Switch Mode Power Supply Question. - Page 1
Forum section: electronics / projects

author: kvresto
author_rank: Regular Contributor
timestamp: March 22, 2015, 11:21:52 pm
content: Hi Everyone.I've come back to a project I want to eventually finish, it involves a logger looking into automotive signals. this will run on a cars battery, and as I would expect the battery voltage to get below say 10V if its gone flat, I still want this logger to work. So I've been looking into some designs, and the two I thing will do the job are the flyback type of switching power supply, and the sepic type. My cct will have the usual op-amps powered from +-15V, ADC's@+5V, and the obligatory micro@+3.3V.My question is what is the best choice? are there other more suitable? there's a bit of info around on flyback, and sepic, but sometimes it leaves me with more questions than answers. I don't want to become a switching power supply magician, there's just too much else to do. I hope someone who has dealt with stuff can help out.cheerskv

author: moffy
author_rank: Super Contributor
timestamp: March 22, 2015, 11:27:33 pm
content: 1. You don't need +/-15v supplies these days as you can get precision op amps that run off a single supply, +5v typically. That would obviate the need for the +/-15v supplies, unless your sensors require it.2. If you need the +/-15v then a simple flyback with two output windings, one for the +15v and another for the -15v would suffice.

author: kvresto
author_rank: Regular Contributor
timestamp: March 22, 2015, 11:36:58 pm
content: Thanks moffy, in such a design what do you think the output ripple voltage would be, and do you have any thoughts on sepic converters.kv

author: rs20
author_rank: Super Contributor
timestamp: March 22, 2015, 11:52:48 pm
content: But why do you think you need +/-15V? We're not just saying "use single supply +5V op amps" as a joke. Modifying your design to use single-supply op amps ismuchless trouble than dealing with ripply switching converters.How much ripple can you tolerate? Don't just say "really good" or "dead smooth". Calculate what you need based on the supply rejection ratios of your op-amps and your desired accuracy.The amount of ripple you'll see from a flyback/sepic will depend enormously on the operating frequency, inductance, and capacitance. Datasheets will give these formulae. However, if increasing inductance, capacitance and operating frequency don't get your ripple down to where you need it; just use a +/- 18V flyback followed by +/-15V linear regulators. The linear regulators will reject the ripple (again, how much is given in the datasheet).

author: rs20
author_rank: Super Contributor
timestamp: March 23, 2015, 12:21:22 am
content: Quote from: blueskull on March 23, 2015, 12:10:12 amWhy on earth would you use an LDO to smooth and regulate a supply, and then feed thatintoa boost converter??

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 23, 2015, 01:37:27 am
content: To the OP and the group,Take a look at this project:https://www.eevblog.com/forum/projects/tektronix-tekprobe-power-supply-for-agilent-rigol-and-other-oscilloscopes/msg632320/#msg632320In this project I made +/-5V and +/-15V from 5V (USB) but the circuit will work with a 12V input without modification.The original poster doesn't say how much current is required and if there are special requirements for low noise etc.Regards,Jay_Diddy_B

author: kvresto
author_rank: Regular Contributor
timestamp: March 23, 2015, 02:10:02 am
content: Thanks all for your comments.rs20, modifying to use single supply op amps is on the cards, but I want to stick with the +-15V just for now, because I have actually struck a problem which I would also like to solve to avoid the same issue again. I've included images of a SEPIC +15V design I was testing, but the results were disastrous, see for yourself. The max current is about 1.5A, but what ripple I can tolerate has not been calculated out I admit, but the less the better (sorry for that). the input signal max I expect to see the output of inductive pulse sensors which measure speed generally at ~ +-batt voltage, not a precision type of measurement, but still reasonable overall system accuracy is the goal.I have also been looking at some of TI references about a +/- 18V flyback designs as well.kv

author: rs20
author_rank: Super Contributor
timestamp: March 23, 2015, 02:36:02 am
content: Like I said earlier, adjust inductance, capacitance,operating frequency. Your converter appears to be operating at around about 1.2 kHz, I'd have thought that one hundred times that (120kHz) would have been more typical.

author: kvresto
author_rank: Regular Contributor
timestamp: March 23, 2015, 03:01:14 am
content: This is the design spat out my WEBENCH, I didn't notice that the op freq was so low. I need to go back and inspect what components I soldered on the board??Hmmmm??thanks rs20, Ill post what I found. Actually Lout is 100uH, not that it makes much difference.

author: rs20
author_rank: Super Contributor
timestamp: March 23, 2015, 03:25:20 am
content: Quote from: kvresto on March 23, 2015, 03:01:14 amWhat did WEBENCH say the peak-to-peak ripple and operating frequency were expected to be? Is it actually different to reality? If you dragged the slider to "maximum efficiency", you may well end up with huge ripple and low frequency (low frequency --> low switching losses). Also, you can force webench to use a particular frequency of switching if you want to overrule it.

