Looking for a Low Cost Way of Measuring DC-DC Converter Control Loop Response - Page 1
Forum section: electronics / testgear

author: bendras
author_rank: Contributor
timestamp: August 06, 2018, 08:56:36 pm
content: I would like to measure control loop response (loop gain and phase) of a DC to DC switching regulator. Various resources online suggest using a Vector Network Analyser together with an injection transformer. VNAs are very expensive and I am not sure if a full blown VNA is really necessary. Assuming that I already have a suitable injection transformer, could someone recommend alternative solutions under $1200?

author: Keysight DanielBogdanoff
author_rank: Unknown
timestamp: August 06, 2018, 09:33:06 pm
content: It'spossiblethat a InfiniiVision 1000 X-Series scope could work for you, it does gain and phase up to 20 MHz if you get a "G" model.https://www.keysight.com/en/pdx-2766207-pn-DSOX1102G/oscilloscope-70-100-mhz-2-analog-channels?cc=US&lc=eng

author: Hydron
author_rank: Super Contributor
timestamp: August 07, 2018, 11:10:25 am
content: The new Siglent 4ch low cost SDS1x04X-E scope series might do it too if you have/buy a suitable sig-gen (in a similar way to Keysight, scope has bode plot function built in). If this is a one-off then maybe you could do it manually, measuring with scope FFT + sig-gen at a range of frequencies. Bandwidth requirement won't be that large, a few MHz at the very most. The thread about Dave's Bode 100 teardown video also has info about a bunch of DIY and low-cost isolation transformers (short answer - wind one yourself or find a video isolation transformer).

author: capt bullshot
author_rank: Super Contributor
timestamp: August 07, 2018, 11:15:04 am
content: If you don't need that fancy frequency / phase response plot, I'd suggest testing the DC/DC by applying a step load change.Nothing more required than a homebrew step load generator and a scope. Look up Jim Willams' app notes regarding this technique.

author: coppice
author_rank: Super Contributor
timestamp: August 07, 2018, 11:28:16 am
content: Quote from: capt bullshot on August 07, 2018, 11:15:04 amThere really isn't much to look up. All you need is an additional load resistor, a transistor to switch it and and out of circuit, and something to drive the transistor. You might get away with just a resistor and a switch, but switch bounce might spoil the results with that setup. You don't need anything finely calibrated. You only need something to abruptly increase or decrease the load somewhat, and a scope to capture the effect on the power rail.

author: JohnG
author_rank: Frequent Contributor
timestamp: August 07, 2018, 02:44:51 pm
content: If you don't need to measure much beyond the audio band and you don't mind figuring out probing, etc, you can get very nice VNA measurement capability from a USB sound card and some software. I find that ARTA (http://www.artalabs.hr/) gives some really powerful analysis capability. It's especially nice because sometimes a swept sine input to a converter breaks things, but a PRNS does not.John

author: exe
author_rank: Unknown
timestamp: August 07, 2018, 03:31:47 pm
content: Quote from: coppice on August 07, 2018, 11:28:16 amIs it possible to estimated loop bandwidth and phase/gain margin from this?

author: BravoV
author_rank: Super Contributor
timestamp: August 07, 2018, 05:17:31 pm
content: Quote from: coppice on August 07, 2018, 11:28:16 amNoob question, will something like this ->Dynamic Electronic Load Projectis enough ?I mean, especially for "low cost".

author: coppice
author_rank: Super Contributor
timestamp: August 07, 2018, 05:31:16 pm
content: Quote from: exe on August 07, 2018, 03:31:47 pmWhy do you want to know the frequency and phase response? So I can work out the impulse response. Why do you want to know the impulse response? So I can work out the frequency and phase response.Its one of those "Is the Fourier transform half empty or half full" kinda things.

author: exe
author_rank: Unknown
timestamp: August 07, 2018, 08:16:15 pm
content: Quote from: coppice on August 07, 2018, 05:31:16 pmI'm designing a PSU and I want to know if it is fast and stable and how different components affect its performance.

author: dcarr
author_rank: Regular Contributor
timestamp: August 07, 2018, 09:22:08 pm
content: This can be done very cheaply:1. Read these two app notes:-http://www.ti.com/lit/an/slva381b/slva381b.pdf-https://www.richtek.com/Design%20Support/Technical%20Document/AN0382. Get a suitable load power resistor to do the load step test- Note that if you immerse the resistor in water you can probably run 2-5x rated power for a short time3. Hook up your DSO and run the test. The "switch" can be as simple as manually touching an alligator clip/wire to the PS output terminal.

