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Posted on 13 January 2019

Design Tips for Even Better Class-D Amplifier Performance

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Examination of some of the finer points of class D amplifiers, and how engineers can optimize audio performance and keep improving their designs.

By Jun Honda, Director of Audio Systems Engineering at International Rectifier

The efficiency and size advantages of class-D amplifiers, which enable improvements in system form factor, styling and battery life, are generally understood. However, class D operation also provides inherently low distortion and more stable dynamic response than class AB. Whereas class AB forces designers to trade power efficiency for audio fidelity, efficiency and fidelity theoretically improve together in class D; as semiconductor advances boost efficiency, audio performance also increases.

Recap on Class D Operation

As figure 1 illustrates, the class-D amplifier converts the input audio signal into a series of pulses which have instantaneous average value proportional to the input signal. This binary signal switches the power MOSFET, creating an amplified version of the PWM. A passive low-pass filter attached to the PWM switching stage removes highfrequency components to recover the amplified audio signal.

Overview of class-D amplifier and principles

Class D Under the Skin

The inherently high efficiency of a class-D amplifier allows parts such as heatsinks and cables to be downsized enabling audio equipment to be far smaller than has historically been possible. This not only helps improve aspects such as styling and portability, but also contributes to improved audio performance. Every current loop inside an amplifier forms magnetic couplings with other current loops, effectively creating many small transformers that can interfere with nearby circuits and components. When the amplifier is physically smaller, the magnetic flux due to these current loops is reduced. The susceptibility to flux is also lower. In practice, the impact of magnetic flux is inversely proportional to the square of size.

Compared to class AB, class D exhibits inherently superior response to changes in power demand, resulting in highly dynamic sound characteristics. This superiority comes from the completely different manner in which the class-D amplifier controls its output power. Even when the amplifier is idling, the output MOSFETs are turned fully ON or OFF alternately every 1.2μs. Hence the timing of the switching controls the power flow; to change from zero output power at idle to a half rated power output takes only 0.6μs until the next switching event is triggered. Hence the switching timing regulates output power, with no additional energy needed to commence the event. In other words the output power is controlled independently of the current or voltage demanded by the speaker, and is simply determined by when the PWM signal switches from one state to another. This ensures robust control of the power output to the speakers, and reduces distortion caused by the speakers' back EMF.

In addition, there is no temperature-sensitive operating bias point in a class-D power stage. This enables the amplifier to maintain stable operation under dynamic power changes. The gain of the class-D stage is simply a function of supply voltage and a ratio of the ON time between the output MOSFETs, and is not related to temperature. One big challenge in class AB design used to be how to maintain the stability of the bias current under dynamic loading conditions, since the transistor gain when used in the linear region is strongly dependent on bias current, which in turn is highly sensitive to temperature. Class D eliminates this interdependency.

Key Semiconductor Components

Although class D allows a theoretically perfect amplifier, with 0% distortion and 100% efficiency, the actual performance is dependent on the quality of the components used. In particular, the output power MOSFETs and the controller IC each have a major influence.

MOSFET

A perfect MOSFET would allow the amplifier to have 100% efficiency resulting in zero heat generation. Today's state-of-the-art MOSFETs allow a practical Class D to achieve above 90% power efficiency. Examining the On resistance (RDS(ON)) x gate charge (Qg) figure of merit (FOM) illustrates how closely the performance of today's MOSFETs now approaches that of an ideal power switching device.

Consider the evolution of 200V rated MOSFETs over the last couple of decades. To achieve the highest efficiency in a Class-D amplifier, the conduction loss from RDS(ON) and switching loss dictated by gate charge Qg should both be as low as possible. The IRF640 from the 1980s, which has a planar structure, has RDS(ON) of 180mΩ with Qg of 70nC. The latest trench structure MOSFET IRFB4227 has RDS(ON) of 20mΩ with the same 70nC Qg. The FOMs has been reduced from 12,600 to 1400; a nine-fold improvement.

Control IC

As far as the controller IC is concerned, effective noise isolation is a key requirement. IR has paid special attention to this aspect in the design of its class-D amplifier controllers, and uses proprietary techniques to prevent switching noise being coupled into noise-sensitive error-compensation circuitry. This has enabled the built-in noise-sensitive analogue error amplifier in the IRS2092 controller, for example, to provide a clean low-noise output despite the fact that the other side of the silicon chip just 1mm away is switching between -60V and +60V at very fast transition speed. This error amplifier is also well isolated from the switching of the external output power MOSFET.

To achieve low distortion, precise timing control of the PWM gatedrive signal is as crucial as ensuring precision analogue signal processing in the error amplifier section. A stable dead-time control and jitter-less level shifting is also needed to control the gate signals to the output MOSFETs. IR has developed special circuits that attain very low jitter in the signal path.

The IRS2092 combines these noise-isolation and timing-control features with robust protection circuitry to create a class-D amplifier that surpasses the audio performance possible with a conventional class AB approach in many aspects.

Internal structure of IRS2092

Integrated Class-D amplifier

Building on the same MOSFET and controller IC construction, IR's PowIRaudio™ IR43xx ICs combine both components in a small surface-mount package.

2x135W (2Ω) design example with PowIRaudio™ IR4321

Instead of integrating the controller and output power MOSFETs on a single piece of silicon, the IR43xx family features separate controller IC and MOSFET chips. This approach takes full advantage of the wide operating voltage range and strong noise immunity of the IRS2092, as well as the latest-generation application-optimised MOSFETs, to deliver the high efficiency and high audio quality achievable with class D within an even smaller footprint. Evolution is set to continue further, for example using new semiconductor materials such as Gallium Nitride (GaN) which can potentially bring a ten-fold performance increase.

The PowIRaudio family comprises the IR4301M, IR4311M, IR4321, IR4302M, IR4312M and IR4322, supporting full-bridge and halfbridge topologies from 20W to 320W per channel.

Summary

Class D is redefining the tradeoffs in audio power amplifier design, simultaneously delivering smaller size and higher power density with better sound. The roadmap for IR's latest IR43xx PowIRaudio™ ICs show how smaller solution size for the same output power enables new generations of products delivering higher energy efficiency and better audio performance. Smaller really is better with class D.

The power stage is the key factor governing the performance of a class-D amplifier. Component selection holds the key to achieving great power and sound, taking advantage of the exceptionally high linearity and energy efficiency inherent in the class-D operating principle. Newer device technology moves these amplifiers closer towards the ideal by increasing efficiency and improving distortion. The evolution of class D never stops.

 

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