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热水器恒温控制系统文献综述专业：电子科学与技术 班级：06级1班 作者 任虎俊 指导老师：曾旺辉1.前言热水器是一种可供浴室，洗手间及厨房使用的家用电器，具有卫生、方便并且加热迅速的特点，因此得到广泛的应用。目前市场上热水器主要品种有电热水器、太阳能热水器、燃气热水器.就中国的具体情况而言,由于太阳能热水器的使用受天气原因的限制,使用范围狭窄；燃气热水器由于以石油、天然气为燃料,而燃料供应量又难以满足人们日益增长的需求，且不利于环境，因此电热水器越来越受到消费者的青睐.目前市场上的电热水器按结构分连续水流式和储水式，前者虽具有加热速度快和体积小的优点，但需要的功率大，大多数家庭供电线路难以承受。而市场上传统的机械式电热水器控制功能不完善,而且精度低、可靠性差，生活质量的提高使得消费者对电热水器要求越来越趋向于智能化和数字化，通过上网查阅电子文献、论坛看贴、图书馆借阅相关书籍的方式来搜集有关电热水器智能控制器相关方面的资料。发现现阶段电热水控制器在控制精度、可靠性好，安全性、实用性、节能性等方面比较完善。但在控制功能方面还需进一步完善，如智能预约，电话远程控制功能等还是刚刚起步，因此我们采用单片机作为控制中心设计家用电热水器智能控制器，其将具有调温、恒温、防干烧、防超高温、防漏电等功能，且配备遥控器，方便操作。2.电热水控制器的主要方案2.1电热水控制器的功能简介从文献1246中可以看出，目前电热水控制器主要有两种形式，一种是直热式(快热式)电热水控制器，另一种是储水式电热水控制器。贮水式电热水器是用电热器件把贮存在水箱内的冷水加热到所需要的温度后供人们，贮水式电热水器一般由箱体系统、制热系统、控制系统和进出水系统四大部分构成；流动式是冷水直接流过电热器件，使冷水在流动中被加热至所需的温度。它们的主要功能也基本相同，如：数码显示档位及水温，方便任意调节；防漏电保护装置，采用微电脑控制，自动安全检测，出现意外时热水器自动切断电源有效地保护人身安全。超温保护装置，水温恒定，防止烫伤；防干烧保护装置，确保安全等。2.2电热水控制器主要功能方案2.2.1 温度检测电路方案文献中关于温度检测的方案主要有三种。（1）李建国在微电脑电热水器的控制采用高性能的温敏电阻实时采集热水器内水温,将温度信号转变为电压信号后送单片机处理（热敏电阻将温度信息转换为电压信息，经A/D转换模块输入CPU,查表后得到实时水温）。（2）雷建龙在基于模糊控制的水温自动调节器一文中利用温度传感器及有关电路将温度转化为电脉冲的脉宽，单片机将脉冲宽度的值转换为与之对应的温度值与设定温度相比较后，控制电热水器的开启与关闭就可达到水温自动调节的目的。对任意温度的脉宽值还可以进行自动检测，并加以显示。（3）张鹏在分辨率可编程的一线总线数字温度计DS18B20及其应用中采用单总线温度传感器DS18B20。与传统的热敏电阻温度传感器不同的是，DS18B20能够直接读出被测温度，无须进行复杂的计算，并且该芯片在检测点已把被测信号数字化了，因此在单总线上传送的是数字信号，因而无须进行A/D转换，可以直接给单片机进行处理。选择了该芯片也使得整个系统的温度采集模块的硬件极其简单，只占用单片机的一个数据I/O口,再加上一个上拉电阻即可。2.2.3 开机方式刘海英等基于单片机的电热水器定时控制器设计文中的定时控制器主要是以单片机AT89s52作为核心控制元件，通过外围电路控制热水器的电源，以达到定时开关机的目的。系统加电后即进入正常计时状态，用户可以随时进行校准时间动作以及设定热水器开关的时间，控制器将会在设定的开关时刻通过单片机的输出端口控制输出继电器动作，进而控制热水器的启闭。2.2.4 漏电检测和保护电路方案（1）梁国华在家用电热水器模糊控制器的设计中将漏电检测线圈的输出经过比较器后送给单片机中断口，一旦漏电超过规定的阀值，单片机立即响应中断，切断整个系统的电源。报警电路是在传感器故障、超温时报警提示。（2）张静，陈新微电脑控制多功能热水器的漏电检测功能为在正常情况下，流过磁环的电流大小相等，方向相反，磁环检测线圈无感应电流信号，漏电检测集成电路输出低电平。当出现漏电电流时，由了流过磁环的电流不平衡，于是磁环检测线圈感应出漏电信号，经集成电路M54123L放大输出高电平，经三极管倒相后输出至CPU。CPU接收到漏电信号，则停止加热保温及键盘操作，结束工程程序并发出报警信号，电源指示灯闪烁警示，蜂鸣器连续呜响。正常情况下，CPU每隔10ms就发出一个漏电保护可靠性自检脉冲，代替普通漏电开关的试验按钮，控制晶体管导通，于是磁环流过一个大于10mA的电流，该电流作为模拟漏电信号被磁环检测，经M54123L放大及三极管倒相后，输出至CPU。CPU自动判断是否为自检信号以及自检是否合格，有信号则自检合格，继续执行程序；无信号则自检不合格，自动停止加热保温及键盘操作，结束工作程序并发出报警 信号，电源指示灯闪烁警示，蜂鸣器连续呜响，从而确保电热水器的安全使用。在漏电保护及自检不合格情况下，只有关闭电源及排除故障后，重新接通电源才能工作。3.主要特点及研究趋势3.1主要特点电热水器一般分为储水式和速热式两大类。速热式加热较快，但因为功率太大，一般家庭的电表很难承受。而储水式具有出水温度均匀，加热功率适中，安全系数较高等特点，是家庭使用首选。但储水式电热水器体积大，占用卫生间空间；使用前需要预热，不能连续使用超出额定容量的水量。要是家庭人多，洗澡中途还得等；内部温度达到80后，容易生垢，为加热管破裂、漏电埋下了安全隐患，需一年除垢一次。此外，许多老式住房的电路接地不可靠，目前采用的漏电保护器仅解决了产品本身的安全问题，但用户仍面临由外部环境引入的水带电等触电隐患.。而市场上传统的机械式电热水器控制功能不完善,而且精度低、可靠性差，已经很少使用。早期的电热水控制器因控制功能简单、智能化程度低、保护功能少，已不适应现代生活的需求。文献9 中控制器采用高性价比单片机，运用软件控制技术，融合智能化控制思想，增加了漏电检测、干烧报警等多重保护功能，大大提高了电热水器的智能化、安全性，并采用LCD显示温度，使热水器具有良好的人机界面，方便用户操作，使电热水器显得更加经济实用。3.2 电热水控制器的研究趋势现今电热水控制器的研究主要朝着提高安全性、实用性、节能性的方向发展，出现智能防电墙技术，拥有恒温设计、智能预约、电话远程控制等功能的新型电热水器，对生活舒适度提出了新的要求。(1)智能防电墙技术智能防电墙技术不但实时采集、检测电流、电压等动态信号，确保沐浴安全，更针对中国家庭普遍存在的环境带电隐患，全面监控家庭用电环境，发现异常及时闪亮警示灯，蜂鸣报警，严防危电伤人。(2)防烫伤恒温功能拥有恒温设计，不仅在使用中不会忽冷忽热，而且其特有的防烫伤装置能保证出水温度最高不超过50，避免水温过高造成意外的烫伤。(3)智能预约功能智能预约功能提前为用户准时备好热水。智能预约功能根据设定的洗浴时间自动计算加热时间，保证在用户洗浴前刚好加热完毕，避免了反复加热。(4)电话远程控制功能通过电话线实现对家用电器的远程集中控制，打个电话就可以遥控家里的热水器提前加热洗澡水。这就克服了电热水器最大的弊病洗澡前约一个小时的无聊等待，从而使人们的生活更加更加轻松方便。4. 结语热水器恒温控制系统采用ATMEL公司的单片机为核心控制系统进行设计。结合外围温度采集电路、加热电路、时钟电路、显示电路、保护及报警电路。有机的结合起来达到预定的目标。通过各个模块的组合，完成此次的设计。此设计结合数字电路，模拟电路，微机接口电路，单片机运用等方面的知识。电脑辅助设计软件的运用，达到了学以自用，完整的结合起来，通过所学的知识完成此次的设计任务。对完整的系统设计和对知识的搜索、整理和理解。提高自身的知识的连贯性。对整体的把握有了很深的认识。参考文献1 李建国，微电脑电热水器的控制，浙江：浙江大学出版社，2000.72 雷建龙. 基于模糊控制的水温自动调节器J.单片机与嵌入式系统应用，2003，(8):10-11.3 张 鹏. 分辨率可编程的一线总线数字温度计DS18B20及其应用J.电子产品世界，2002，(4):20-24.4 梁国华. 家用电热水器模糊控制器的设计J.应用技术，2004，(12)：89-90.5 刘筱明. 电脑电热水器继电器非正常状态下的保护措施，北京：北京大学，1999.96 张 静，陈新，微电脑控制多功能热水器，上海：上海交通大学，2002.97 牛金生. 电热电动器具原理与维修，福建：厦门大学,1998.128 易继锴，侯媛彬，智能控制技术，北京：北京工业大学出版社，1999.99 高 明. 