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Old 21st July 2008, 05:02 AM   #1
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Default Is bypassing PSU capacitors effective?

Just a silly question from a newbie.


The scenario

Let us consider a linear power supply with a 100uF electrolytic capacitor at the output. The capacitor becomes inductive at high frequencies so the PSU impedance rises. To flatten the power supply impedance a film capacitor (e.g. 0.01uF to 0.1uF) is used to bypass the electrolytic capacitor.


My hypothesis

When there is a sudden current demand from the circuit, the voltage drops at the output of the regulator. PSU capacitors are reservoir capacitors so when the voltage drops at the output of the regulator, the capacitors that are fully charged will then supply the current, before the regulator recovers.

For the 100uF electrolytic capacitor, due to the larger capacitance it can supply the current without much voltage drop. It is like a large battery. However, for the film capacitor of 0.01uF, the capacitance is so small and it is just like a tiny battery that runs out immediately. When the circuit draws current at high frequencies, due to the lower impedance of the film capacitor it always gets used before the electrolytic capacitors, and its "battery" life is quickly run out before the regulator recovers. So the current must be drawn from the electrolytic capacitors that have higher impedance at higher frequencies.

In other words, if the above hypothesis is correct, film capacitor bypassing would be fairly ineffective.


Is this right? Please excuse me for asking such a simple question.

Regards,
Bill
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Old 21st July 2008, 06:09 AM   #2
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As far as I can tell,that's standard practice,For some of the reasons you mention,and more.
Another reason I can think of is for bypassing RF/HF noise.
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Old 21st July 2008, 06:23 AM   #3
Eva is offline Eva  Spain
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Forget all this pseudo-scientific analysis, it's all wrong. Time is continuous and all possible delays come from transmission line effects only, which you may consider negligible for very short distances. Output current will come from *all* components at *all* times, it's only the ratio what changes depending on time and stimulus.

You have to learn to model everything as R, L and C elements and check impedance in the frequency domain. Results may seem a bit counter intuitive at first, but it's the only way to predict exact circuit behaviour.

For example, a small film capacitor in parallel with an electrolytic capacitor not having high ESR will actually result in higher overall output impedance, ringing and very bad performance in the low Mhz range.

The other way to learn about the subject is to build a current pulse generator with very fast rise and fall times and use it to see the actual pulse behaviour for whatever circuit configuration you want on the oscilloscope. I use to check my class D and SMPS designs that way.
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Old 21st July 2008, 08:21 AM   #4
golam is offline golam  Bangladesh
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Is bypassing PSU capacitors effective?

A sily like question to Experts but very useful to Newbie.

Hi HiFiNutNut,
Let us forget all pseudo-scientific analysis and move logically.

The scenario- What is the meaning of Bypass? It is the route that is used to avoid the main route. Here, the definition is perfect. This film capacitors act as an alternative route for the high freq. spikes/RF/HF signals in the circuit if any, known as noise which can not be handled by Bigger Electrolytic Capacitors. These film capacitors are made in such a way that they are having low impedance for RF/HF freq. and so, by handling those, keep the main circuit safe for lower freq. and linear power supply is meant for that.

Analysis -- Your 100 uf Electrolytic capacitor only will act as main reservoir capacitors as linear power supply is meant to handle low frequencies only and film capacitors will handle occasional high freq. spikes/RF/HF sigs. if any. So, small film capacitors are not intended in the circuit for acting as reservoir capacitors combining with Electrolytic Capacitors in your assumed Linear Power Supply Circuit.

DigitalJunkie is almost correct.

Thanks.
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Old 21st July 2008, 10:31 AM   #5
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I usually always use a small cap in parallel with the larger electrolytic cap(s) in PSU circuits,and I've never noticed any negative effects.
The main reason that _I_ use them,is for RF/HF bypassing.
Recently I've been using SMD 0.1uf caps,as they have better performance up into the high-Mhz range than their leaded counterparts. -That's not much of a concern for audio,but the smaller size is helpful for getting the bypass caps *right on* the IC pins underneath the PCB.

Alot of regulator IC's (78/79xx,etc) require small caps in addition to the larger electrolytic caps to prevent instability.
Bypassing is very critical for alot of chip-amps,and (fast)op-amps.

IME/My 0.02cents.

Edit:
And in RF circuits (VHF and above) good bypassing is *mandatory*.
No excuses!
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Old 21st July 2008, 12:49 PM   #6
Jozua is offline Jozua  South Africa
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Default Theory vs Experience

Gentlemen

I have added 6uF Kimber Caps to my 100 watt class A amp and the sonic impact was most impressive. Much cleaner and natural sound.

Possibly the least expensive "ugrade" with the biggest sonic impact on my system.