author: exe
author_rank: Unknown
timestamp: August 07, 2018, 09:32:55 pm
content: Quote from: dcarr on August 07, 2018, 09:22:08 pmThanks for the links. As for switching, I just directly drive a mosfet from my signal generator (using square wave). By regulating upper and lower voltages I can simulate not just load ON / OFF scenario, but also, e.g., transient 0.1A => 1A (and back). Of course mosfet drifts a bit as it heats up and 50-ohm drive will not switch mosfet lightning fast, but it was good-enough to characterize PSUs I have. As for heating, I just keep duty cycle low, this helps a lot.I think manually touching wires will create a lot of ringing and not repeatable. Perhaps, NE555+mosfet will be a good and cheap alternative.

author: dcarr
author_rank: Regular Contributor
timestamp: August 07, 2018, 10:54:00 pm
content: MOSFET + siggen is definitely a more elegant solution. However, manual alligator switching is an empirically verified method.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 07, 2018, 11:37:38 pm
content: Quote from: dcarr on August 07, 2018, 09:22:08 pmYou're only interested in the step response, so load time can be very short and with a low duty cycle. Unless the power level is such that there's a risk of the resistor rapidly combusting or exploding ( yes, this happens, guess how I know), then average dissipation in the resistor can be negligibleI think it was Bob Pease who phrased step-load testing as "just bang on the output and see what happens"

author: JohnG
author_rank: Frequent Contributor
timestamp: August 08, 2018, 07:31:25 pm
content: While it might be hard to get a clean step response using an alligator clip, it is not a bad test for robustness. I once saw a test setup in a lab that designed very reliable converters. It was a giant "computer grade" electrolytic cap with a big knife switch. Good for testing transient robustness of inputs and outputsJohn

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 08, 2018, 07:56:40 pm
content: Quote from: JohnG on August 08, 2018, 07:31:25 pmIt can be good for robustness testing, as it will catch things like overvoltage spikes due to inductive ringing. This on the input of a small DC/DC with only ceramic input caps can kill a converter running close to its max input voltage.Been there, Done that , added a zener

author: exe
author_rank: Unknown
timestamp: August 08, 2018, 08:27:48 pm
content: Quote from: mikeselectricstuff on August 08, 2018, 07:56:40 pmDid you connect the zener between the output terminals or accross the input capacitor? (I think it should be on output).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 08, 2018, 10:48:34 pm
content: Quote from: exe on August 08, 2018, 08:27:48 pmInput - I had a situation where a 32V max input converter IC would literally explode if you connected a 24V supply via direct contact contact ( as opposed to turning the PSU on). Scope showed input spike up to 40V from combination of cable inductance and ceramic input filter cap.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2018, 01:39:36 am
content: Quote from: exe on August 07, 2018, 03:31:47 pmYes, the load step response works and is also the easiest but you need an oscilloscope which can differentiate the response and then return magnitude *and* phase response from the FFT which the mentioned Keysight and Siglent oscilloscopes won't do. I do not know of any current production oscilloscopes that can do this for less than the cost of a new car; since it is just firemware, I suspect market segmentation is involved.This article from EDN covers the three ways (swept, noise, impulse) that oscilloscopes can make this measurement:https://www.edn.com/design/test-and-measurement/4441000/1/Measure-frequency-response-on-an-oscilloscopeThe load step response is not quite the same test but has the advantage of measuring large signal response which may be more relevant than the small signal response.Quote from: exe on August 07, 2018, 08:16:15 pmThe load response test above will do exactly what you want but like I said, no current DSOs even close to your price range will work.Two other options for DSO based VNAs include Cleverscope which it designed exactly for this application but is just outside your price range and Syscomp Design which is well within your price range:https://cleverscope.com/https://www.syscompdesign.com/products/

author: Hydron
author_rank: Super Contributor
timestamp: August 09, 2018, 06:44:51 am
content: Quote from: mikeselectricstuff on August 08, 2018, 10:48:34 pmLinear tech (as usual) has an informative app note about this:http://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdfHave definitely seen this as well, can also be solved with a series resistor on the input (before capacitors) if you can eat a little loss. Bonus points when someone goes through a stack of boards "trying to find a good one", managing to blow them all up in this manner (board was designed to go into something with a PSU directly upstream and no switch).

