Amplifier designers crafting high-fidelity audio systems increasingly are facing the same challenges as those in the power industry: the constant quest for smaller size and greater efficiency. To address this challenge, engineers are utilizing new integrated Class D amplifiers alongside the latest MOSFET technology in both discrete and modular formats.
This article will consider the benefits of Class D amplifiers in general and discuss how the migration of key aspects of technology from power applications can enable designs and products that were not viable with the Class AB approach.
Class D Audio
Designers of high performance medium and high power audio applications traditionally have opted to use Class AB designs to meet efficiency and linearity goals. But advances in semiconductor process technologies and circuit techniques are making class D solutions more attractive for a wider variety of high power, low noise audio applications. We are now seeing Class D audio amplifiers used in high fidelity sound equipment with the ability to handle hundreds of Watts of power with higher efficiency and linearity while simultaneously attaining total harmonic distortion (THD) that is far below 0.05 percent.
Class D amplifiers operate as pulse-width modulated (PWM) switching devices. The analog signal is converted to a stream of pulses that switches amplifying MOSFETs fully on or off. MOSFETs have the lowest resistance when fully on, which gives Class D amplifiers their high efficiency characteristics. In the past, Class D amplifiers did not have the same quality as Class AB and were primarily used in compact, portable audio applications, or in line-powered audio applications with less demanding specs (such as powering subwoofers). More recently, high quality Class D amplifiers have reached market that present new options for high-end designs.
One of the significant aspects of Class D amplifiers is that the efficiency and the performance of a Class D amplifier is a function of Figure of Merit of the power device, which evolves from one generation to the next. However, designing and building a high power class D audio amplifier that matches the performance of a traditional class AB linear amplifier is not a simple task.
Design Challenges for Today’s Amplifier Designers
The major challenges for today’s amplifier designers are much the same as other design disciplines. Customers are looking for greater performance, more features, smaller packages, and lower costs. Most importantly, with the market trend toward adoption of wireless speakers, designers need to achieve performance goals while integrating the amplifier with speakers. This calls for design with relatively few components and a single-sided printed circuit board (PCB).
The challenge is compounded by the difference between Class D amplification and the characteristics of linear designs. Amplifier designers typically have a skill set based around low frequency analog design and the Class D amplifier acts and operates much more along the lines of a modern switch-mode power supply (SMPS). Thus amplifier designers need to adopt a whole new design approach and component set to get the job done.
How Class D Is Enabling Applications That Were Not Possible Before
There are numerous examples of Class D amplification being used to solve a design challenge in a way that enables a product category that did not exist before. Examples of such groundbreaking products include 500 W speakers with an integrated amplifier, a 3 kW amplifier that, at 7 lbs, weighs less than 10 percent of a traditional Class AB design, and a four-channel amplifier which fits easily behind the dash in mid-range cars.
By moving to Class D amplification (Figure 1), the efficiency of each design is increased. This means that smaller components can be used and often heatsinks are not required. More power can be packed in a smaller space, and the amplifier can be put in places it would not fit before. It also allows designs to be sealed; this affords protection from general dirt and dust and enables more reliable, maintenance-free and fan-less designs.
Similarities with Advances in Power System Design
The similarities between Class D amplifier design and SMPS becomes even more relevant when we understand that amplifier performance is directly influenced by the quality of components used to build it – particularly the PWM-control IC and the audio MOSFETs.
Improvements in control ICs are mostly directed towards better noise immunity. Even when using the best quality components some noise will inevitably be present in the switching stage. It is crucial to have a noise-immune control IC to correct errors accurately. Simply put, the better the immunity, the better the listening experience for the end user.
As far as MOSFETs are concerned, amplifier designers benefit from the steady pace of improvements at the silicon level to enhance switching speed and accuracy. One of the main ways this is achieved is improvement in the on-resistance (RDS(ON)) and gate charge (Qg). The product of these two key parameters (RDS(ON) x Qg) is the Figure of Merit (FoM) for the MOSFET, which is now an equally important performance benchmark in both audio systems and power supplies.
How The Latest Technology Addresses Design Challenges
Looking at the required success factors for component suppliers in the Class D market, it is clear that audio designers should consider the combined performance of both the PWM and MOSFET technology when evaluating components. Integrated amplifier solutions available now combine the audio controller IC and audio-optimized MOSFETs to achieve improved efficiency, THD, and EMI while allowing the system to operate without a heatsink (Figure 2).
The transition from linear amplifier design to PWM-based Class D is eased with the availability of a comprehensive range of reference design kits. These kits are available for discrete designs with separate controller and MOSFET and also integrated designs based upon the company’s PowIRaudioTM technology. There are sixteen kits available including two, three and four channel designs with multiple options covering the 35-1700 W range. Additionally, two reference designs for audio power supplies are offered.
By selecting one of these kits as a starting point for a Class D design, designers can be assured that they are starting with a well thought out design that incorporates the latest technology to help meet design challenges.