Low esr capacitor possible oscillation in power supply

Thread Starter

john1211

Joined Sep 17, 2018
3
Hi And thanks for reading this post.

I am building an amplifier power supply and am going to parallel connect the capacitors to take advantage of lower esr.

But what is the esr goes too low and the power supply begins to oscillate, what is an easy way to tell if that problem is occuring.

Will there be a buzzing sound coming from the capacitors or from the speakers. Or will the power supply simply oscillate and damage the amplifer without any warning.

Many thanks.
 

danadak

Joined Mar 10, 2018
3,793
Will there be a buzzing sound coming from the capacitors or from the speakers. Or will the power supply simply oscillate and damage the amplifier without any warning.
All the above could be true, depends on how amp was designed.

Using an oscilloscope would make detecting this a lot easier.

Is this a class D amp ?

Regards, Dana.
 

Thread Starter

john1211

Joined Sep 17, 2018
3
Hi. Thanks for the reply, it is a class b consisting of a pair of boards ( see attached jpeg ) and and am now trying to build a power supply for them.
Sorry, i do not have an oscilloscope. So far i have paralleled six 100uf capacitors together, and my esr meter states the esr in total is 0.0002 ohm.
Is there a value for esr where it is considered dangerous to go any lower.
Because the combined capacitance is only 600uf i am hoping only the higher frequencies will be decoupled.

Thanks for your help
 

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MrAl

Joined Jun 17, 2014
6,970
Hi. Thanks for the reply, it is a class b consisting of a pair of boards ( see attached jpeg ) and and am now trying to build a power supply for them.
Sorry, i do not have an oscilloscope. So far i have paralleled six 100uf capacitors together, and my esr meter states the esr in total is 0.0002 ohm.
Is there a value for esr where it is considered dangerous to go any lower.
Because the combined capacitance is only 600uf i am hoping only the higher frequencies will be decoupled.

Thanks for your help
Hello there,

The effect of adding capacitance to a power supply depends highly on how the power supply is constructed electrically. Some are very sensitive to cap ESR and others are not. In general for example, boost converters are sensitive and buck converters are not.

Yes oscillation can harm connected components, no doubt, because the output voltage can go orders of magnitude higher than the set point. It's hard to tell what is going to happen without the load too, so you need to connect some kind of load as well as test the output with at least a meter.
The meter will show little or no AC voltage if the output is smooth DC, but if it oscillates the AC voltage will go up.

If you dont have a meter you would have to get one. There are inexpensive meters out there these days, just be sure it has a low AC voltage scale. With some meters you may have to add a small capacitor in series with it too in order to get a good AC reading. Unfortunately many of these meters do not do well at higher AC frequencies either, so you may have to use a diode detector to convert the AC into DC and then use the DC scale.
When units are tested normally an oscilloscope i used, but unfortunately many people dont want to have to buy one of these just to test one power supply.
 

Thread Starter

john1211

Joined Sep 17, 2018
3
Hi!

Thank you for the reply.

I am going to put the power supply together in the next few days. Unfortunately, i have not purchased all of the components yet
so will have to wait 48 hours for delivery - Thank you for your patience and please do keep an eye on this thread.
I do have a decent Mills multimeter that has ac and dc voltage function.
When the power supply is completed and the dummy load attached should i monitor the power supply for ac voltage?
How much ac should be present on the output if the power supply is working correctly and how much if oscillation is taking place?
The dc voltage should be 40 volts. What do you think this will rise to if oscillating, will it be a few volts, or tens of volts.
Finally, is the oscillation volume dependant, in that if i raise the volume more power will be pulled from the supply and more chance of oscillation?

Many thanks.
 

MrAl

Joined Jun 17, 2014
6,970
Hi,

Some of those questions are hard to answer without seeing the schematic, and even then it could be hard to tell. The way to determine any problem is to measure the output as mentioned, and that give you an idea what is happening.

The oscillation amplitude, if present, is sometimes dependent on the load and sometimes on the capacitance and sometimes on the ESR of the capacitor(s). If you are adding caps then i would think you could measure before and after, and compare results.

