Where is the "honest" place to measure ripple/noise/PARD for SMPS spec - Page 1
Forum section: electronics / projects

author: rx8pilot
author_rank: Super Contributor
timestamp: January 21, 2015, 01:19:45 am
content: I am sure that this varies a lot based on the credibility of the manufacture, but I am trying to learn where is the most typical location to measure performance specs like ripple and noise on a new SMPS design. I have done most of the testing across the last output cap with a very low-inductance probe. Is this fair or should I be measuring at the device output terminals where there is definitely more noise?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 21, 2015, 02:53:15 am
content: What do you mean by "honest"?The accepted (de facto or IEC or ??) standard?Or what would be most reasonable for unstated usage?The latter of course is difficult to guess. The usual test is, as you say, with a relatively low impedance probe. The connection is made at a specific distance from the power supply, with the leads terminated by a capacitor (usually 10 to 100uF). Bandwidth 20MHz.This isn't representative of the total noise of the power supply, especially since there can be a mess beyond 20MHz that they conveniently skip with this test. But it's commonly done, for better or for worse.Tim

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2015, 03:13:37 am
content: The Chinese for their published SMPS specs turn on scope Averaging which will reduce any ripple measurement to ~1/3 of Peak Detect.Honest?But it can definately be replicated in any subsequent checks/tests.

author: BravoV
author_rank: Super Contributor
timestamp: January 21, 2015, 03:36:26 am
content: Not an expert, but here my 2 cents, if the word "honest" here means a common practice or standard that is recommended to produce consistent and trusted results, I guess you have to follow those particular standard in your SMPS measurement as they vary too much from different applications.For example, one of the most used SMPS standard in the world is Desktop PC's PSU, the recommended measurement standard is clearly written here ->ATX12V Power Supply Design Guide by FormfactorsQuoting from above documentation :

author: rx8pilot
author_rank: Super Contributor
timestamp: January 21, 2015, 03:39:24 am
content: I guess by "honest", I mean a figure that does not hide anything. This is a design that will ultimately be used in a high-end professional environment connected to very expensive and sophisticated equipment that can be in very remote locations.If there is any problem at all, I want the specifications to be honest when/if my device is blamed for a problem at the north pole (literally).

author: BravoV
author_rank: Super Contributor
timestamp: January 21, 2015, 03:42:34 am
content: Quote from: rx8pilot on January 21, 2015, 03:39:24 amI guess you will have to list down in "detail" what are the requirements or expectations in that particular application and environment, as usual, the devil is always in the details.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 21, 2015, 03:55:38 am
content: The ATX guidelines seem easy... 20Mhz limit and 1%-ish noise lets you getaway with a modest performance. Conducted emissions are probably the bigger challenge.Maybe all I need to do is describe the basic test criteria on my data sheet that an engineer can measure and verify.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 21, 2015, 08:07:12 am
content: measure for what purpose? to publish specs (I.E. see what you can get away with) or gain insight, in confidence?this is my preferred method for a general "spec" (see pic)...however it's not the only place to gain insight and look for potential problemsanother option is to strap two ballast resistors across the outputs and measure across them.....another option is directly on the output, but that can get a bit whacky, depending on what kind of capacitive load exists there....also you can reference sense to the POL output, or to the "ballast" I referred to above...each scenario here is going to be dependent on the specific topology of each DUT

author: AndyC_772
author_rank: Super Contributor
timestamp: January 21, 2015, 08:38:00 am
content: Just be clear about how the measurement was made. I'd be perfectly happy to see a spec which described the total ripple + noise:- across a 100nF ceramic capacitor- located 30cm away from the PSU using an unshielded twisted pair- with a bandwidth of 20 MHz- with a current of 5A dc flowing through a load resistorThere's no "right" way to describe it. Whatever you do will be different to how the PSU will actually be used in service, and it's probably up to whoever is designing the system to ensure it's compatible with the load.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 22, 2015, 10:31:22 pm
content: Quote from: TunerSandwich on January 21, 2015, 08:07:12 amYes, for general published spec. For my own insight purposes, I have a whole series of test points that I use to reference new design tweaks with older revisions. That is also the confidence builder I guess.Since I do not get to control the source of power or the ultimate load, I am always guessing. What bad things can happen and how do I need to react to them. For a starting point, I will simply publish a spec and the general method of acquiring the data that led to the spec. I will never be able to guarantee a certain level of performance, but will offer a base to start from.Quote from: AndyC_772 on January 21, 2015, 08:38:00 amI have to say that I am a bit surprised that 20Mhz is acceptable. One tiny flaw in a switcher operating at 750Khz or 1Mhz can blast the adjacent circuits out of commission at frequencies way above 20Mhz.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 11:06:42 pm
content: Quote from: rx8pilot on January 22, 2015, 10:31:22 pmBy "in confidence" I meant confidential....I.E. only you know20Mhz is the "accepted standard", but when I see bandwidth limited measurements I always assume anything above is "too scary to publish". I would have a ton of respect for a company that published results out to the bandwidth, where artifact rolloff is practically nil....even if those figures scared away people who don't know better. I am always skeptical about RMS + noise measurements, or those with no stated load.If it was me and i was stamping my name on the product....i would show full derating curves, as well as the real noise etc specs......for the absolute worst case scenarios, and also the "best" case scenarios. I think anyone with any experience would appreciate that. There is nothing worse than buying a product based on a "spec"....getting it on the bench and then discovering it's orders of magnitude "worse" than stated.I recently bought some Acopian PSUs. They stated 0.5-1 mV pk-pk ripple (depending on unit). I got them on the bench and measured more than 200mV ripple at 600 MHz. I ripped them apart to figure out why, and discovered...that their design and topology, and component selections would NEVER meet their stated specs. Hell even when I filtered the output to 10hZ they didn't meet "spec". That is bad business. I will never buy their products again (looked like EE101 students at the local JC put them together) and will be sure to tell everyone I know about them. They could have actually met their specs, with $5 more worth of PCB quality and another $10 in labor costs. Again, that is just bad business, and a waste of some really nice parts (they all have top notch IC's transformers and caps in them).If I were in your shoes, I would avoid the above scenario at all costs. Even if it makes your products "look worse than the competition" on paper. Honesty will buy you word of mouth business, and shady "specs" will buy you exactly the opposite. No amount of marketing can buy you back out of bad will.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 22, 2015, 11:15:57 pm
content: For sure, 20Mhz RMS noise looks pretty good on paper.I am at 1% p-p noise of output voltage at full load and 500Mhz. At 20Mhz its about .15%-ish and throw in RMS and it looks more like a battery (not really). Pretty big range.I will be conservative and prevent problems.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 22, 2015, 11:30:34 pm
content: Quote from: rx8pilot on January 22, 2015, 11:15:57 pmThat's not bad at all. 9mV at 9V @max load out to 500Mhz is decent for a switching PSU. I think it would be great if manufacturers also showed recovery times after the device clips.....what is your current ripple and do you also state the capacitor specs for handling current ripple? That says a lot about longevity.....

