Oscillating Volt Regulators?

[DigiTan]

I've been reading from previous posts about volt regulators like the 7805 oscillating in certain situations. Naturally, I'd like to avoid this as best I can, so can someone explain what causes oscillation and how to prevent it? National Semiconductor breifly mentioned the load capacitor and ESR but didn't go into the details other than saying ESR was critical. The reason I ask is because I've used dozens 78XX's withing knowing the ESR, and never had this problem. :?

 

[audioguru]

Hi Digitan,
I've also never seen 7805 regulators oscillate if they have the recommended ordinary input and output ceramic disc capacitors. Ceramic disc caps usually have a low ESR that the IC likes the best.
Some low-dropout regulators are very fussy about the ESR of their "must-have" output capacitor and oscillate if it is too high or too low. Low dropout regulators use a PNP transistor or P-channel Mosfet that has voltage gain and phase-shift unlike the no-gain and low phase-shift of an emitter-follower pass transistor of an ordinary regulator. Lots of gain and phase-shift= lots of chances for oscillation.

 

[Oznog]

Some cap types, in particular garden variety electrolytics, have an impressively high capacitance but the internal resistance is high. Thus it can't quickly absorb high current transients, but it can still absorb and provide normal currents over long periods of time due to its high capacitance.

If the ESR of the cap isn't high enough, a good solution is often to put an ceramic disc cap (which is typically a very low ESR, but also low capacitance) in parallel with the electrolyic. Best of both worlds, unfortunately a bit more expensive.

 

[audioguru]

If the ESR of the cap isn't high enough, a good solution is often to put an ceramic disc cap (which is typically a very low ESR, but also low capacitance) in parallel with the electrolyic. Best of both worlds, unfortunately a bit more expensive.

That's why the regulator manufacturers recommend using a ceramic disc cap at the input and output.

 

[Roff]

Hi Digitan,
I've also never seen 7805 regulators oscillate if they have the recommended ordinary input and output ceramic disc capacitors. Ceramic disc caps usually have a low ESR that the IC likes the best.
Some low-dropout regulators are very fussy about the ESR of their "must-have" output capacitor and oscillate if it is too high or too low. Low dropout regulators use a PNP transistor or P-channel Mosfet that has voltage gain and phase-shift unlike the no-gain and low phase-shift of an emitter-follower pass transistor of an ordinary regulator. Lots of gain and phase-shift= lots of chances for oscillation.

78XX regulators also have high gain error amplifiers (although it may be less than LDOs). They just don't have voltage gain in their output transistor. It would be impossible to get milliohm-level output resistance (see graph below from Nat'l Semi datasheet) without feedback.

 

[Dr.EM]

Ive used metallised polyester on mine in the past, I presume those are good too, but more expensive :?:

 

[audioguru]

Hi Dr. EM,
I have heard that metalized polyester caps are coiled, therefore have inductance. They are excellent as audio coupling capacitors. Ceramic disc capacitors should be used as a high frequency supply bypass due to their extremely low inductance. Tantalum caps also have low inductance but I and others find them to be unreliable.

The inductance of a capacitor makes it ineffective at high frequencies where its low impedance capacitance is needed the most.

 

[Dr.EM]

Ah right. Actually, i'm about to use tantalum caps in a project, they are working fine now and give marginally better performance than standard electros. When you say unrelieable, do you mean they fail after they have been installed, I might not use them if thats likely. They are quite new if that makes much difference (I heard its the older ones that can be more troublesome).

 

[Styx]

Tantilum caps are good, cant get much better for the capacity:size ratio.

You can get good ones and poor ones. I have to use the -55:125C variant and they are extreamly reliable, mind I do pay for it

Also please bear in mind that every tant cap you buy you are helping to kill Gorilla's

The mineral used for tantilum is mined in the Congo in open pits (nice and cheep) decimating the Gorilla's environement - blame the mobile industry

IF you can use Niobium. It has better performance than Tant and is in larger quantities

 

[Nigel Goodwin]

Ah right. Actually, i'm about to use tantalum caps in a project, they are working fine now and give marginally better performance than standard electros. When you say unrelieable, do you mean they fail after they have been installed, I might not use them if thats likely. They are quite new if that makes much difference (I heard its the older ones that can be more troublesome).