单片微机接口与系统设计哈尔滨：哈尔滨工业大学出版社，19950 余永权. ATMEL89系列（MCS51兼容）Flash单片机原理及应用北京：电子工业出版社，199711 王建校等编著 51系列及C51程序设计 科学出版社 2002.412 高 鹏等编著 Protel99 入门与提高 人民邮电出版社 2000.213 梁军等编 单片机原理及应用东南大学出版社 200014 何立民编著 单片机高级教程 北京航空航天大学出版社 2000第 5 页 共 5 页 Analog Dialogue 40-06, June (2006) Class D Audio Amplifiers: What, Why, and HowBy Eric Gaalaas eric.gaalaasClass D amplifiers, first proposed in 1958, have become increasingly popular in recent years. What are Class D amplifiers? How do they compare with other kinds of amplifiers? Why is Class D of interest for audio? What is needed to make a “good” audio Class D amplifier? What are the features of ADIs Class D amplifier products? Find the answers to all these questions in the following pages.Audio Amplifier BackgroundThe goal of audio amplifiers is to reproduce input audio signals at sound-producing output elements, with desired volume and power levelsfaithfully, efficiently, and at low distortion. Audio frequencies range from about 20 Hz to 20 kHz, so the amplifier must have good frequency response over this range (less when driving a band-limited speaker, such as a woofer or a tweeter). Power capabilities vary widely depending on the application, from milliwatts in headphones, to a few watts in TV or PC audio, to tens of watts for “mini” home stereos and automotive audio, to hundreds of watts and beyond for more powerful home and commercial sound systemsand to fill theaters or auditoriums with sound.A straightforward analog implementation of an audio amplifier uses transistors in linear mode to create an output voltage that is a scaled copy of the input voltage. The forward voltage gain is usually high (at least 40 dB). If the forward gain is part of a feedback loop, the overall loop gain will also be high. Feedback is often used because high loop gain improves performancesuppressing distortion caused by nonlinearities in the forward path and reducing power supply noise by increasing the power-supply rejection (PSR).The Class D Amplifier AdvantageIn a conventional transistor amplifier, the output stage contains transistors that supply the instantaneous continuous output current. The many possible implementations for audio systems include Classes A, AB, and B. Compared with Class D designs, the output-stage power dissipation is large in even the most efficient linear output stages. This difference gives Class D significant advantages in many applications because the lower power dissipation produces less heat, saves circuit board space and cost, and extends battery life in portable systems.Linear Amplifiers, Class D Amplifiers, and Power DissipationLinear-amplifier output stages are directly connected to the speaker (in some cases via capacitors). If bipolar junction transistors (BJTs) are used in the output stage, they generally operate in the linear mode, with large collector-emitter voltages. The output stage could also be implemented with MOS transistors, as shown in Figure 1./analogdialogueOUTPUTSTAGEMHMLSPEAKERVOUTVINGROUND0VVSSVDDFigure 1. CMOS linear output stage.Power is dissipated in all linear output stages, because the process of generating VOUT unavoidably causes nonzero IDS and VDS in at least one output transistor. The amount of power dissipation strongly depends on the method used to bias the output transistors.The Class A topology uses one of the transistors as a dc current source, capable of supplying the maximum audio current required by the speaker. Good sound quality is possible with the Class A output stage, but power dissipation is excessive because a large dc bias current usually flows in the output-stage transistors (where we do not want it), without being delivered to the speaker (where we do want it).The Class B topology eliminates the dc bias current and dissipates significantly less power. Its output transistors are individually controlled in a push-pull manner, allowing the MH device to supply positive currents to the speaker, and ML to sink negative currents. This reduces output stage power dissipation, with only signal current conducted through the transistors. The Class B circuit has inferior sound quality, however, due to nonlinear behavior (crossover distortion) when the output current passes through 0 and the transistors are changing between the on and off conditions.Class AB, a hybrid compromise of Classes A and