author: exe
author_rank: Unknown
timestamp: August 09, 2018, 07:38:54 am
content: Quote from: David Hess on August 09, 2018, 01:39:36 amI'll try to export raw data from the scope and do analysis in Python.

author: Fungus
author_rank: Super Contributor
timestamp: August 09, 2018, 11:51:21 am
content: Any 4-channel oscilloscope should be able to do it.Put a current shunt in series with the load and use two channels to look at the difference across it (i.e. the current reaching the load).Use the other two channels to look at the input and output voltages.It won't produce pretty graphs automatically but grabbing the data from the scope over USB/Ethernet is usually easy and you can make graphs using excel (or whatever).

author: nctnico
author_rank: Super Contributor
timestamp: August 09, 2018, 11:54:44 am
content: Quote from: Fungus on August 09, 2018, 11:51:21 amThat is not going to work because you'll be looking at milli-Volts on top of several Volts (or more). The resolution of an oscilloscope won't be enough. A better way is to put the current shunt at the ground point with one end and use 2 channels: one for voltage and one for current.

author: Fungus
author_rank: Super Contributor
timestamp: August 09, 2018, 12:19:11 pm
content: Quote from: nctnico on August 09, 2018, 11:54:44 amNot if you use the math function.,But yes, putting one end of the shunt on GND is easier and saves a channel.

author: coppice
author_rank: Super Contributor
timestamp: August 09, 2018, 12:23:39 pm
content: Quote from: nctnico on August 09, 2018, 11:54:44 amConnecting one end of the shunt to ground only works moderately well if you are careful to ensure the ground point for the scope is right on the shunt. The ground bounces around too much for millivolt level sensing to work unless the ground for the measurement is well placed.

author: nctnico
author_rank: Super Contributor
timestamp: August 09, 2018, 01:43:37 pm
content: Quote from: Fungus on August 09, 2018, 12:19:11 pmNo, because you are still limited to the ADC resolution and dynamic range of your oscilloscope. If you need a total range of 5V then each bit of the ADC is 19mV. If you use a 0.1 Ohm shunt then each bit of the ADC is equal to 190mA.

author: exe
author_rank: Unknown
timestamp: August 09, 2018, 02:54:39 pm
content: Quote from: coppice on August 09, 2018, 12:23:39 pmThat's why I'm gonna use analog discovery with differential inputs.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 09, 2018, 05:52:00 pm
content: It's possible to get a bode plot of the feedback loop using a scope and function gen if you have the time.search app note AN-1889.Also I remembered Robert Bolanos has a video also showing this .Regards.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 09, 2018, 06:45:14 pm
content: Quote from: nctnico on August 09, 2018, 01:43:37 pmwhat about AC coupled?

author: nctnico
author_rank: Super Contributor
timestamp: August 09, 2018, 06:48:41 pm
content: Quote from: Mechatrommer on August 09, 2018, 06:45:14 pmBut then you introduce a high-pass filter from the AC coupling which makes the measurement harder to interpret. AND if the load step causes a significant amount of ringing and/or voltage drop you'll overdrive the oscilloscope input anyway causing all kinds of weird effects which aren't there.

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2018, 07:31:51 pm
content: Quote from: exe on August 09, 2018, 07:38:54 amThis Tektronix article gives an idea of what is required. I have considered buying one of their old TDS DSOs which support magnitude and phase FFTs just to do this:http://www.testunlimited.com/pdf/an/55W_8815_2_2009.01.07.10.50.04_2765_EN.pdf

