Enhancing Performance and Reliability in High-Power Class-D Audio Amplifier Designs-EEWORLD

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Green energy standards, lower costs, and the need for higher audio fidelity are driving the use of Class D amplifiers in high-power audio. Traditional analog implementations (such as Class AB topology) are complex and inefficient, but have dominated the high-end audio market due to their high-fidelity performance for audio. Class D systems are simpler to design, more efficient, and offer high-fidelity capabilities comparable to analog amplifiers, and are rapidly closing the gap in the high-end audio market.

A typical Class D audio system first converts the analog audio input signal into a digital PWM signal, performs power amplification in the digital domain, and then converts the digital signal into an analog audio signal for output. As shown in Figure 1, the input audio signal is sent to a pulse width modulator (PWM), which consists of an operational amplifier and a comparator. The modulator digitizes the audio by generating a modulated duty cycle signal that is proportional to the instantaneous value of the audio input signal.

Figure 1: Basic block diagram of a Class D amplifier

The PWM signal is appropriately level-shifted and sent to the gate driver, which controls a two-state power circuit consisting of MOSFETs (M1 and M2). The amplified signal then passes through an output filter (eliminating the PWM carrier frequency), and ultimately only the amplified analog audio signal drives the speaker. External loop filtering is performed by feeding the filter input signal back to the error amplifier input, reducing distortion and noise, further improving audio output fidelity.

Class D Amplifier Design

effect

Traditional analog power amplifiers rely on linear amplification circuits, which are prone to high power losses. In contrast, Class D amplifiers can achieve power efficiencies of 90% or more (depending on the design). This high efficiency benefit is inherent in Class D amplifier technology, where the amplification mechanism uses binary switches (usually power MOSFETs). These switches are either fully on or fully off, with only a small amount of time spent in state transitions. The discrete switching action and low MOSFET on-resistance reduce I ² R losses and improve efficiency. However, in practice, the switch transition time (dead time) must be long enough to avoid a sharp drop in efficiency when both switches are running simultaneously.

High Fidelity

Audio fidelity can be defined as the integrity of sound after reproduction. For audio systems, fidelity has always been synonymous with sound quality. At the same time, other indicators are also used to measure fidelity, and the measurement of some indicators is particularly challenging for designers. The two most challenging indicators are: total harmonic distortion (THD) and noise (N), collectively referred to as THD+N.

THD is an accurate measurement of an audio system, much like hi-fi itself. Errors in the reproduced signal come from harmonics of the input frequency produced by other components, which are noticeably different from the pure output signal. THD is the ratio of all unwanted harmonic frequency energy to the fundamental input frequency energy, typically measured at half power for a given system. THD performance is typically less than 0.1% for most non-hi-fi audio applications, with discerning listeners often requiring THD levels as low as 0.05% or even lower.

The output noise level is a measure of the noise floor level at the output of an amplifier with no signal input. For most speakers, a noise floor of 100-500µV is inaudible at a normal listening distance, and a noise floor of 1mV is too loud, so THD+N is a good indicator of the audio fidelity of an amplifier.

Class D Driver ICs: Features and Benefits

Programmable dead time

The dead time of a Class D amplifier (i.e., the period of time when both switches are off) directly affects efficiency and THD. A dead time that is too short will cause shoot-through current and reduce efficiency, while a dead time that is too long will increase THD, which will adversely affect audio fidelity.

Dead time must be set precisely to find the "sweet spot" that optimizes both power efficiency and THD. Today's typical high voltage audio drivers have imprecise, overlapping dead time settings (i.e., 1/n delay values). As a result, most designers choose to handle dead time with discrete components, which is both expensive and time consuming. A simple and economical solution is to integrate a gate driver with a high-precision dead time generator.

Level conversion

Implementing a two-state Class D amplifier can be challenging due to the input level shifting requirements. In high power Class D amplifiers, it is best to provide a high voltage supply rail (±VSS) for the power MOSFET stage. In a practical Class D amplifier design, ±100Vdc voltages can produce up to 600W of audio power into an 8Ω load .

大多数现有高电压IC（HVIC）D类驱动器缺乏将低压调制部分转为高压电源部分的能力。能够提供电平转换的驱动器有也有其他不足，这使得它很难成为D类操作的理想选择（例如，驱动器输出接地端子采用负电压轨，要求输入驱动信号电平转换到负电源）。通过分立器件添加该项功能，成本高、设计难度大且占用大量空间，具备高电压双极供电接口的电平转换解决方案是D类设计的显著优势。

Typically, most driver solutions do not provide input-output isolation, nor isolation between drivers, so additional components are required to provide a level conversion mechanism.

Figure 3: Level translation required to interface between a low voltage digital modulator and a high voltage bipolar output supply

Reliability and noise suppression

Typical existing gate driver ICs are prone to latch-up on high voltage transients of 20V/ns or greater and typically have no suppression of high slew rate transient noise that couples from the power stage feedback to the precision digital inputs. This lack of noise suppression is a major disadvantage when trying to achieve the best audio fidelity while keeping the noise floor as low as possible.

High frequency operation

One of the best features of a Class D gate driver is the ability to operate at high switching frequencies with minimal propagation delay. These features allow the total loop delay in the feedback path to be very low, resulting in the best possible noise performance. Operating at a higher frequency also increases the "loop gain," improving the amplifier's distortion performance. Most existing HVIC drivers only support modulation frequencies up to 1MHz.

Integration

In today's competitive global market, a solution that integrates all of these features will provide Class D amplifier designers with great benefits, allowing them to get their products to market sooner by reducing design time, component count, insertion cost, and lower reliability due to higher device count.

summary

D类放大器的特性远远超越了传统模拟放大器，包括更低的THD、更小的电路板空间、更高的功率效率和更低的BOM成本。高集成的栅极驱动器IC对系统构架和音频性能都有显著的积极作用。Silicon Labs公司的Si8241/8244音频驱动器是首个集成所有特性到单一IC封装的高功率D类放大器解决方案。这些栅极驱动器的优点包括：为最低THD和最佳功效提供高精度死区时间设置；无需为输入信号电平转换而增加复杂设计和器件数量；隔离的输出驱动器，简化双态开关器实现；对瞬变电源有较高抑制力。要想了解有关Si824x D类音频驱动程序的更多内容，以及如何利用这些栅极驱动器为高保真音频市场提供新的设计思路，请访问： www.silabs.com/audio-driver 。

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