B, uses some dc bias current, but much less than a pure Class A design. The small dc bias current is sufficient to prevent crossover distortion, enabling good sound quality. Power dissipation, although between Class A and Class B limits, is typically closer to Class B. Some control, similar to that of the Class B circuit, is needed to allow the Class AB circuit to supply or sink large output currents.Unfortunately, even a well-designed class AB amplifier has significant power dissipation, because its midrange output voltages are generally far from either the positive or negative supply rails. The large drain-source voltage drops thus produce significant IDS 3 VDS instantaneous power dissipation.Thanks to a different topology (Figure 2), the Class D amplifier dissipates much less power than any of the above. Its output stage switches between the positive and negative power supplies so as to produce a train of voltage pulses. This waveform is benign for power dissipation, because the output transistors have zero current when not switching, and have low VDS when they are conducting current, thus giving smaller IDS 3 VDS. MODULATORSWITCHINGOUTPUTSTAGELOSSLESSLOW-PASSFILTER(LC)SPEAKERVINVOUTFigure 2. Class D open-loop-amplifier block diagram.2 Analog Dialogue 40-06, June (2006)Since most audio signals are not pulse trains, a modulator must be included to convert the audio input into pulses. The frequency content of the pulses includes both the desired audio signal and significant high-frequency energy related to the modulation process. A low-pass filter is often inserted between the output stage and the speaker to minimize electromagnetic interference (EMI) and avoid driving the speaker with too much high frequency energy. The filter (Figure 3) needs to be lossless (or nearly so) in order to retain the power-dissipation advantage of the switching output stage. The filter normally uses capacitors and inductors, with the only intentionally dissipative element being the speaker.MHAMLAMHBMLBLLCCVSSVDDFigure 3. Differential switching output stage and LC low-pass filter.Figure 4 compares ideal output-stage power dissipation (PDISS) for Class A and Class B amplifiers with measured dissipation for the AD1994 Class D amplifier, plotted against power delivered to the speaker (PLOAD), given an audio-frequency sine wave signal. The power numbers are normalized to the power level, PLOAD max, at which the sine is clipped enough to cause 10% total harmonic distortion (THD). The vertical line indicates the PLOAD at which clipping begins.Significant differences in power dissipation are visible for a wide range of loads, especially at high and moderate values. At the onset of clipping, dissipation in the Class D output stage is about 2.5 times less than Class B, and 27 times less than Class A. Note that more power is consumed in the Class A output stage than is delivered to the speakera consequence of using the large dc bias current. Output-stage power efficiency, Eff, is defined asEffPPPLOADLOADDIS=+At the onset of clipping, Eff = 25% for the Class A amplifier, 78.5% for the Class B amplifier, and 90% for the Class D amplifier (see Figure 5). These best-case values for Class A and Class B are the ones often cited in textbooks.10.0010.010.10.010.11POWER EFFICIENCYNORMALIZED LOAD POWER (PLOAD/PLOAD MAX)CLASS A IDEALCLASS B IDEALCLASS D AD199xMEASUREDFigure 5. Power efficiency of Class A, Class B, and Class D output stages.The differences in power dissipation and efficiency widen at moderate power levels. This is important for audio, because long-term average levels for loud music are much lower (by factors of five to 20, depending on the type of music) than the instantaneous peak levels, which approach PLOAD max. Thus, for audio amplifiers, PLOAD = 0.1 3 PLOAD max is a reasonable average power level at which to evaluate PDISS. At this level, the Class D output-stage dissipation is nine times less than Class B, and 107 times less than Class A.For an audio amplifier