Analysis of RC Resistor-Capacitor Absorption Circuit

shaorc

【Ask if you don't understand】 As shown in the figure, the output of the power amplifier chip forms an analog audio signal (approximately a sine wave) after LC filtering Before sending it to the speaker load,it also has to pass through the RC resistor-capacitor circuit in the green box I don't know what its function is Seeing this statement

The RC resistor-capacitor circuit is an absorption circuit that prevents high pulse voltage from damaging the speaker and plays a role in protecting the speaker

[p=20, null, RC value determines the time constant, which is generally between 1-10ms

3.3R absorbs resonant energy

【1】Where does the high pulse voltage come from? Hasn't LC filtering been done before?

【2】What is the RC time constant used for delaying here?

【3】The load is inductive, and the capacitor is used to compensate for the change in inductive reactance. However, as the frequency changes, parallel resonance is formed. Therefore, at the resonant frequency, the maximum energy is absorbed by the resistor (3.3R)?

[attach]4111 27[/attach]

topwon

Which company did you use as your reference design? I followed TI's practice and added a 10R resistor in series with a 100pF capacitor to the ground on the left side of the inductor in the figure, which is the chip output pin, to damp the PWM waveform. Your RC circuit in parallel with the speaker should be close to the legendary "Joubert circuit" in terms of function, but the value is said to be fine-tuned according to the speaker. The following information is randomly excerpted from the Internet. If there are any errors or omissions, I am not responsible. Newbies should be careful to avoid being misled (to avoid being innocently criticized, I guess you will have to add this disclaimer in future replies). In the power amplifier circuit, most commercial machines have a resistor of several ohms to tens of ohms connected to the ground at the output end in series with a 0.1 microfarad capacitor. Some friends think this is the "Joubert network". In fact, the role of this thing is to absorb high-frequency spikes, avoid high-frequency self-excitation, and play a stabilizing role. If it plays the role of a Joubert network, it is minimal. The Joubert network is a series connection of a resistor and a capacitor connected in parallel to the low-frequency speaker, which makes the bass speaker present a near-pure impedance in a fairly wide frequency range, thereby stabilizing the crossover point, improving damping, and improving phase distortion. The values of this resistor and capacitor are: the resistor R is the impedance of the speaker at the frequency that needs to be compensated, and the capacitive reactance of the capacitor C is the inductive reactance of the speaker L/RR. The actual R is generally a nominal value. The C of a 4-ohm speaker is about 4-10 microfarads, and that of an 8-ohm speaker is 10-20 microfarads. The function of the "Joubert circuit" is to offset the inductive reactance of the speaker (to be precise, the bass speaker), so that the speaker presents a near-pure impedance in a fairly wide frequency band. This circuit should be connected after the crossover and directly connected to the low-frequency speaker, not the output of the power amplifier. The RC network connected there should not be called a "Joubert network", it is just a vibration elimination circuit.

topwon

The following information is randomly excerpted from the Internet. We are not responsible for any errors or omissions. Newbies should be careful to avoid being misled (to avoid being criticized innocently, I will probably have to add this disclaimer in future replies). 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 consists of a resistor and capacitor in series from the ampli- fier output rail to ground. It is always fitted on the inside (ie, 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 reaction 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 Blame less 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

The following information is randomly excerpted from the Internet. If there are any errors or omissions, I am not responsible. Newbies should be careful not to be misled (to avoid being criticized innocently, I will probably add this disclaimer to my future replies). The design information of the two RC circuits is collected from the Internet and is for reference only. It has nothing to do with me. ZobelNetworkDesign：茹贝尔网络设计.pdf (216.75 KB, downloads: 103)

2019-4-26 09:03 上传

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Class-D Output Snubber Design Guide sloa201.pdf (58.22 KB, downloads: 94)

2019-4-26 09:04 上传

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topwon

This TI technical document is probably the most detailed and practical one I have seen so far on this part of the circuit. I hope it will be helpful (I also learned a lot in the process of looking for information). 8838.Class D功放高次谐波过流保护分析和解决方法.pdf (285.46 KB, downloads: 133)

2019-4-26 09:37 上传

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topwon

The following information is randomly excerpted from the Internet. We are not responsible for any errors or omissions. Newbies should be careful not to be misled (to avoid being criticized innocently, I will probably add this disclaimer in future replies). I found a simplified equivalent circuit of an 8Ω speaker unit on the Internet, and then used the free TINA-TI V9 simulation software, and added an AC impedance meter to plot the corresponding relationship curves of impedance, phase and frequency. The top picture is the original characteristics of the speaker unit, the middle picture is connected to the common RC vibration elimination network (commonly known as the Joubert network) at the output end of the power amplifier, and the bottom picture is the regular Joubert network hidden in the crossover network in the speaker. Take a look. The middle picture should be the solution mentioned in the TI document, using a smaller capacitor to achieve the purpose of suppressing high-frequency gain (no need to use a large capacitor like the standard Joubert circuit to achieve a completely flat effect).

shine9547788

Typical of Rupel, but you need to adjust the parameters appropriately. If the parameters are appropriate, you can eliminate the inductance of the load, no, you can weaken the inductance of the load without causing too many other problems.

topwon

Finally, here is a foreign article about the calculation and function analysis of Zobel circuit parameters. I added a translation written by a Taiwanese (I also made minor changes in some places). The following information is randomly excerpted from the Internet. I am not responsible for any errors or omissions. Newbies should be careful not to be misled. Experienced users should not criticize if they don’t like it. Just point out the problem. IC Audio Power Amplifiers and Zobel Networks---One Size Does Not Fit All 利用Zob.pdf (450.35 KB, downloads: 76)

2019-4-29 13:34 上传

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shaorc

topwon posted on 2019-4-29 13:36 Finally, here is a foreigner's article on the calculation and function analysis of Zobel circuit parameters. I added a translation written by a Taiwanese (I also did some translations in some places...

Thank you for downloading and reading it

lqhqq

topwon posted on 2019-4-26 10:02 The following information is randomly excerpted from the Internet. If there are any errors or omissions, we are not responsible. Newbies should be careful to avoid being misled (to avoid being innocently criticized, future replies will probably have to add...

Learned.

hk6108

Is this amplifier a BTL topology?

Azi-

Is this compensation, or absorption?

jacen

Learned

不惑吗