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Enhancing Performance and Reliability in High-Power Class D Audio Amplifier Designs
Green energy standards, the continuing need to reduce costs and the demand for higher audio fidelity are driving the adoption of Class D amplifiers in high-power audio applications. Traditional analog implementations, such as Class AB topology, are more complex and less efficient, and yet have dominated the high end of the audio market due to their high-fidelity performance.
A typical Class D audio system converts audio signals at its input to a digital PWM signal, adds power amplification in the digital domain, and then converts the digital signal back to analog at its output. This modulator digitizes the audio input by varying the modulator duty cycle in direct proportion to the instantaneous value of the audio input signal.
Class D amplifier design
Power efficiency
Historically, analog power amplifiers have relied on linear amplification circuits that are prone to high-power losses. By comparison, Class D amplifier power efficiencies can be 90% or higher, depending on the design. This high efficiency benefit is intrinsic in Class D technology where binary switches, usually power MOSFETs, are the amplification mechanism.
High fidelity
Audio fidelity can be defined as the faithfulness with which sound is reproduced. For audio systems, fidelity is a proxy for the all-enveloping term "sound quality." While various specifications are used to quantify fidelity, some of these measures are especially challenging for designers.
THC is a measure of accuracy of an audio system, very much akin to high fidelity itself. Inaccuracies in signal reproduction create additional signal components at harmonic multiples of the input frequencies, which obviously distract from the purity of the output signal.
Output noise level is a measure of the noise floor level of the amplifier outputs with no signal input. For most speakers, a noise floor of 100-500 µV is inaudible from most normal listening distances, while a noise floor as high as 1 mV will prove to be quite annoying.
Class D driver IC: features and benefits
Programmable dead-time
Class D amplifier dead-time (i.e., the period when both switches are off) directly impacts efficiency as well as THD. An overly short dead-time causes shoot-through currents that decrease efficiency a dead-time that is too long results in increased THD, which negatively affects audio fidelity.
The dead-time period must be set precisely to hit the "sweet spot" that optimizes both power efficiency and THD. The typical high-voltage audio drivers available today have coarse and overlapping dead-time settings. As a result, most designers choose to implement the dead-time period using discrete components, which can be expensive and time consuming.
Level shifting
Implementing a two-state Class D amplifier can be difficult due to input level shifting requirements. In high-power Class D amplifiers, it is desirable to have high-voltage supply rails (± VSS) for the power MOSFET stage. For practical Class D amplifier designs, a voltage of ±100 Vdc can deliver an astounding 600 watts of audio power into 8Ω.
Most available high-voltage IC (HVIC) Class D drivers lack the capability to provide level shifting from the low-voltage modulation section to the high-voltage power section. Drivers that provide level shifting have other deficiencies, making them less than ideal for Class D operation. This functionality is added through discrete components, which can be costly and difficult to design and take up an inordinate amount of space. Level shifting solutions that provide an interface to high-voltage bipolar supply rails would be a significant advantage in Class D designs.
Most driver solutions typically do not offer input-to-output isolation or isolation between the drivers.Thus, it becomes necessary to provide a level shifting mechanism with extra components.
Reliability and noise immunity
Typical gate driver ICs available today have a tendency to latch-up at high voltage transients of 20 V/ns or greater and typically do not have any immunity to high slew rate noise transients coupling back from the power stage to the precision digital input side. This is a major disadvantage when trying to keep the noise floor as low as possible for the best audio fidelity.
High-frequency operation
One of the best attributes a Class D gate driver can have is its ability to operate at high switching frequencies with minimum of propagation delay. These attributes allow the total loop delay in the feedback path to be exceptionally low for the best possible noise performance.
Integration
With today's highly competitive global markets, a solution that integrates all of these features would provide a much needed advantage to Class D amplifier designers, enabling them to get their products to market early by minimizing costly design time, component count, insertion costs and the implied lower reliability associated with higher parts count.
About the Author
Silicon Labs – Wireless RF and Optocoupler
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