with 10-W PLOAD max, an average PLOAD of 1 W can be considered a realistic listening level. Under this condition, 282 mW is dissipated inside the Class D output stage, vs. 2.53 W for Class B and 30.2 W for Class A. In this case, the Class D efficiency is reduced to 78%from 90% at higher power. But even 78% is much better than the Class B and Class A efficiencies28% and 3%, respectively.100.010.110.010.11NORMALIZED POWER DISSIPATION (PDISS/PLOAD MAX)NORMALIZED LOAD POWER (PLOAD/PLOAD MAX)NOCLIPPINGAMPLIFIERCLIPSMAX POWER THE AMP CAN DELIVER.OUTPUT IS CLIPPED AT THIS POWER LEVEL (THD = 10%)CLASS A IDEALCLASS B IDEALCLASS D AD199x MEASUREDFigure 4. Power dissipation in Class A, Class B, and Class D output stages. Analog Dialogue 40-06, June (2006) These differences have important consequences for system design. For power levels above 1 W, the excessive dissipation of linear output stages requires significant cooling measures to avoid unacceptable heatingtypically by using large slabs of metal as heat sinks, or fans to blow air over the amplifier. If the amplifier is implemented as an integrated circuit, a bulky and expensive thermally enhanced package may be needed to facilitate heat transfer. These considerations are onerous in consumer products such as flat-screen TVs, where space is at a premiumor automotive audio, where the trend is toward cramming higher channel counts into a fixed space.For power levels below 1 W, wasted power can be more of a difficulty than heat generation. If powered from a battery, a linear output stage would drain battery charge faster than a Class D design. In the above example, the Class D output stage consumes 2.8 times less supply current than Class B and 23.6 times less than Class Aresulting in a big difference in the life of batteries used in products like cell phones, PDAs, and MP3 players.For simplicity, the analysis thus far has focused exclusively on the amplifier output stages. However, when all sources of power dissipation in the amplifier system are considered, linear amplifiers can compare more favorably to Class D amplifiers at low output-power levels. The reason is that the power needed to generate and modulate the switching waveform can be significant at low levels. Thus, the system-wide quiescent dissipation of well-designed low-to-moderate-power Class AB amplifiers can make them competitive with Class D amplifiers. Class D power dissipation is unquestionably superior for the higher output power ranges, though.Class D Amplifier Terminology, and Differential vs. Single-Ended VersionsFigure 3 shows a differential implementation of the output transistors and LC filter in a Class D amplifier. This H-bridge has two half-bridge switching circuits that supply pulses of opposite polarity to the filter, which comprises two inductors, two capacitors, and the speaker. Each half-bridge contains two output transistorsa high-side transistor (MH) connected to the positive power supply, and a low-side transistor (ML) connected to the negative supply. The diagrams here show high-side pMOS transistors. High-side nMOS transistors are often used to reduce size and capacitance, but special gate-drive techniques are required to control them (Further Reading 1).Full H-bridge circuits generally run from a single supply (VDD), with ground used for the negative supply terminal (VSS). For a given VDD and VSS, the differential nature of the bridge means that it can deliver twice the output signal and four times the output power of single-ended implementations. Half-bridge circuits can be powered from bipolar power supplies or a single supply, but the single-supply version imposes a potentially harmful dc bias voltage, VDD/2, across the speaker, unless a blocking capacitor is added.The power supply voltage buses of half-bridge circuits can be “pumped” beyond their nominal values by large inductor currents from the LC filter. The dV/dt of the pumping transient can be limited by adding large decoupling capacitors between