Adjustable switch mode power supply tutorial

A tutorial on feedback resistors in DC-DC converters and how to build a high current adjustable power supply using an LM2678.
Check out my webpage for more electronics! www.afrotechmods.com
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Parts:
LM2678T-ADJ:
octopart.com/lm2678t-adjnopb-t...
Input electrolytic cap:
octopart.com/eeu-fm1h221-ecg-2...
Output electrolytic cap:
octopart.com/eeu-fr1c102b-pana...
Input and output ceramic caps:
octopart.com/partsearch#!?q=UM...
Bootstrap cap:
octopart.com/c420c104k5r5ta720...
Schottky diode:
octopart.com/stps5l60-stmicroe...
22uH inductor:
octopart.com/aiap-03-220-k-abr...
Potentiometer:
octopart.com/p090s-14t20br10k-...
Schematic:
i.imgur.com/XkGTV09.jpg
Twitter: / afrotechmods
Facebook: / afrotechmods

[@Afrotechmods]

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[@greatscottlab]

As always really nicely explained!

[@JFrap]

your no noob to voltage dividers. or.... /a subscriber of yours.

[@craveskill2574]

OMG your project is awesome wifi rgb led show me in video :)

[@markb1911]

The way you explained the voltage feedback comparison process for regulation in the dc-dc ic is simply art. The art of explaining complexity so well that it suddenly becomes simple things in cooperation. Great video.

[@TheAmmoniacal]

Love that Back To The Future reference!

[@ppdan]

Checked lots of SMPS tutorials but your series was the most comprehensive I've found. I don't understand why you SMPS tutorial series doesn't get more views.

[@KX36]

A few ponits if I may, The usual reason for keeping the value of feedback resistors low is to reduce the error due to the feedback pin's input bias current. The variable resistor will present less risk to loop stability if it is in the lower part of the voltage divider as the upper resistor is in the AC transfer function while the lower resistor (for the most part) is not. X1 mode on a X10 probe has a MUCH lower bandwidth than the X10 mode. You should use the X10 mode where possible. If you want to minimise noise, you may be able to use the short ground spring clip rather than the long croc clip if you have a close enough ground point.

[@happyhippr]

This video is so well put together. Clear, Consice, informative, and just enough extra info to lead you to further research. So helpful.. Thank you! --SO EE preparing for summer internship

[@luciddreams5677]

I thought I would never understand the function of electronic components. Thanks to your channel. You put things in simple words, in witch I could understand & not sound like my class instructor speaking in a foreign-language. Thank you

[@charlielu05]

Absolutely love your videos! Please do make more! Learning so much from just watching them!

[@misterannonymus7560]

You really saved my day after i heared the Linear Voltage regulators are not very efficient, unless you want to heat your whole room. At first i was very irritated what component values i should use, but you explained the Voltage Divider very well, thank you very much 👍

[@RC_Engineering]

Insanely helpful, thank you from an EE student.

[@L2.Lagrange]

Really awesome video, especially going into detail about the EMI stuff

[@rosspainting9499]

hey i love your videos you taught me so much you cant even imagine how thankful i am

[@arthurcascardo7202]

Very good video, i have been looking for an explanation on that for a long time, and i'm really glad i found yours

[@uvlight]

Great video, very easy to understand. combined nicely with your op amp & buck converter tutorial video.

[@Migueldeservantes]

You did help me way back when.. and I'm very grateful, I always enjoy a lot your videos, thank you very much

[@noweare1]

Excellent video, good advice just using the switcher chip. Keep 'em coming.

[@revealingfacts4all]

Very happy you're back to making these videos. I absolutely love your teaching style and the format. Love the humor injections. I'm also a fan of EEVBlog, but prefer your content more so. Dave's channel, while good at times, waste too much time with nonsense like mailbag segment and tours of maker fairs at other times. I hope the patreon thing works out for you.

[@Ale_Dev]

This video is excellent and it was made 9 years ago... I liked that 1.21GWatts reference xD

[@thuyavansathiamurthi3592]

This is a super solid tutorial, thank you so much!!!

[@Prayimario]

Ah, so this explains how internal self-caused interference screws stuff up? Thanks very much for this tutorial, and as always, they are extremely helpful.

[@aspadeisaspadee7847]

awesome video. keep it up. i learned a lot watching your video which i couldnt in a year spent in college. thanks so much!

[@amontodd551]

Awesome video! Do you have a circuit for constant current control? It'd be super handy for driving LED's as well as over current protecting for testing prototype circuits.etc

[@visionofwellboyofficial]

I like your turtorials

[@dko4882]

2:35 - Electronics have never been funnier. So good, these videos!