author: kvresto
author_rank: Regular Contributor
timestamp: March 23, 2015, 05:30:25 am
content: Switching Freq = 405Khz, and Vripple pp = 173.375mV or there abouts:)In fact heres the output from WEBENCH

author: kvresto
author_rank: Regular Contributor
timestamp: March 23, 2015, 08:22:37 am
content: Also the data sheet has Rfreq at about the same value that WEBENCH calculated for that 400 or so KHz, 37.4Khz, which as I checked is the value I installed, so it should be switching at that freq, all other values are ok as well, but I cant measure the Rsence with my MM. Could the output capacitor which is 33uF be too small?, and what size inductor(s) should I go to if I was to replace them trying to get a better output?cheers rs20.kv.

author: kvresto
author_rank: Regular Contributor
timestamp: March 23, 2015, 09:47:19 pm
content: bump?

author: rs20
author_rank: Super Contributor
timestamp: March 23, 2015, 10:40:25 pm
content: I dunno. Anyone else? How do you debug a SEPIC converter that's operating orders of magnitude away from its nominal operating frequency?

author: kvresto
author_rank: Regular Contributor
timestamp: March 24, 2015, 12:31:29 am
content: just another one, looking into the flyback type and not being a power supply guru, what are the good texts/app notes that discuss transformer selection/design and other, for putting together a +-18V supply?cheers

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 24, 2015, 01:50:46 am
content: Hi kvresto,Are you sure that the scope isn't aliasing? Try setting the timebase to 1us/div and taking another picture.Jay_Diddy_B

author: kvresto
author_rank: Regular Contributor
timestamp: March 24, 2015, 01:58:28 am
content: Sure Jay_Diddy_B, here's the picEDIT: and one at 100uS

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 24, 2015, 03:36:17 am
content: Hi kvresto,This scope shot shows typical switching spikes with a period of 2.5usThis indicates that the switching frequency is 400 kHz, same as the design. Note there are two spikes per cycle, one when the switch turns on, the other when the switch turns off.The other waveform:Could be the result of:1) Control loop instability (most likely)or2) a pulsating load current.Did you follow the recommendations for the type of capacitors for the input, output and coupling capacitor?Jay_Diddy_B

author: rs20
author_rank: Super Contributor
timestamp: March 24, 2015, 04:09:31 am
content: Quote from: Jay_Diddy_B on March 24, 2015, 03:36:17 amGood point. Just to expand on that, many regulator designs actually rely on the capacitors having an ESRabovea certainminimumvalue*, believe it or not. Even the output capacitors are actually part of the control loop. Even plain old simple linear regulators often have this restriction (if you're interested,here's a whole report on LDOs needing specific ESRs). So if, for example, you're using an MLCC where an electrolytic is called for, or even using the wrong model of electrolytic, you can encounter issues.* It goes without saying that there'll be a maximum value too, but that's perhaps more obvious.

author: kvresto
author_rank: Regular Contributor
timestamp: March 24, 2015, 05:14:02 am
content: I've used the correct output cap as per WEBENCH design(see pdf previous reply), but I used what I had in my draw on the input cap. WEBENCH called up an ESR of 0.4R@35V PN:35TQC3R9MYF, and the one I used is ESR=0.7R@35V PN:TPSC335K035R0700.kvEDIT: Here are two captures of the input voltage taken across the input cap.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 24, 2015, 11:39:42 am
content: Hi,The voltage across the input cap, confirms that the switching frequency is the correct value.You have a loop stability issue.Jay_Diddy_B

author: kvresto
author_rank: Regular Contributor
timestamp: March 24, 2015, 11:18:43 pm
content: Hi Jay_Diddy_B, thanks for your input and rs20 has been very helpful. How are you determining the switching freq, is it by the ripple on the peaks?kv

author: rs20
author_rank: Super Contributor
timestamp: March 25, 2015, 12:41:34 am
content: It's the overall waveform -- in your first diagram, with the timebase set to 1us/div, the peaks are 2.5 divisions apart, aka 2.5 us. 1 / (2.5 us) = 400 kHz.

author: kvresto
author_rank: Regular Contributor
timestamp: March 25, 2015, 03:15:14 am
content: ok. I'm planning on replacing the input cap just as soon as I order the same as WEBENCH BOM. If I remember this was a bit of a pain to get. I'll post my results when I get it and install it.cheers,and thanks.kv

author: kvresto
author_rank: Regular Contributor
timestamp: April 03, 2015, 01:29:43 am
content: Hi All, just an update.I fitted 1 new 35TQC10M series input capacitor a 10uF 35V Tant, it was the only one available from RS, and apart form its size and ESR its the same family of cap called up in WEBENCH BOM. Purchasing the cap from anyone else meant a delivery charge making this a very very expensive cap. Its got an ESR=120mOhm. As you can see from the pics, its a different waveform, but no better than before. I'll have to do some more research, and possibly start removing and replacing/re-soldering components to try and find out what going on.cheerskv

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2015, 02:24:16 am
content: Your grounding or probing technique is highly questionable. Also, the zoomed-out waveform is dubious to useless, as the scope appears to be aliasing noise, not viewing the signal correctly at that sample rate. Turn on bandwidth limiting or hi-res mode at slow sweep rates.Tim

author: kvresto
author_rank: Regular Contributor
timestamp: April 03, 2015, 07:48:10 am
content: Thanks T3sl4co1l glad to get your input. There isn't any aliasing, I've checked.kv