Best plug-and-play products for getting Bode plots of voltage regulators?  - Page 1
Forum section: electronics / general

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: February 10, 2023, 06:59:28 pm
content: I am a power supply engineer and at my lab my team currently has no dedicated equipment for doing Bode plot analysis of regulators, which I think is a great weakness of ours. We are generally dealing with typical point of load LDOs and switchers (Fsw up to 1 or 2 MHz), which go for example from 12V to 5V, 5V to 3.3V, 1.8V, etc. Vin typically no greater than 48V and currents generally no greater than a few amps. Ideally I am looking for equipment which is relatively cheap (less than 10k USD) and easy to use since my team doesn't have much experience (myself included). In my research I have come across the Bode 100 from Omicron Lab which seems like a good option:https://us-shop.omicron-lab.com/collections/frequency-response-analysisAs I understand it I basically need 2 things: The main "analyzer" unit and then the injection transformer. I know there are various ways to sort of "homebrew" Bode analyses but my goal is relatively simple plug and play dedicated equipment. I'm just wondering if there is anything else out there on the market that would suit my needs comparable to the offering from Omicron Lab. Thanks

author: jonpaul
author_rank: Super Contributor
timestamp: February 11, 2023, 02:05:22 am
content: We used,analog computers and pôle zéro plotters in 1960s.The stability of a second order system is,very well documented see Control Systems Engineering by DelToro.Our old friend Deane VENABLE in 1980s did the the Pioneering work on bode plotting for power suppliesYours can DIY your own setup, with a computer, power amp, and transformer.Nowadays several off the shelf systems like Ridley's are available for Windows, rather costly HW and SW.But, SMPS Bode plots were never an issue as most SMPS controllers are well behaved and unconditionallly stable with just the mfgs reccomended comp network.Subcycle oscillations and current limit cycling are much more likely than classic 2ond order loop instability.Bon courageJon

author: Faringdon
author_rank: Super Contributor
timestamp: February 12, 2023, 01:36:27 am
content: SMPS Feedback loop measurementsThe Ridley engineering group make a gain phase analyser ( AP300) which is good for measuring the gain and phase margin of an SMPS…http://www.ridleyengineering.com/analyzer.htmlThe following 4 videos go through use of the AP300 frequency analyser, which you can use to measure gain and phase margin of the SMPS feedback loop.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: February 13, 2023, 10:35:38 pm
content: Quote from: jonpaul on February 11, 2023, 02:05:22 amWe've run into a very uncooperative switcher with subharmonic oscillation at 1/4th the switching frequency. And using the datasheet recommendation for the RC network on the COMP pin somehow makes things much worse for us... :'(

author: jonpaul
author_rank: Super Contributor
timestamp: February 14, 2023, 12:11:32 am
content: Jim, whats the IC, switcher mfg, model?Can you post a schematic?one pc or many have subcycle 1 /4?Jon

author: uer166
author_rank: Super Contributor
timestamp: February 14, 2023, 12:51:52 am
content: Quote from: Jim from Chicago on February 10, 2023, 06:59:28 pmThat is what I use regularly for lots of things from component characterization to PDN to stability analysis. Worth every penny.

author: Faringdon
author_rank: Super Contributor
timestamp: February 14, 2023, 01:41:05 pm
content: QuoteHave you tried LTpowerCAD designs from analog.com...it designs the whole circuit for you and gives you the bode plots and everything.Presumbaly you have no isolation requirement, so you are just using Bucks....?...do you have a requirement for high feedback loop bandwidth?...if not then i am surprised you are having trouble...you can usually just hack them to be stable no problem. If you post schem with your values i bet i can fix it for you over the web.Is it noise issues due to pcb layout that you trouble with?..and maybe youre switching the fet on too quick?...maybe you need to staunch the drain node transition via adding more gate series resistance.....maybe your sense connections are over the switching node, etc etc?If you have SHO, then thats strange because your duty cycle must be <<0.5 at 48 to 3.3v.Maybe your fet on time is too short...maybe you need to look into emulated current mode control a la texas

author: jonpaul
author_rank: Super Contributor
timestamp: February 14, 2023, 02:18:27 pm
content: Rebonjour:Sub-cycle SMPS oscillations relates to the PWM control system slope compensation.Bode plots give a clew but do not get to the root issue.The non-linear behavior creates a multivibrateur type of divider, like a Morrison Quartz clock.https://www.monolithicpower.com/recognition-and-elimination-of-subharmonic-oscillationsThus I would check the type of current control (if used) and presence or absence of slope comp in the control loop or IC.Here is a TI note on current controlled SMPS controllers.https://www.ti.com/lit/an/snva555/snva555.pdfBon ChanceJon

author: Weston
author_rank: Regular Contributor
timestamp: February 14, 2023, 07:31:21 pm
content: The injection transformers or alternative line injectors are pretty simple but expensive if you want to buy off the shelf ($400+ for a $10 mu-metal core with some twisted pair on it).Picotest and Bode make injectors / injection transformers.For taking the measurements themselves the Bode 100 is probably the gold standard. Its very nice but very expensive. Its basically a low frequency VNA and has very high accuracy.If you are characterizing SMPS feedback loops you mostly care about the gain and phase margin, which do not have that wide a dynamic range. A lot of recent oscilloscopes with build in function generators can take bode plots. The Analog discovery 2 also can take bode plots and has a pretty wide dynamic range and differential inputs.I use my RTB2004 and get quite good loop response measurements, even with low injection signals or measuring switched signals (measuring the control to switch current response). I don't know if they explicitly list the specs for the bode plot signal processing, but the scope has a 10 bit ADC and has internal oversampling and then the bode plot routine does some additional sampling, so the dynamic range is pretty good. I would expect most scope based bode plots to be pretty similar. The Bode 100 has hardware mixers to achieve better rejection of noise, but I don't think thats needed in most cases.I dont seem to have any bode plots from SMPS stored on my laptop at the moment, but here is a bode plot from when I was using it to characterize the open loop gain of an op-amp on an IC I designed.

author: Jim from Chicago
author_rank: Regular Contributor
timestamp: March 17, 2023, 06:07:32 pm
content: Thanks for replies, about the specific problem with a switcher I was having, we just changed to a different part that didn't cause problems.