As i was saying before, you may have to use a diode and cap to measure the AC because the meter may not respond to the higher frequency AC if it is present. This requires a high speed diode and capacitor maybe 0.1uf. You may get away with a rectifier diode, but a high speed diode would be better.
The idea here is the diode and cap will produce a DC output approximately the same as the output DC of the supply, so if it is 40v then you'll see around 40v or a little less. Then as you add output load and/or caps if the oscillation starts you should see an increase in that DC voltage across the 0.1uf cap. That will tell you right away.
It's too bad you cant get a scope because that is the best test instrument for this purpose, but the diode + small cap should give you some indication anyway.

You can also check your meter manual and see what frequencies the AC range responds to. Most lower cost meters respond to 50 to 400Hz and the more expensive ones go up to maybe 500kHz, but you can check that.
 

MisterBill2

Joined Jan 23, 2018
4,894
What sort of power supply circuit would oscillate if the ESR of the output capacitors was low? That is a new one on me. Where did such a concept come from?? If the ESR were too high, possibly. If you have access to a simulator program put in the amplifier circuit, add a load resistor, and then experiment with a resistance in series with the supply voltage, plotting the voltage at the input to the amplifier. The benefit of doing it on a simulator is no parts are damaged if it does not work.
The one potential issue that I see is the inrush current of the capacitor bank at first turn-on, because discharged capacitors appear to be a short circuit. That does indeed have the potential to cause a problem if the supply protection system goes into a shutdown mode. But if the power supply is not regulated then there will be no problem since there would be no protection circuit.
The classic work-around for capacitor charging current is a current limiting resistor that is bypassed once the capacitors gain a full charge. Typically it is a relay with a few seconds of delay before energizing.
 

MrAl

Joined Jun 17, 2014
6,970
What sort of power supply circuit would oscillate if the ESR of the output capacitors was low? That is a new one on me. Where did such a concept come from?? If the ESR were too high, possibly. If you have access to a simulator program put in the amplifier circuit, add a load resistor, and then experiment with a resistance in series with the supply voltage, plotting the voltage at the input to the amplifier. The benefit of doing it on a simulator is no parts are damaged if it does not work.
The one potential issue that I see is the inrush current of the capacitor bank at first turn-on, because discharged capacitors appear to be a short circuit. That does indeed have the potential to cause a problem if the supply protection system goes into a shutdown mode. But if the power supply is not regulated then there will be no problem since there would be no protection circuit.
The classic work-around for capacitor charging current is a current limiting resistor that is bypassed once the capacitors gain a full charge. Typically it is a relay with a few seconds of delay before energizing.
Hello there,

One i can think of right off is a linear voltage regulator with a PNP output stage.

There are some on the market that show the right ESR values on the data sheet. They are of the very low drop out voltage variety. Perhaps someone else here can quote a part number.

The boost converter also has unusual behavior with different values of ESR. That is harder to understand however.
 

MisterBill2

Joined Jan 23, 2018
4,894
I looked at the pictures of the circuit boards again and I see that the supply is dual 40 volts, positive and negative. Most of those amplifiers did not use regulated supplies, at least they did not used to use them. And if it is being used for high quality audio, such as music, the load does not vary much with a class B amplifier, and so you really gain not much, except heat from a regulated supply. A transformer, rectifiers, and capacitors will be what you need. Of course, the turn-on current surge is still a potential problem, so the best advice is to select the current rating for the rectifiers based on that, rather than the much lower current drawn by the amplifier.
 

Picbuster

Joined Dec 2, 2013
990
A few things are important;
a: The output impedance from the power supply should as low as possible for all frequencies generated from power supply and amplifier.
b:The load spectrum on the power supply will depend on the frequencies amplified including the harmonics.
Power supplier regulator could be disturbed when power supplies output impedance is to high for a frequency caused by amplifier load resulting in an oscillation or malfunction of the total system.
c:you have to make sure that each amplifier stage is suppressing frequencies above the human hearing levels ( 25KHz approx.).
This will ensure that the ESR from capacitors in the power lines don't have to deal with high frequencies.
Feedback < 25KHz which is easy to handle/ suppress by the power supply.

Picbuster
 
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