[@TECHNO_TURK]

@1:27 1.21 Giga Watts

[@yahnsolo]

3:05 also

[@braincelluniverse]

Fantastic Tutorial! I have been looking for a regulator that can be controlled via digital potentiometer but am having trouble finding one with a high wiper current. The end goal is to send messages to the digipot via Arduino to manipulate voltage.

[@stephenirwin2761]

Nicely done!

[@kokosdame]

Hi. Very nice and interesting videos for me. My question is how can I increase the Ampers on the same circuit?

[@Ghostpalace]

Nice video! Keep up the good work!

[@amingholizad]

thanks Can you teach us how to add current limiter to this that can limit output current from 0 to 3A in a new video??

[@LukasBonny]

Great tutorial!

[@TheSupertecnology]

That reference from Back to the Future, awesome hahaha

[@TheBobbyB86]

Would a shielded inductor help with the interference?

[@TraceWaters]

Love the videos. I'm pretty new to electronics and am trying to figure out a way to make an efficient high powered LED driver. Right now I use a boost converter to get the 44 volts and 1.5 amps I need. Any suggestions? Oh and all audible frequencies should be in the key of D minor, the saddest of all keys.

[@kamalchahlaoui8242]

nice video bro . thanks . but what is the input and max output voltage ?

[@uberDoward]

Great video, love it!

[@mahmoudmetwally1920]

Can you please start a series on batteries with your excellent way of explanation. I mean its types, characteristics and how to charge them etc... Thank you.

[@amingholizad]

thanks. can you take the build a step further and make a power supply with adjustable current and voltage together?

[@adama1294]

Cool video. How can this be adapted for very high currents to charge very big batteries?

[@heshamhamed1712]

I am making a buck converter with an input of 48 volts and outputs 24 volts 10 amperes, in that case i just need a different IC and component values right?

[@ProXicT]

Could you make a tutorial for bench power supply with voltage and current regulator? It would be excelent!

[@stemmaster1568]

One point twenty-one gigawats! GREAT SCOTT!!!

[@stemmaster1568]

I'll have to move to the UK then lol, where I live there are people going mayhem with plutonium guns and Delorians.

[@CoolDudeClem]

Goodbye LM317!

[@robot797]

true that

[@mrjohhhnnnyyy5797]

Not really, for small power linear regulators is OK. Another advantage is absence of HF noise caused by switching

[@PaganiTypeR]

MrJohhhnnnyyy plus lm317 is way cheaper than lm2678. So in a lot of projects it'll be more preferable than lm2678

[@mrjohhhnnnyyy5797]

MelectroU Of course, and it's not picky to construct. As you saw in this video, you have to lay out components properly to get good performance. I use switchers too, but sayin' that linear ones is not needed anymore is just... wrong

[@thorstimon]

depends from delta V.

[@TheVVane]

Nice video thnx :) Can I add current limiter to this that can limit output current from 0 to 3A in this circuits ??

[@dougholsted348]

I have a big heavy transformer from an old camper battery charger. With a large bridge rectifier and capacitor I can get DC (crude). Will this work as the V in for the Buck converter? Love your videos, thanks for all you do.

[@Afrotechmods]

+doug holsted Probably, yes. To be safe make sure you measure the unloaded peak voltage of your rectified DC and make sure that's within the range of the buck converter's max input.

[@ilmionomefattimiei]

Good video! 👍 Afrotechmods can you please do a video also on a FM transmitter receiver? a while back you've done one in which you explain the AM transmitting principle.... So can you do a video in which you explain the theory behind the FM transmitter and the receiver... Maybe just a simple circuit would be more than enough to understand correctly the working principle because I have already read on wikipedia the frequency modulation metod... But i want to have some formulas on which i can relate on... For example the exact coil and capacitor dimension... etc.. Thanks anyway :)

[@milejukic3297]

Is there such circuit for 10 or more amps, or is it possible to add powerful FET to increase output current?

[@FreebitNL]

Really good videos! I wonder it is also possible to use a 555 timer and control the PWM automatically?

[@cameraman1234567890]

I imagine yes, but you need a feedback IC that could control the 555 timer

[@Mladjasmilic]

Please do a videos about UPS , DC-DC step up and simple controler for up to 20kw DC electric car with regen .

[@DigGil3]

Ahh! In previous videos, you'd recommend using KiloOhm range resistors for voltage reference, but you had never really explained what was that for. Now I understand! It's a signal-to-noise thing, then.

[@stephenbrao9203]

Would you be able to achieve a similar result with a MOSFET instead of a simple switcher?

[@ali10163]

Can you do a video on buck-boost converters?

[@assalachraf4110]

Very informative Thanks (y)

[@amikhimji]

Great video thanks

[@Razor2048]

What about those variable boost converters. e.g., some will take in 5V but output 6-15 volts? Also if you keep adding capacitance (e.g., multiple electrolytic and ceramic caps), will you reach a point where you can get rid of all of the ripple, or is it impossible to filter out all ripple from the switching regulator?

