Buck battery charger....power stage poles and zeros? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: February 13, 2022, 06:40:11 pm
content: Hi,What are the poles and zeros of a Buck battery charger that regulates output current?Its a current mode Buck.Its in CCM.It has slope compensation.Duty cycle is 0.5 to 0.78.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 14, 2022, 04:25:27 pm
content: Hi Faringdon,that stuff can be found in many introductory books about control and power systems (I still have mine from uni).I don't mind helping you but please show us what you got and try to make a little of effort in solving the problem before.

author: Faringdon
author_rank: Super Contributor
timestamp: February 14, 2022, 08:04:06 pm
content: Thanks, i have the attached from Basso... (Equn 2A-5)There is a power stage zero from the output cap and its ESRThere is a power stage pole from the load and the output capacitor.Also a power stage pole fromt he slope compensation.I think the same exists with a Buck battery charger.....though i am in two minds , because i think the current regulation stops the battery from looking like the massive capacitor that it is?...***...In relation to this, please see bottom pf page 8 of this..https://www.microsemi.com/document-portal/doc_view/124786-voltage-mode-current-mode-and-hysteretic-control"Unfortunately, DC gain does tend to fall at high load currents"..this epitomises what i mean...high DC gain is desirable in most SMPS's....so why ever drop DC gain by using a type 2b error amplifier?...***...Also, i have this...https://www.ti.com/lit/an/slyt308/slyt308.pdf?ts=1644871005431&ref_url=https%253A%252F%252Fwww.google.com%252F..it shows that in current regulated SMPS's, the dynamic resistance of the load (battery or load) comes into the load pole.The ESR zero is still there, but must be at extremely low frequency, since a battery is a huge capacitance.....such a low frequency zero will need a low frequency compensatory pole to compensate for it....otherwise instability will ensue.

author: zeke.
author_rank: Contributor
timestamp: February 15, 2022, 03:19:28 pm
content: I think you’re better off trying to measure the transfer function of your circuit rather than use the transfer functions from the links below. Not all systems are the same, even if they have the same design goal.A common way to do this is to inject a sweep of frequencies (different ranges of frequencies) onto the open loop plant function. You could do this with a signal generator and measure your input and outputs with your oscilloscope.Break the feedback path of your SMPS with the input from your signal generator (don’t directly connect the signal generator to the circuit)Use the microsemi VMC and CMC paper to check your results.This is a good link to follow:https://ridleyengineering.com/hardware/ap310-analyzer/ap300-application/ap300-loop-injection.htmlOnce you get your plant transfer function you can solve for the poles and zeros of your circuit.The poles are when the denominator equals zero and the zeros are when the numerator equals zero (take the numerator and denominator equations, set them equal to zero and solve for s).You could follow the tutorial below from MIT if you want to test the stability of the SMPS, they do a great job explaining all this:http://www.mit.edu/afs.new/athena/course/2/2.010/www_f00/psets/hw3_dir/tutor3_dir/tut3_e.htmlI hope this helped, your probably aware of most of what I mentioned anyways but good to talk about it in this post.

author: Faringdon
author_rank: Super Contributor
timestamp: February 15, 2022, 09:30:41 pm
content: Thanks, please could you show where to break the loop and inject the frequency sweep into this 1kW Buck battery charger?PDF schem and LTspice sim attached

author: Faringdon
author_rank: Super Contributor
timestamp: February 16, 2022, 08:51:37 pm
content: Hi,The attached is a Buck battery charger , 1kW.The one with “unstable” in the name is unstable.The other is stable , and has had a feedback loop injector added. Is this a good point to do feedback loop injection testing for gain and phase margin assessment?PDF schems and LTspice sims as attached.

author: Faringdon
author_rank: Super Contributor
timestamp: February 18, 2022, 05:25:37 am
content: Are power stage poles and zeros for a buck regulating to a constant voltage output, the same as for a buck regulatig to the same-ish voltage, but doing current output regulation as its a battery charger?

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 18, 2022, 02:34:56 pm
content: Hey Faringdon,like I said, at least show us that you did some of the necessary study of how the converter works and then we'll be able to talk in more or less the same level of terms.English is not my primary language but sometimes your posts come across as if you want all the effort served to you on a plate.

author: Faringdon
author_rank: Super Contributor
timestamp: February 19, 2022, 02:26:53 pm
content: ......Thanks, i see your very good point, but it may be a few years for me to get the necessary first principles together and then calculate the modulator and power stage poles and zeros of a Buck battery charger which does output current regulation, as opposed to output voltage regulation. Though thankyou for re-igniting some interest in this thread.(As you know, unfortunately , the customer will have given up on us by then...and i beg your pardon in that becuase i am not being funny in that.)....Its interesting, because this particular information, is not available on the internet anywhere, and i dont see it on first reading of both of the great Basso's books....(but i would say that Basso probably covers the theory needed to arrive at the required information). Basso does literally "serve up on a plate" the modulator/power stage poles and zeros for a output voltage regulated Buck. (as in my above thread).If somebody asks on this forum, "what is the relationship between vout, vin and Duty for a CCM buck", then i will answer them , and serve them on a plate "Vout = D*Vin".....but this can not be done for the above information requirement pertaining to a current regulated switch mode battery charger, as this work is not in any book or website.....anywhere in the world.Mind you, i think the feedback loop injection point as depicted in the above post #5, is a gem of golden information for people.....because it shows how to assess the gain and phase margin of your actual converter...i was just wondering if anyone sees any gremlins in the method...?...other than the extreme cost of the kit needed to do it...eg, cheapest is Pico 5444B at a couple thousand pounds/dollars.

author: Someone
author_rank: Super Contributor
timestamp: February 19, 2022, 10:31:50 pm
content: Quote from: pardo-bsso on February 18, 2022, 02:34:56 pmIndeed, as the reply says clearly:Quote from: Faringdon on February 19, 2022, 02:26:53 pmIts not a pre-packaged answer as it is so little used/needed and ends up being outrageously specific to the application. General solutions = commodity designs unskilled people can deploy. If you want to compete with the skilled people and earn $$$, then you have to learn how to do it...... or hire a consultant to do it for you!The joke here being a misguided focus on poles and zeros (the thread title and repeated through the replies) which are just academic approximations/models although convenient for simple analysis, fail/fall apart when trying to use them as the only analysis of higher order regulators. So its the usual problem of not knowing enough to ask a sensible question, which makes it obvious any assistance provided is just going down the toilet (and providing commercial assistance to you).

author: zeke.
author_rank: Contributor
timestamp: February 20, 2022, 01:01:43 am
content: Break where your LTC6101 current sensor is outputting a voltage reference to your DC/DC converter. At that break inject your frequency sweep.Measure what the current sensor is outputting with your oscilloscope.Your still going to need to need to transform your current sensor output and injected signal into the laplace domain to get your transfer function. A frequency response analyzer does all the signal injections and transfer function generation but it’s pretty pricey, worth the money.I'm not sure if I'm helping with the problem, trying my best lol.