VDD and VSS. Full-bridge circuits do not suffer from bus pumping, because inductor current flowing into one of the half-bridges flows out of the other one, creating a local current loop that minimally disturbs the power supplies.Factors in Audio Class D Amplifier DesignThe lower power dissipation provides a strong motivation to use Class D for audio applications, but there are important challenges for the designer. These include: Choice of output transistor size Output-stage protection Sound quality Modulation technique EMI LC filter design System costChoice of Output Transistor SizeThe output transistor size is chosen to optimize power dissipation over a wide range of signal conditions. Ensuring that VDS stays small when conducting large IDS requires the on resistance (RON) of the output transistors to be small (typically 0.1 V to 0.2 V). But this requires large transistors with significant gate capacitance (CG). The gate-drive circuitry that switches the capacitance consumes powerCV 2f, where C is the capacitance, V is the voltage change during charging, and f is the switching frequency. This “switching loss” becomes excessive if the capacitance or frequency is too high, so practical upper limits exist. The choice of transistor size is therefore a trade-off between minimizing IDS 3 VDS losses during conduction vs. minimizing switching losses. Conductive losses will dominate power dissipation and efficiency at high output power levels, while dissipation is dominated by switching losses at low output levels. Power transistor manufacturers try to minimize the RON 3 CG product of their devices to reduce overall power dissipation in switching applications, and to provide flexibility in the choice of switching frequency.Protecting the Output StageThe output stage must be protected from a number of potentially hazardous conditions:Overheating: Class Ds output-stage power dissipation, though lower than that of linear amplifiers, can still reach levels that endanger the output transistors if the amplifier is forced to deliver very high power for a long time. To protect against dangerous overheating, temperature-monitoring control circuitry is needed. In simple protection schemes, the output stage is shut off when its temperature, as measured by an on-chip sensor, exceeds a thermal-shutdown safety threshold, and is kept off until it cools down. The sensor can provide additional temperature information, aside from the simple binary indication about whether temperature has exceeded the shutdown threshold. By measuring temperature, the control circuitry can gradually reduce the volume level, reducing power dissipation and keeping temperature well within limitsinstead of forcing perceptible periods of silence during thermal-shutdown events.Excessive current flow in the output transistors: The low on resistance of the output transistors is not a problem if the output stage and speaker terminals are properly connected, but enormous currents can result if these nodes are inadvertently short-circuited to one another, or to the positive or negative power supplies. If unchecked, such currents can damage the transistors or surrounding circuitry. Consequently, current-sensing output-transistor protection circuitry is needed. In simple protection schemes, the output stage is shut off if the output currents exceed a safety threshold. In more sophisticated schemes, the 4 Analog Dialogue 40-06, June (2006)current-sensor output is fed back into the amplifierseeking to limit the output current to a maximum safe level, while allowing the amplifier to run continuously without shutting down. In these schemes, shutdown can be forced as a last resort if the attempted limiting proves ineffective. Effective current limiters can also keep the amplifier running safely in the presence of momentarily large transient currents due to speaker resonances.Undervoltage: Most switching output stage circuits work well only if the positive power supply voltages are high enough. Problems