[@Afrotechmods]

Variable boost converter video is a Patreon milestone and will likely come soon. It's impossible to filter out ALL ripple but after a certain point it becomes so small you won't even care. Adding more capacitance will help but adding additional LC filtering is a more cost effective method. You could also add a linear regulator stage.

[@Aboamer31]

Thank you, u r better than university

[@Avionics1958]

thank you very much

[@karlacha8560]

aside from noise, what are the limitations in choosing the practical values of the feedback resistors especially the amount of current the power supply would allow. also, in reality, the voltage divider equation only accounts to a no-load condition. it would be different when loaded.

[@norwinsolano7799]

I'd like to build a 50W AC to DC transformer. What do you recomend?

[@faridkhan7854]

Hi Plz can u make a current regulator. To deliver stable current to drive of led or other loads with precise controlled currents

[@pasty734]

can you please tell me the difference between this PWM and the one you made with an NE555 ?

[@powapumas38FTW]

Very interesting video :) but, I've a question. How can i control this circuit with an Microcontroller (Aka Arduino)? I was thinking to use digital potentiometer but this isn't a good idea because they have few steps. Thank you :)

[@codebeard]

How would you go about building an adjustable power supply which can drop all the way to 0V or close to it (e.g. 0.1V)? All the circuits I've seen using these chips have a minimum output voltage of 1.2V. I suppose you could try to amplify the signal to the feedback pin so that rather than dividing the voltage you multiply it?

[@fede142857]

Personally, I would use a fixed voltage divider on the output, and then feed its output to a comparator input; the other input would go to the wiper of a potentiometer; its ends would then be connected to a TL431 or similar voltage reference. The comparator output could then be fed into a pass transistor for a linear power supply (I tested this on a simulator and it works flawlessly) or, I suppose (I could be wrong so don't quote me on that), that it could also go to the feedback pin of a regulator IC such as the LM2678 shown on the video. If, for example, your voltage reference is 2.5V, and you want an output range from 0 to 25 volts, you could use a voltage divider with a 9 to 1 resistor ratio (which would divide the output by 10)

[@chethanraju6647]

Hello, i want to design an SMPS with 5v USB input and dual output of 12v and 5V for Battery and Ardunio respectively. I would request you to suggest a PCB board or an IC. Thanks and Regards

[@dth2731]

If the signal from the feedback voltage divider was sent to an Arduino Uno analog input and an output mapped to this signal sent back to the feedback pin of the LM2678T , would the Arduino be able to control the voltage output? Asking because I have no clue.

[@shad0wburn3d]

Greatness. Cheers.

[@HakureiMikko]

Excellent video as always! I only have one question: How about using a potentiometer with it's outer terminals tied to the rails and the viper to the feedback pin? How would it compare to the fixed resistors + pot in your configuration?

[@Afrotechmods]

Don't do that. If you do this you could easily end up shorting the feedback pin to ground or to your high voltage output.

[@HakureiMikko]

Ah, but of course. I didn't think of that. Thank's for the clarification.

[@yangzhang2839]

hi,i have a constant 15v dc power source,and i want to use pwm signal to control the buck converter,then i need the controllable output voltage.add the pwm signal on FB pin? and how to do that?

[@espenbgh2540]

How about ripple and or spikes on the output(like with the buck conv.) - it's the same Circuit 2678?How about the front end(in) just a transformer and a diode bridge?What is the efficiency with this Circuit(?) and the cooling af Circuit(?) any special needs drawing 3 Amp at 5 or 12 V?A complementary Circuit for negative supply?Have a nice day.EB from the other side of the sea.

[@aymanzayed]

if u have smps ac/ dc like cell phone charger do u know how to make it 12 v instead 5v?

[@wewwewew670]

hello is it neccesary to put a diode in the output so that the battery wont flow back to the circuit?

[@mattkingston08]

Can anything be added t this to limit the max current at the output thus making an adjustable cc supply?thanks in advance.

[@Eratas1]

Can i simulate feedback with microcontroller? How? Im thinking building my own Solar MPPT controller.

[@ringbaer8396]

can i load a smartphones lithium ionen akku with it, or is it a bad idea (no current limitation)?

[@Psycho4Ever666]

+Afrotechmods Hi I found not really information about this: I asked myself whether it is possible to directly feed a voltage into the Feedback Input via an Operational amplifier or not?

[@guitarguy3688]

Can you explain bootstrap configuration please

[@glaizadecierdo8154]

hi good day..can i use a 3.7v battery for the input?

[@tmc200527]

What were you using for the input voltage source?

