RC阻容吸收电路的分析

shaorc

【不懂就问】
如图，功放芯片输出，经过LC滤波后，形成模拟音频信号（近似正弦波）
在送给扬声器负载前，还要经过绿色方框的RC阻容电路
不知道它的作用是什么
看到这样的说法

RC阻容电路是吸收电路，防止高脉冲电压损坏喇叭，起到保护喇叭作用

RC值决定时间常数，一般在1-10ms

3.3R是吸收谐振能量

【1】高脉冲电压何来？不是前面已经LC滤波了吗？

【2】RC时间常数在这里是给什么做延时呢？

【3】负载是感性，用电容来补偿感抗的变化，但是随着频率变化形成了并联谐振，所以在谐振频率点，最大的能量要被电阻（3.3R）吸收掉？

topwon

你的参考设计是哪家的？我按照TI做法是在图中电感的左侧也就是芯片输出脚加10R级别的电阻串联100pF级别的电容到地，对PWM的波形起阻尼作用。你这种与喇叭并联的RC电路，功能上应该接近传说中的“茹贝尔电路”，但是取值据说是要根据喇叭情况细调的。
以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导（为避免无辜被喷，今后回帖估计都要加这段免责申明了）。
功放电路里，大多数商品机输出端对地会有一个几欧到几十欧的电阻串联0.1微法电容，有的朋友以为这就是”茹贝尔网络“，其实，这个东西的作用，是吸收高频尖峰，避免高频自激，起稳定作用的。要说起到茹贝尔网络的作用，那也是微乎其微。
茹贝尔网络，是并联在低频喇叭上的一只电阻和一只电容的串联，使得低音喇叭在相当宽的频率范围内呈现近似纯阻，进而使分频点稳定，改善阻尼，改善相位失真。这个电阻跟电容的取值，电阻R取所用喇叭在需要补偿的频率下的阻抗，电容C的容抗取喇叭感抗L/RR，实际R一般取标称值，4欧喇叭C大约是4-10微法，8欧喇叭10-20微法。
“茹贝尔电路”的作用是将喇叭（准确的说，是低音喇叭）的感抗抵消，使喇叭在相当宽的频带里呈现近似纯阻，这个电路，应该接在分频器后，直接并在低频喇叭上，而非功放的输出，接在那儿的RC网络，不应称作“茹贝尔网络”，那只是个消振电路。

topwon

以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导（为避免无辜被喷，今后回帖估计都要加这段免责申明了）。
Zobel networks

      All power amplifiers except for the most rudimentary kinds include a Zobel
network in their arrangements for stability. This simple but somewhat enig-
matic network comprises a resistor and capacitor in series from the ampli-
fier output rail to ground. It is always fitted on the inside (i.e., upstream)
of the output inductor, though a few designs have a second Zobel net-
work after the output inductor; the thinking behind this latter approach is
obscure. The resistor approximates to the expected load impedance, and is
usually between 4.7 and 10Ω. The capacitor is almost invariably 100 nF,
and these convenient values and their constancy in the face of changing
amplifier design might lead one to suppose that they are not critical; in fact
experiment suggests that the real reason is that the traditional values are
just about right.
     The function of the Zobel network (sometimes also called a Boucherot cell)
is rarely discussed, but is usually said to prevent too inductive a reactance
being presented to the amplifier output by a loudspeaker voice-coil, the
implication being that this could cause HF instability. It is intuitively easy to
see why a capacitative load on an amplifier with a finite output resistance
could cause HF instability by introducing extra lagging phase-shift into
the global NFB loop, but it is less clear why an inductive load should be
a problem; if a capacitive load reduces stability margins, then it seems
reasonable that an inductive one would increase them.
At this point I felt some experiments were called for, and so I removed the
standard 10Ω/0.1μF Zobel from a Blameless Class-B amplifier with CFP

