERUS™ MΑ2304DNS evaluation board (Figure 7) demonstrates MERUS™ multilevel amplifier devices for ultra-compact and low-power consumer applications.

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Figure 7. Infineon MERUS™ MA2304DNS evaluation kit featuring a MERUS™ multilevel class D amplifier

The compact solution delivers up to 37 W output power per channel. It has only a tiny EMI filter consisting of an SMD-sized (0806) ferrite bead and an SMD-sized (0402) 1 nF capacitor placed at each half-bridge output of the amplifier. This design comfortably complied with the EN55022-B (radio disturbance characteristics limits and measurement methods) standards for consumer products up to 40 cm cable length at full power, depending on the power supply design. With no need for powerful filtering to combat unwanted interference, multilevel amplifiers can drive speakers directly, commonly without LC and RC filters, even in higher-powered applications, reducing application costs. Even if a designer chooses LC filters, these can be much smaller than those required in traditional class D amplifiers.

Conclusion

This article explored the benefits of using multilevel class D, first introduced to audio applications by Infineon through its MERUS™ range of amplifiers. These devices, available in analog and digital input versions, offer ultra-high energy efficiency and provide designers of consumer audio applications with exceptional flexibility.

Operating from a fixed power supply voltage, MERUS™ multilevel amplifiers drastically reduce power losses at normal listening levels and eliminate the need for LC and RC filters in most applications. As a result, they excel in applications where small size, low heat, and low power consumption are essential, including battery-powered speakers, soundbars, wireless speakers, and multi-room audio systems.

The following third article in this 4-part series discusses the challenges when designing high-power audio applications (>250 W) and presents Infineon solutions to address them. Watch out for it.

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