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Energy Efficiency with Class D Amplifier Modules
By Jun Honda
Director, Audio System Engineering
Class-D switching amplifiers are helping audio designers create personal multimedia devices and home audio/visual systems that demonstrate how compact and stylish equipment can also deliver high sound quality and high audio output power. The key to this breakthrough, providing freedom from the large and bulky boxes housing traditional audio products, lies in the class-D amplifier’s high energy efficiency, which is typically around 90%. This allows designers to reduce or eliminate heatsinks as well as using smaller-sized PCBs and smaller components such as transformers, connectors and power supplies.
However, switching amplifiers also introduce engineering challenges that may be unfamiliar to specialist audio designers, demanding power electronic skills such as the design of high-voltage switching circuitry. The availability of modules that integrate key elements of class-D amplifier circuitry into a single device helps engineers overcome such challenges, while also reducing component count and time to market.
Operating Principles and Design Challenges
In a class-D amplifier, the input audio signal is compared with a high-frequency sawtooth waveform to produce a pulse-width-modulated square-wave representation of the input. The sawtooth frequency is set well outside the audio signal frequency range, in the region of 400kHz.
The pulse-width-modulated equivalent of the audio signal drives the amplifier output stage, which may be a half-bridge or full-bridge topology. A half-bridge requires positive and negative power supply rails, whereas a full bridge is able to operate from a single power supply and also produces a higher output for a given supply voltage.
The amplified audio signal is contained within the square wave present at the output of the MOSFET bridge. A low pass filter removes the out-of-audio frequencies and restores the pure audio signal to drive the speaker.
Before the audio input signal reaches the PWM comparator, an error amplifier compares the input with the output audio signal to correct for imperfections due to factors such as the finite switching time of the output MOSFETs, over/under-shoots associated with switching transitions, and power supply fluctuations. Since the error amplifier must operate within a very noisy environment, selecting a suitable op-amp for the task can be difficult and costly.
Another important task for the designer is to isolate the noise-sensitive analogue circuitry at the input from the potentially disruptive switching noise generated at the output stage.
In addition, when tackling the switching stage, the designer must optimise deadtime insertion. Deadtime compensates for the finite switching time of the output MOSFETs thereby preventing potentially damaging shoot-through currents, which may arise if the on phases of high-side and low-side MOSFETs are allowed to overlap. Precise gate control, with low pulse-width distortion and good matching between the high- and low-side driving signals, is essential so that deadtime can be minimised in order to achieve the lowest possible audio distortion.
Also, since the power transistors are either switched hard on or fully off, the PWM switching stage must be well protected. If the design of the switching stage and protection circuitry are not handled correctly, prototypes may fail to operate or may fail catastrophically when tested.
Simplifying Class-D Design
Audio systems designers can avoid many of these design challenges and potential risks by using a class-D amplifier module that implements key functions such as the error amplifier and protected PWM switching in a single package.
IR’s first-generation integrated class-D module, the IRS2092 audio driver integrates the error amplifier, PWM comparator, switching stage with deadtime insertion, and protection circuitry. It is designed to be connected to discrete output transistors from IR’s range of MOSFETs for digital audio applications from 50W to 500W, allowing a chipset approach to building class-D audio solutions. The audio MOSFETs have low on-resistance as well as optimised gate charge and body-diode reverse recovery characteristics, which serve to maximise energy efficiency and ensure low EMI and Total Harmonic Distortion plus Noise (THD+N).
Other important amplifier features that are closely linked with the design of the power switching stage include measures to eliminate EMI caused by the pulse-width modulator, as well as circuitry to implement click/pop noise reduction during start-up and shut-down. By implementing these features internally, the IRS2092 further reduces design overheads and component count. This approach solves the power electronic design challenges associated with class-D amplifiers and provides a foundation from which engineers can apply specialist audio skills to further improve performance.