author: rx8pilot
author_rank: Super Contributor
timestamp: August 09, 2018, 08:46:28 pm
content: Quote from: Keysight DanielBogdanoff on August 06, 2018, 09:33:06 pmThis can be a challenge on a scope because of the low level signal that is typically buried deep in the noise of the switching elements. The latest software appears to be filtering/averaging, but I cannot see what it is doing in the background. The dynamic range is considerably better than the original application.I have been able to rough it manually using averaging and filter math to get what I needed. I manually step through the frequencies and measure the gain and phase with the scopes measurement tools. In general, you are looking for two distinct points - the phase margin and the gain margin which means youdon'thave to analyze in 1hz steps. This is rather slow and somewhat limiting - but cheap. Forum member Robert Bolanos demonstrates this in his YouTube video posted a few replies up.The next step I took was to use a differential amplifier Preamble/LeCroy DA1855 to better see and filter the low level signal. That requires some time compensation to get an accurate phase measurement but the benefit is that the low-level signal is gained to a point where the scope is not on the edge of its front end performance. This is not critical, but helpful. The main use I have for my 1855A differential amplifiers is to measure current shunts.Next for me was getting a VNA. I found an HP 3577A that was originally used in the Paleolithic period by cavemen trying to analyze the energy transfer at the contact point of two sticks being rubbed together. I searched for quite a while and ended up with a mildly broken one for $200 plus about $75 to fix it. I added a Jensen video isolation xfrmr VB-1BB that was another $20 off of eBay. I am still learning how to get this setup and deliver trustworthy measurements. So far, it seems rather fantastic relative to my previous efforts. It goes down to 5hz and has 1hz RBW, averaging, etc.....lots of relevant features for FRA. The alternative I was looking at was suggested by power integrity guru Steve Sandler was a Keysight E5061B with low-frequency option. This is an amazing tool - but with an appropriately amazing price. The 3577A used as an FRA is a great bang for the buck as long as you have a bench big enough and strong enough to hold it.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 09, 2018, 09:49:51 pm
content: QuoteI'm hoping to have a stab at measuring the loop response of an offline flyback using an Analog Discovery, time permiting. It's got enough dynamic range to do the job but I don't know how well the "back end" software will deal with the unwanted switching frequency. Will have to try it out and see.Robert Bolanos has some excellent tutorials of flyback control loops, he uses a cleverscope for the measurements.

author: _Wim_
author_rank: Super Contributor
timestamp: August 11, 2018, 07:22:20 am
content: This guy made a excellent description how to perform gain phase measurement using a low-cost pico scope and the FRA software tool:http://www.simprojects.nl/images/Gain_Phase_measurements.pdfHe also made a very detailed description how to make your own injection transformer for less than 30€:http://www.simprojects.nl/images/DIY_signal_injection_transformer.pdfI have made 2 of these, one using the large core (Coilcraft as suggested in the article), and one using a slightly smaller core (Würth WE-CMBNC XXL) trying to achieve a bit higher frequency response. Attached are the two results.

author: JohnPi
author_rank: Regular Contributor
timestamp: August 14, 2018, 05:28:45 am
content: I'm trying to do this also. My plan is to use an injection transformer in the feedback loop; drive it with a signal generator, and use a scope to capture input & output waveforms. By capturing the scope data for each waveform (over precisely 1 cycle), I can correlate (dot product) with a sine & cosine and get its phase & amplitude. atan2(sig2, sig1) gives me the phase. For a high gain loop, the signal will be small, but I can get within a few degrees with 4 mVpp on a 200 mV/div range.This method will be slow (sort of like a spectrum analyzer). The Bode 100https://www.omicron-lab.com/products/vector-network-analysis/bode-100/can sweep from 100 to 1 MHz in ~10 s. I don't know if it uses discrete frequency steps, or somehow does a smooth sweep and calculates something. They use a 24-bit ADC inside (see teardown), so can get good SNR.

author: Hydron
author_rank: Super Contributor
timestamp: August 14, 2018, 08:30:01 am
content: The "scope" based units (e.g. cleverscope, keysight/siglent bode plot mode, pico FRA software) use FFTs to do this, not a direct measurement. This allows for low level signals (which you will get in this case) to be pulled well out of the noise, as you're just looking at the magnitude and phase of the FFT output at that frequency, ignoring all the rest. If you're grabbing the data off the scope to a PC in the first place, just do the FFT on the PC as well (with a decent record length for good FFT resolution, though this will slow the sweep).

author: vealmike
author_rank: Regular Contributor
timestamp: August 14, 2018, 08:51:59 am
content: Sig gen, scope, injection transformer.http://www.ti.com/lit/an/snva364a/snva364a.pdfSome of the Keysight scopes with built in sig gens have a bode plot feature, and will do the hard work for you.If you don't have one of these, just adjust the gen & take the readings manually.

author: mzzj
author_rank: Super Contributor
timestamp: August 14, 2018, 10:12:06 am
content: This could be one ready-to-go option if audio frequencies are enough for you:http://www.holmacoustics.com/holmimpulse.php