author: rx8pilot
author_rank: Super Contributor
timestamp: January 23, 2015, 12:20:18 am
content: Unfortunately, 1% of 9V is 90mV.In my case, target voltage is 26V and I would like to be at 130mv p-p out to 500Mhz. I have another pass on the PCB, that should easily meet that goal. Of course .1% would be 26mV and I would be ecstatic to achieve that in the tiny space I have.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 23, 2015, 02:04:40 am
content: Quote from: rx8pilot on January 23, 2015, 12:20:18 amoops....I misread that number....I though it was .1%.I have a package here.....that does 12mV pk-pk @ full load of 15 amps (600 plus Mhz).....and it can cook at 80deg C all day and maintain that.....very small and only needs 2 x 47uF tantalum on the input and 2x 47uF MLC on the output. Send me a waveform if you can......1mS DC coupled....is the peak measured at the switching frequency?

author: rx8pilot
author_rank: Super Contributor
timestamp: January 23, 2015, 02:14:45 am
content: Now THAT is what I am after. I think I will wait for the next rev to be built since it is already on the way and the current rev will never be used.SMPS is a black art. Nailing it requires a sixth sense. It all looks so simple on paper, but reality offers a continuous stream of surprises. I only have 'months' of SMPS experience under my belt. It's like drinking from a fire hose.

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 23, 2015, 02:23:27 am
content: Quote from: rx8pilot on January 23, 2015, 02:14:45 amIt took me years of failures to get to a point where I know exactly what does and what doesn't help these problems along. there are some fundamentals that will help you nail it every single time.I.E. maintaining phase offsets that are divisible in very specific ways, against the switching frequency.....stability of the control line.....proper implementation of V-sense etc....avoid bulk capacitance t all costs....it' a band aid to bad layout....keep your control line clean, isolated and short....etc

author: rx8pilot
author_rank: Super Contributor
timestamp: January 23, 2015, 02:44:36 am
content: Are you referring to bulk capacitance on the input and/or the output? In a synchronous boost topology, is it not required on the output to hold the load while the inductor is charging?For the control line, I have isolated the associated components to the opposite side of the PCB. The FETs and inductor on one side GND plane on layer 2 and signals on layer 3 and 4. This revision is non-sync based on LM3488 but the latest is a sync boost converter based on TPS43060 controller. It is relatively simple schematically, but the layout as you know is unforgiving.Trying to incorporate non-dissapative snubbers gently so the efficiency is not obliterated. This thing needs to operate on battery so efficiency is important, but not quite as important that I keep the noise level at .5% or better at full load full BW. Also moving to a faster frequency to get the inductor size down.Do you ever use a gate resistor to control slew rate and clean up some of the sw node ringing?

author: TunerSandwich
author_rank: Frequent Contributor
timestamp: January 23, 2015, 03:00:20 am
content: Quote from: rx8pilot on January 23, 2015, 02:44:36 amThe faster switching rate is better for noise, worse for efficiency.....Boost regulator topology is a bit more difficult to deal with than buck (is it not possible to step down?)gotta run real quick....I will address the other questions when I get back online.....

author: tautech
author_rank: Super Contributor
timestamp: January 23, 2015, 03:35:02 am
content: Quote from: rx8pilot on January 23, 2015, 12:20:18 amBit of a benchmark for you.Compact mains bench 30V 5A SMPS that I get from Asia are stated at better than 35mv ripple.But as I stated before, the only way that can be met is with averageing on and of course is at switching frequency.They're not bad little units, no overshoot, clean ramp up, smart fan cooled and have taken everything we can throw at them.So you 26mV should be achievable.

author: rx8pilot
author_rank: Super Contributor
timestamp: January 23, 2015, 05:03:48 am
content: I am measuring the switching noise components at full bandwidth which makes something like 26mV seem tough. The ripple is very low and if I BW limit the measurement to 20Mhz the peak-peak noise is about about <100mV. At full bandwidth it is a little over 300mV p-p. It is a DC/DC device so ripple is not really the concern as much as switching noise. If I ever get the switching noise close to the ripple, it would be nice. As I am on the learning curve of SMPS design, I am looking to see how low I can go with p-p noise and how the remedies impact the efficiency. At the same time, I need this circuit to complete a project. In reality, the as-is performance probably wont scare anyone away but it needs to be better for my own sanity.My next project will require a more capable design, so as much as I can learn from this round will benefit the next and more challenging project.