[@Afrotechmods]

I was just using my bench power supply

[@tusharkumarchawda3085]

I have step down transformer of 24V, 1.5A supply using rectifiers and buck converter can i get 5 A supply

[@girishnimje9213]

Can I use an Electrolytic or Ceramic cap in place of Bootstrap cap

[@mediaguardian]

In this video and others (using an LM317) you showed how to regulate voltage and hold it constant, even if the load changed. But how do you regulate current (i.e. hold it fixed) regardless of output voltage or load?

[@Afrotechmods]

You want what is called a constant current source. You can make a linear one out of an LM317 but you can also make a switch mode one.

[@JoaoFerreira-bi8wt]

Afrotechmods Could that be done with another LM267T-ADJ?

[@emilioyahoo]

if my loas isn't grounded, do i still need the divider?

[@MyMinecraftmodz]

Will this work to get 42v out for my battery pack?

[@christyh.7343]

Could you use 2 of these circuits to run say 1 string of LEDs. To have a "low" and a "high" setting? As in for LED tail lights on a car. So you would have the "low" (normal driving lights) then a secondary circuit connected to the brakes that would be set to a higher voltage to emulate the brake lights? I'd imagine there would need to be an added component or two. As in diodes on the outputs where the voltage wouldn't backfeed. Thought about trying this on some custom tail lights I might be building in the near future. or even a possible 3rd for a certain design in the LED's. Also obviously some of the components would have to be upgraded to handle more currently as some components are rated at 5 A

[@Afrotechmods]

Well ideally you'd want to use a constant current LED driver circuit which would be similar to this buck converter but with current feedback instead of voltage feedback. If you want to hack this design, try using some mosfets or a digital potentiometer to change the effective feedback resistor values to get different voltages.

[@SlydinAE86]

Afrotechmods hmmmm that makes sense. Thanks I'll look into that.

[@SlydinAE86]

Afrotechmods hmmmm that makes sense. Thanks I'll look into that.

[@OriginalDeadGuy]

1:26, doc you trying to build a time machine by any chance?

[@SuperBlackReality]

Why do you use 1k resistor after potentiometer? You said that if you connect it straight, it will output 1.21V, so why 'limit it' to minimum 2.5V?

[@leidyyohanarodriguezcalzad4670]

I want to use a LM2679 to design a 1.25 - 30V with adjustable Current limit power supply, does anyone have any idea for where to start?

[@freeflaminginfinite917]

Caught that Back to the Future reference there (1:27). Lucky your not trying to power the flux capacitor or you'd need some kind of lightening power source...

[@jaimebarr8426]

1.21 gigawatts! You kill me.

[@Korupt0]

wouldnt you need a flux capacitor for that? 😁... subscribed

[@espenbgh2540]

1.21 gigawatts yeah man "into the future"

[@szafran1543]

Great video! I have a question. Can we just use potentiometer as voltage divider? Why use voltage divider AND pot? If no, please tell me why. Thanks :)

[@Afrotechmods]

If you think about it, moving the pot to one extreme end or the other will result in a direct connection between the feedback pin and ground or feedback and Vout. You can get away with this in certain situations but I wanted to show a resilient design to my audience.

[@szafran1543]

Ok, thanks. Can you tell, just for curiosoty, why it can be a bad idea to connect this connection directly to Vcc or ground?

[@Afrotechmods]

szafran Depends on how the feedback circuitry is designed. Could damage it. It's the type of thing you need to read about in the datasheet.

[@szafran1543]

Understood, thanks

[@peshmadscientist1833]

I doubt it would hurt to just use a pot for feedback, unless data sheet gives a minimum resistance between output and feedback. This looks like a really interesting chip. wonder if more powerful versions have been since developed.....

[@andrekissoon5047]

can i run an amp on this and can i purchase it

[@rheyjen006]

So I can make a 12V-1.5V output out of 15VDC?

[@DolezalPetr]

yes

[@budstran2121]

What should be changed to make 1.5v or even lower?

[@kyleg.5041]

can you do a video one LC low pass filters

[@imthe107]

can be a similar circuit with a LM2679?? I would like to use it because give more current in the output

[@Afrotechmods]

Yes! The LM2679 will be very similar.

[@imthe107]

@@Afrotechmods ok thanks for the answer i will they with the same components or similars thanks and great video

[@philipvernejules9926]

.......the switchode power supply can interfere with itself .... Naw ,, I'm too mature for purile humour. I'll leave it alone .

[@Phrennie]

She lives WAY up in Lisa's attic in The Old Executive Office Building.

[@minutz99]

at 3:50 minutes the output resistor are 10k-1k-1k but at 4:00 minutes are 10M-1M-1M what is the correct value?

[@user-gu4qg1ed2h]

Your explanation is beautiful and wonderful, but if translated into Arabic so that we understand and thank you

[@DolezalPetr]

wat

[@j5892000]

So the variable resistor is controlling the transistor