Jozua
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Old 21st July 2008, 02:19 PM   #7
golam is offline golam  Bangladesh
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Hi all,

All practical application should be based on theoretical knowledge. Experience based on theoretical and practical knowledge matters. Otherwise in all spheres of Power Electronics test and trial may not solve all the problems all the time.

Dear Jozua, I am not clear about your statement. If you made an 100 watt class A amplifier with only a 6uF capacitor, then you have proved all the theoretical knowledge so far we have gained is wrong. Then elaborate in detail so that we can learn.

Or, you have purchased a 100 watt class A amplifier and made the sound clearer by adding a 6uF capacitor. That is possible. Because inside the Amplifier lot of capacitors may be there and adding that additional 6uF capacitor may have solved the internal problem which created noise/problem with sound quality. Even than, your effort and experience is praiseworthy. Keep it up.

Thanks.
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Old 22nd July 2008, 03:45 AM   #8
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Thank you wholeheartedly for all of your posts.

I can understand all but this one:

Quote:
For example, a small film capacitor in parallel with an electrolytic capacitor not having high ESR will actually result in higher overall output impedance, ringing and very bad performance in the low Mhz range

I can think of bypassing a capacitor to be equivalent to parallelling two series L, R and C circuits. Is that the reason for a "higher overall output impedance" in the low Mhz range? Would it mainly depend on the capacitors?

I can imagine there would always be steep peaks and nulls in any bypassing as their L, R and C would be so different.

If the ringing occurs above the audioband, would it have any sonic impact?
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Old 22nd July 2008, 04:41 AM   #9
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It matters where the load is. If the load is any distance from the 100uF cap, lead inductance will be a huge factor at high frequencies. You need sufficient high frequency bypass close to the circuitry. Going the other way, if you have a noise problem from the rectifiers, you need to solve the problem right at the rectifiers with some high frequency bypass caps, not even inches downstream where once again lead inductance stymies your efforts. IMO, once those things are taken care of, bypass caps right at the filter caps won't have dramatic effects unless there's some other blunder with the layout and grounding strategy. As said above, it's a good idea to put some high speed edges into the system and see what happens. Sometimes what seems like a good idea turns out to ring like a bell.
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Old 22nd July 2008, 04:41 AM   #10
Eva is offline Eva  Spain
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Thie following simulation is not just a textbook example, it's a system that I had to solve in practice. The bode plotter shows resulting impedance in ohms versus frequency.

Click the image to open in full size.
The 1500uF 250V electrolytic alone. It's 40mm D and 50mm H with 10mm pin spacing (for PCB mount).

Click the image to open in full size.
Adding 10nF 500V ceramic chip in parallel, what a disaster. Imagine all those naive people so proud of their 10nF bypass caps... Impedance is only low at 35Mhz.

Click the image to open in full size.
Adding 100nF 500V ceramic chip in parallel, not so bad, but these high voltage chips are very expensive and impedance at 4Mhz is still twice as high as with the electrolytic alone. In practice any impedance peak like this translates into ringing, excess EMI in SMPS, distortion in class-D and instability in audio power amplifiers.

Do you really need very low impedance at 10Mhz and above at the expense of messing things up in the 4Mhz range? Maybe in a RF transmitter, but never for audio.

Click the image to open in full size.
This is a more or less optimum solution with class-D and SMPS in mind, the most demanding applications. Two 100nF chips are required in order to get low impedance up to 50Mhz. Also, five 100nF film capacitors with 1ohm resistors in series are required to tame the resonant peak around 3Mhz (SMD chips are too expensive to use so many).

The root of the problem is exactly the fault that we are trying to solve: Inductance. The series inductance of any capacitor (not just electrolytics) will resonate with whatever other capacitor you put in parallel. You can assume 1nH per mm of pin spacing. A large-can electrolytic with 10mm pin spacing soldered to a PCB usually exhibits 10nH to 15nH. A film capacitor with 15mm terminal spacing exhibits approx 15nH too. Bigger films with 40mm or 50mm spacing exhibit approx 50nH (they are completely useless for bypassing). SMD chips may exhibit 2nH or so.

There is no rule of thumb for capacitor paralleling, every system has to be analyzed as the RLC elements it's made of. Then a custom solution that "complements" the existing RLC values is easily found.

Finally, note that I'm assuming that all the capacitors are mounted very close together in a double sided PCB with ground and power planes, with the load (be a class-AB amp, a class-D amp or a SMPS) sharing the same PCB and very close to the capacitors so that path inductances become negligible.

Even just 2 cm of wiring or conventional PCB tracks (with no ground or power planes) will add enough series inductance (5nH per 1 cm) to render any "remote" bypassing useless.

There is nothing more useless and ridiculous than all those remote capacitor boards (with all sorts of bypasses) traditionally used in class AB amplifiers. This can only come from people having zero understanding about high frequency electronics. Every 3 cm of wiring (approx 15nH) is as inductive as a big electrolytic capacitor.
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