I've been repairing domestic electronics professionally for 35 years, at various times tantalum capacitors have been used by manufacturers - from a service point of view they have proved to be EXTREMELY unreliable!. If you're trying to find a fault, and you spot some tantalum capacitors, they are the first thing to check - doing so will often find the fault, without requiring any effort :lol:

It's rare to find them used these days, their unreliability caused manufacturers to stop using them.

 

[Dr.EM]

Thanks for the information, certainly didn't know about the Gorilla thing :shock: , didn't know they had been around that long either.

I think that i'll use electros then, these will be difficult to replace so I dont want them failing.

 

[Nigel Goodwin]

Thanks for the information, certainly didn't know about the Gorilla thing :shock: , didn't know they had been around that long either.

I think that i'll use electros then, these will be difficult to replace so I dont want them failing.

I've NEVER had an electrolytic fail in anything I've ever built, generally the ones you buy are better quality than those used in commercially made equipment (where they buy the cheapest they can find!).

I've also NEVER seen a small electrolytic fail on the input or output of a 78xx voltage regulator - although the large electrolytic that follows the rectifier sometimes fails (low ESR or low value) - but that's nothing to do with the regulator.

 

[chemelec]

In 30 years of Manufacturing numerous products, (all these producs using Tanalums as Bypass caps in the power supplies) I have never had one fail yet.

I love them.

But I will also comment that Polarity on these caps is critical. They do not like reverse volages. And I don't find them very good for AC Coupling.

 

[Nigel Goodwin]

In 30 years of Manufacturing numerous products, (all these producs using Tanalums as Bypass caps in the power supplies) I have never had one fail yet.

I love them.

So you're the person who uses them? :lol:

 

[audioguru]

I used to make things with tantalum caps. A few I saw after a few years of operation had their tantalum caps looking wierd: just two thin wires sticking up. The "meat" of the capacitor evaporated into thin air.
Hee, hee. My circuits worked fine like that. :lol:

 

[Styx]

Tants really are fine, as stated they really cannot cope with any reverse voltage on them (seen some that cant tolerate -1V for a 16V part)

When they fail yes they fai land make a nice mess of your board, but that tend's to be due to design error's and not technology fault.

They have a very low ripply current as well, and the voltage rating comes screaming down alot faster then otehr electrolytics with respect to temp.
BUT the main thing that ends up killing them is inrush!!!!

At powerup it is either a very bad designer or a cheap designer who does not take into concideration the inrush into their boards.

THAT is what kills tants more often then not

 

[Nigel Goodwin]

I used to make things with tantalum caps. A few I saw after a few years of operation had their tantalum caps looking wierd: just two thin wires sticking up. The "meat" of the capacitor evaporated into thin air.
Hee, hee. My circuits worked fine like that. :lol:

Presumably they had gone S/C (the usual failure mode) and 'exploded' :lol:

Unfortunately those I've found faulty didn't have enough power in the PSU's to blow them apart :cry:

Perhaps 'chemelec' buys a better spec of tantalum?.

 

[audioguru]

I used ITT brand tantalum caps in the late '70's and early '80's. Blue ones.

 

[chemelec]

I'm Presently doing some design work with an LT1371 Switching Regulator.

In the Data book there is a Good write-up about Tantalums.

Particularly Interesting is the Last Paragraph concerning "physical size verses ESR".

I did some ESR Tests on the tantalums I have, using my Sencore LC102 Capacitor/Inducor Analyser. It Definately appears to be true.
Physically Bigger tantalums with the same voltage and capacitance ratings, have Lower ESR.

 

[Styx]

Thank chemelec, that backup what I said about it was inrush that kills Tants