What exactly does it mean for a regulator to be "unstable"? - Page 1
Forum section: electronics / general

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: March 27, 2023, 03:58:04 am
content: In my limited experience, I've heard people used the word "unstable" to describe problematic regulators, but I haven't yet grasped a precise definition of what it means. I've heard some people say that a regulator is unstable if there is ringing on the transient response, but even then it's unclear to me how much ringing makes it "unstable", because don't most regulators display some ringing/oscillation on their transient response? I've also heard people describe certain regulators as unstable when they are simply noisy. And lastly I've heard people say regulators are unstable when their gain or phase margin doesn't meet a certain spec.So I consult the wisdom of the forum... what is your understanding of an "unstable" regulator?Thanks

author: DimitriP
author_rank: Super Contributor
timestamp: March 27, 2023, 04:08:23 am
content: All of the above.To what extent, depends on the datasheet

author: james_s
author_rank: Super Contributor
timestamp: March 27, 2023, 05:50:04 am
content: I once had a 78xx or maybe it was a 79xx that was oscillating at around 1MHz due to insufficient capacitance on the output. It was powering an audio circuit and created some strange distortion.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 27, 2023, 06:20:01 pm
content: I would only describe a regulator as unstable if it is oscillating or it it could enter oscillation under normal operation. Ringing on the transient response can indicate a low phase margin which means that with a change in load could cause it to start oscillating. Loop gain depends on both the load impedance and the load current which causes transistors to operate at different points on the transimpedance curve, so it's often ncessary for a regulator to be tested for stability over a wide range of conditions.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 27, 2023, 08:56:43 pm
content: How much a regulator rings depends on the load. With a difficult load (usually low ESR capacitance) most regulators will ring to some extend or even oscillate.Ideally one would like a regulator to not ring, but this only works OK for a well behaved load. For the general case including high capacitive loads it is already hard to have it not oscillate.For a regulator being unstable this usually means oscillation. In a few cases a large drift with temperature is also called not stable.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 31, 2023, 07:02:29 am
content: Unstable = can't maintain regulation because the output voltage does things it's not supposed to do, in an attempt to regulate with wrongly tuned feedback loop.There is no exact definition but I'd say anything that exceeds the rated minimum Vout ... maximum Vout range without an allowed reason (like output overload current limiting), is instability.

author: Someone
author_rank: Super Contributor
timestamp: March 31, 2023, 07:41:07 am
content: I'd say such a poorly considered question is ripe for a (human edited) AI reply:Unstable (adj.): A term used to describe a system, or situation that is prone to sudden and unpredictable changes, often resulting in chaos, turmoil, or disaster. Synonyms include erratic, and unreliable. Common analogies include wobbly tables. In short, unstable is a state of being that is best avoided if one wishes to maintain any semblance of operation.

author: Faringdon
author_rank: Super Contributor
timestamp: March 31, 2023, 06:23:00 pm
content: Unstable.....well flat DC is stable, anything else, other than switching frequency ripple, is unstable.If it rings after a transient, but the ringing dies away...then i usually say thats stable but with poor phase marginAnd if it has poor phase margin, then it may in future go unstable if its components change with time...eg, the opto feedback opto looses CTR with time.But yes, instability is usually some kind of oscillation on the output....its usually at a few khz or lessThen theres loads of converters which have subharmonic oscillation..and sometimes they still do the job they are supposed to do just fine so people dont bother about it.But the trick of the trade...is not just to make the PSU stable......its to give it stability, with a good MARGIN...such that its components can vary with age, and it will still remain stable.So math wise, we talk of gain margin and phase margin....these are read from the bode plots.If you want a good intro to feedback loop...read the first 2 Chapters of owen bishops control loop book....its only a small book (i think its called electronics control)https://www.amazon.co.uk/Understand-Electronic-Control-Systems-Bishop-ebook/dp/B001PIJ700Some people have a psu with say 2v pkpk oscillation on it, but low and behold it still does the job ok...still powers whatever...but the thing is...the instability may get worse, since its unstable, and obviously has no stability margin built in.f) “Understand Electronic Control Systems”by Owen BishopNewnes…First two chapters, ‘introducing control systems’ and ‘control strategies’ are an excellent description of the general workings of feedback loops…this is very useful for getting a feel for SMPS feedback loops. This is at most half an hour’s reading and well worth it.

author: David Hess
author_rank: Super Contributor
timestamp: April 01, 2023, 02:31:04 am
content: Quote from: Jim from Chicago on March 27, 2023, 03:58:04 amIn the common case all three of those are the same thing. The transient response is directly correlated with the gain and phase margin; it is possible to calculate one from the other.These can vary with output current, load impedance (capacitive or inductive), and an external feedback network for setting the output voltage if present.QuoteMany do, but it is possible to adjust the frequency compensation to produce a critically damped response, or even an over-damped response so no ringing is present. With careful design, a regulator can also be made unconditionally stable no matter what the load current and load impedance is.QuoteI would think more in terms of "drifty", which may be indistinguishable from low frequency noise.

author: jonpaul
author_rank: Super Contributor
timestamp: April 01, 2023, 11:39:18 am
content: poor control loop design, poles in RHPSubcycle oscillationFlicker in referenceConditionally stable, Oscillation or out of loop control for certain loads eg capacitive values, inductors, negative R.j

author: Faringdon
author_rank: Super Contributor
timestamp: April 01, 2023, 07:05:02 pm
content: Also as attached

author: AnalogTodd
author_rank: Regular Contributor
timestamp: April 03, 2023, 01:46:39 pm
content: I've designed regulators for just shy of three decades, and 'unstable' can mean different things to different customers.From a design standpoint, a 'stable' regulator has enough phase margin and gain margin across all characteristic ranges of load, line, temperature, process, and expected output capacitance such that no single frequency signal will be measurable compared to the overall noise. Yes, that's a lot of variables. Yes, designing these things is not easy. Oh, don't forget that things should operate in an expected fashion even under fault conditions.From a customer standpoint, 'stable' is often defined as the ability to provide a consistent output under the conditions of their application. This is often a subset of the design criteria listed above as there are often many of those criteria above that are limited in the customer application. Some customers add in minimum amounts of gain margin and phase margin for a typical use case as that is often enough to guarantee potential corner cases do not arise where products do not function properly.People often throw in things like long-term drift as part of stability. Technically, as long as the regulator still maintains the output within data sheet specifications, it is not unstable as a function of long-term drift. If you need a voltage that is rock solid long-term, you should buy a reference, not a regulator.In reality, there are still a lot of people out there who use products outside of the 'normal' data sheet operating conditions and consider their application 'stable'. Data sheet says minimum output capacitor is 10uF? Well, this customer has a fixed load and adds a resistor in series with a 0.1uF capacitor instead and has excellent stability for what they need. Costs less and takes up less space, so it's what gets used. Another finds that minimum output capacitance isn't enough on one product to keep transient response within acceptable levels and has to add more capacitance or else switch to a different product.So, in the end, 'stability' is really what the customer needs it to be in their use case.n.b. Do be aware, almost all regulators require an output capacitor to provide both a critical pole in the loop as well as energy storage for transient performance. Load characteristics can often cause instabilities in a loop if not well understood. A pole-zero pair (often an LC filter) that comes in at a low enough frequency compared to the loop can cause a 180 degree phase shift while there is still gain in the loop and make a circuit oscillate. When a circuit is oscillating, guarantees on the loop operating at proper voltages go out the window (as the loop is not operating under proper conditions).