output and the usual NFB factor of 32 dB at 20 kHz. With an 8Ω resistive
load the THD performance and stability were unchanged. However, when
a 0.47mH inductor was added in series, to roughly simulate a single-unit
loudspeaker, there was evidence of local VHF instability in the output
stage; there was certainly no Nyquist instability of the global NFB loop.
I also attempted to reduce the loading placed on the output by the Zobel
network. However, increasing the series resistance to 22Ω still gave some
evidence of stability problems, and I was forced to the depressing conclu-
sion that the standard values are just about right. In fact, with the standard
10Ω/0.1μF network the extra loading placed on the amplifier at HF is not
great; for a 1 V output at 10 kHz the Zobel network draws 6.3 mA, rising
to 12.4mA at 20 kHz, compared with 125mA drawn at all frequencies by
an 8Ω resistor. These currents can be simply scaled up for realistic output
levels, and this allows the Zobel resistor power rating to be determined.
Thus an amplifier capable of 20 V rms output must have a Zobel resistor
capable of sustaining 248mA rms at 20 kHz, dissipating 0.62 W; a 1W
component could be chosen.
       In fact, the greatest stress is placed on the Zobel resistor by HF instability, as
amplifier oscillation is often in the range 50–500 kHz. It should therefore be
chosen to withstand this for at least a short time, as otherwise faultfinding
becomes rather fraught; ratings in the range 3 to 5W are usual.
To conclude this section, there seems no doubt that a Zobel network is
required with any load that is even mildly inductive. The resistor can be
of an ordinary wire-wound type, rated to 5W or more; this should prevent
its burn-out under HF instability. A wire-wound resistor may reduce the
effectiveness of the Zobel at VHF, but seems to work well in practice; the
Zobel still gives effective stabilisation with inductive loads.

topwon

以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导（为避免无辜被喷，今后回帖估计都要加这段免责申明了）。
两种RC电路的设计资料，都是收集于网络，仅供参考，与本人无关。
ZobelNetworkDesign：茹贝尔网络设计.pdf (216.75 KB, 下载次数: 103)

2019-4-26 09:03 上传

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Class-D Output Snubber Design Guide sloa201.pdf (58.22 KB, 下载次数: 95)

2019-4-26 09:04 上传

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topwon

这篇TI的技术文档应该是我目前看到对这部分电路讲得最详细最实用的了，希望有所帮助（找资料的过程我也学到了不少知识）。 8838.Class D功放高次谐波过流保护分析和解决方法.pdf (285.46 KB, 下载次数: 134)

2019-4-26 09:37 上传

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topwon

以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导（为避免无辜被喷，今后回帖估计都要加这段免责申明了）。

我在互联网上找了一个８Ω喇叭单体简化的等效电路，再利用免费的 TINA-TI V9 仿真软体，加挂个交流阻抗表测绘出阻抗丶相位与频率三者之相应关系曲线，最上图是喇叭单体原本特性，中间图并接上功放输出端常见的 R-C 消振网路(俗称也是茹贝尔网路)，最下图则是隐没在音箱内分频网路中的正规茹贝尔网路，参考看看。


中间那张图应该就是TI文档中提到的解决对策，使用较小的电容，达到抑制高频增益目的即可（不必像标准茹贝尔电路用的大电容来达到完全平坦的效果）。

shine9547788

典型的茹蓓尔，但是要调合适的参数就是了。
参数合适，就可以消除负载的感性，不对，是削弱负载的感性同时不会带来太大的其他问题。

topwon

最后再上一篇老外关于Zobel电路参数的计算和作用分析。文中我添加了一个台湾人写的翻译（有些地方我也做了细微改动）。
以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导，老手不喜勿喷，指出问题即可。
IC Audio Power Amplifiers and Zobel Networks---One Size Does Not Fit All 利用Zob.pdf (450.35 KB, 下载次数: 76)

2019-4-29 13:34 上传

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shaorc

topwon 发表于 2019-4-29 13:36
最后再上一篇老外关于Zobel电路参数的计算和作用分析。文中我添加了一个台湾人写的翻译（有些地方我也做了 ...

谢谢 下载在看了中

lqhqq

topwon 发表于 2019-4-26 10:02 以下信息来自网络随意摘抄，如有错漏概不负责，新手请注意避免被误导（为避免无辜被喷，今后回帖估计都要加 ...

学习了。

hk6108

这功放是 BTL 拓扑吗？

Azi-

这是补偿，拉来的吸收一说？

jacen

学习了

不惑吗