result if there is an undervoltage condition, where the supplies are too low. This issue is commonly handled by an undervoltage lockout circuit, which permits the output stages to operate only if the power supply voltages are above an undervoltage-lockout threshold.Output transistor turn-on timing: The MH and ML output stage transistors (Figure 6) have very low on resistance. It is therefore important to avoid situations in which both MH and ML are on simultaneously, as this would create a low-resistance path from VDD to VSS through the transistors and a large shoot-through current. At best, the transistors will heat up and waste power; at worst, the transistors may be damaged. Break-before-make control of the transistors prevents the shoot-through condition by forcing both transistors off before turning one on. The time intervals in which both transistors are off are called nonoverlap time or dead time.SWITCHINGOUTPUTSTAGEMHMLVOUTVSSMHONMHOFFMLONMLOFFNONOVERLAP TIMEMLOFFVDD Figure 6. Break-before-make switching of output-stage transistors.Sound QualitySeveral issues must be addressed to achieve good overall sound quality in Class D amplifiers.Clicks and pops, which occur when the amplifier is turning on or off can be very annoying. Unfortunately, however, they are easy to introduce into a Class D amplifier unless careful attention is paid to modulator state, output-stage timing, and LC filter state when the amplifier is muted or unmuted.Signal-to-noise ratio (SNR): To avoid audible hiss from the amplifier noise floor, SNR should typically exceed 90 dB in low-power amplifiers for portable applications, 100 dB for medium-power designs, and 110 dB for high-power designs. This is achievable for a wide variety of amplifier implementations, but individual noise sources must be tracked during amplifier design to ensure a satisfactory overall SNR.Distortion mechanisms: These include nonlinearities in the modulation technique or modulator implementationand the dead time used in the output stage to solve the shoot-through current problem.Information about the audio signal level is generally encoded in the widths of the Class D modulator output pulses. Adding dead time to prevent output stage shoot-through currents introduces a nonlinear timing error, which creates distortion at the speaker in proportion to the timing error in relation to the ideal pulse width. The shortest dead time that avoids shoot-through is often best for minimizing distortion; see Further Reading 2 for a detailed design method to optimize distortion performance of switching output stages.Other sources of distortion include: mismatch of rise and fall times in the output pulses, mismatch in the timing characteristics for the output transistor gate-drive circuits, and nonlinearities in the components of the LC low-pass filter.Power-supply rejection (PSR): In the circuit of Figure 2, power-supply noise couples almost directly to the speaker with very little rejection. This occurs because the output-stage transistors connect the power supplies to the low-pass filter through a very low resistance. The filter rejects high-frequency noise, but is designed to pass all audio frequencies, including noise. See Further Reading 3 for a good description of the effect of power-supply noise in single-ended and differential switching output-stage circuits.If neither distortion nor power-supply issues are addressed, it is difficult to achieve PSR better than 10 dB, or total harmonic distortion (THD) better than 0.1%. Even worse, the THD tends to be the bad-sounding high-order kind.Fortunately, there are good solutions to these issues. Using feedback with high loop gain (as is done in many linear amplifier designs) helps a lot. Feedback from the LC filter input will greatly improve PSR and attenuate all non-LC filter distortion mechanisms. LC filter nonlinearities can be attenuated by including the speaker in the feedback loop. Audiophile-grade sound quality with PSR 60 dB and THD 60 dB PS